[1-41]
[1] Singlet oxygen in the environmental sciences. VIII. Absolute rates
of deactivation of O2(1Dg) by terminal olefins, tetramethylethylene, and
methyl chloride
R. A. Ackerman, J. N. Pitts, Jr., R. P. Steer, Journal of Chemical Physics
1970, 52, 1603-1604
Abs. rate consts. (kg) for deactivation (or reaction) of O2-(1Dg) by various
mols. were detd. directly by measuring intensity of the O2(1Dg) -> O2(3Sg) emission at 1269 nm in a discharge flow system. Deactivating mols. and dg
values (1./mole-sec) are: MeCl 9.6 * 102,C2H4 1.1 * 103, propylene 1.3 *
103, 1-butene 1.4 * 103, 1-pentene 1.9 * 103, 2,3-dimethyl-2-butene 3.0 *
105. For terminal olefins, kg increases with increasing mol. wt. as expected
when deactivation occurs via electronic to vibrational energy transfer.
Contrary to behavior of O2(1Sg+)-quenched systems, deactivation of
O2(1Dg) is little affected by the polar mol. MeCl, and is relatively
inefficient. In polluted urban atms., the rate of quenching of O2(1Dg) by
ground-state O2 is several orders of magnitude faster than the rates of
reaction of O2(1Dg) with simple monoolefins. [on SciFinder (R)] asdfgf
[2] Chemiluminescent reactions of ozone with olefins and organic
sulfides
J. N. Pitts, Jr., B. J. Finlayson, H. Akimoto, W. A. Kummer, R. P. Steer, Int.
Symp. Identification Meas. Environ. Pollut., [Proc.] 1971, 32-37
Detailed studies to elucidate the emitting species and mechanisms of the
chemiluminescent reactions of O3 with some S compds. and with olefins
were carried out at 1 torr and their spectra recorded. The intensity of the
emission relative to ethylene from the reaction of O3 with tri- and
tetramethylethylene, 2-butene, H2S, methyl mercaptan and dimethyl sulfide
was 50, 50, 10, 25, 2000, and 200, resp. These results may have important
applications for monitoring of O3, S compds., or olefins in an urban atm. [on
SciFinder (R)] asdfgf
[3] Chemiluminescent reactions of ozone with olefins and sulfides
J. N. Pitts, Jr., W. A. Kummer, R. P. Steer, Environmental Science and
Technology 1971, 5, 1045-1047
Chemiluminescence spectra from the gas-phase reactions of ozone with
several olefins and org. sulfides in the mm pressure range have been
recorded, and the integrated emission intensities relative to ethylene have
been calcd. Possible uses of these reactions in monitoring low concns. of
ozone and S-contg. pollutants are discussed. [on SciFinder (R)] asdfgf
[4] Effect of pressure on the rate of reaction molecular oxygen (1Dg)
with tetramethylethylene
R. A. Ackerman, J. N. Pitts, Jr., R. P. Steer, Chemical Physics Letters 1972,
12, 526-528
In the reaction of O (1Dg) and 2,3-dimethyl-2-butene in a discharge flow
system, the abs. bimol. rate const. k increases with decreasing total pressure
at 0.5-3 mm. Above 3 mm, k = (4.9 +- 0.3) * 105 l.-mole-1-sec-1. A kinetic
mechanism is developed. The pressure behavior explains discrepancies in k.
[on SciFinder (R)] asdfgf
[5] Chemiluminescent reactions of ozone with olefins and organic
sulfides
J. N. Pitts, Jr., W. A. Kummer, R. P. Steer, B. J. Finlayson, Advances in
Chemistry Series 1972, 113, 246-254
Spectra from the chemiluminescent gas phase reactions at 0.5 torr, of O3
with ethylene, tetramethylethylene, trans-2-butene, and MeSH at room temp.
are presented. The general features of the emissions obtained from reaction
in the gas phase of O3 with 14 different olefins are given. The emitting
species in the O3-olefin reactions have been tentatively identified as electron
excited aldehydes, ketones, and a-dicarbonyl compds. The reaction of O3
with H2S MeSH, and Me2S produces SO2 in its singlet excited state. [on
SciFinder (R)] asdfgf
[6] Formation of unscavengeable diradical intermediates in the
photolysis of cis- and trans-3-ethyl-2-propylthietane
D. R. Dice, R. P. Steer, Journal of the Chemical Society, Chemical
Communications 1973, 106-107
Gas-phase photolysis of either cis- or trans-3-ethyl-2-propylthietane gave
EtCH:CH2, cis-and trans-EtCH:-CHPr, and isomerized substrate. The yields
were unaffected by addn. of NO or CO2. A short-lived diradical
intermediate was indicated. [on SciFinder (R)] asdfgf
[7] Primary and secondary photolytic processes in the
photodecomposition of thietane vapor
D. R. Dice, R. P. Steer, Journal of Physical Chemistry 1973, 77, 434-438
The photolysis of thietane vapor at 254 and 313 nm and at temps. between
22 and 200 Deg was examd. The primary photodecompn. process forms
C2H4 and thioformaldehyde. Thioformaldehyde monomer is long lived and
undergoes secondary photolysis to produce propylene in a sequence of
reactions involving both the photoexcited monomer and a diradical..
Photolysis of thioformaldehyde yields He with a quantum yield ?0.08 at
temps. ?200 Deg and is therefore much more resistant to photodecompn.
than its O analog. [on SciFinder (R)] asdfgf
[8] Hot 1,4-biradicals from the photodecomposition of 3-ethyl-2propylthietane vapor
D. R. Dice, R. P. Steer, Journal of the American Chemical Society 1974, 96,
7361-7362
Interaction of Hg 6(3P1) with 3-ethyl-2-propylthietane (I) results in cleavage
of C4-S and C2-C3 bond to produce 1-butene and thiobutyraldehyde or of
C2-S and C3-C4 bond to produce 3-heptene and thioformaldehyde. When
either the cis or the trans isomer of I is photolyzed with Hg sensitization,
both cis- and trans-3-heptene are produced and their ratio varies with
pressure, indicating that hot triplet 1,4-biradicals are their precursors.
Comparison of triplet and singlet biradicals from photosensitized and direct
photolysis resp. reveals that the triplets are longer lived than the singlets,
likely as a result of a slow intersystem crossing step which the triplets must
undergo prior to decompn. or ring closure. [on SciFinder (R)] asdfgf
[9] Photolysis of thietane-cyclopentadiene mixtures. Reactions of
monomeric thioformaldehyde
D. R. Dice, R. P. Steer, Canadian Journal of Chemistry 1974, 52, 3518-3522
The 313 nm photolysis of thietane-cyclopentadiene mixts. in the vapor phase
at temps. between 25 and 235 Deg was studied. 2-Thiabicyclo[2.2.1]hept-5ene(I) results from the concerted, thermal, 1,4-Diels-Alder cycloaddn. of
thioformaldehyde (II) to cyclopentadiene. Other products arise via
nonconcerted processes involving biradical intermediates. II is scavenged
quant. by cyclopentadiene and thus the yield of I may be used as a measure
of the amt. of monomeric II produced in the system. [on SciFinder (R)]
asdfgf
[10] Fluorescence from the second excited singlet state of thiophosgene
vapor
S. Z. Levine, A. R. Knight, R. P. Steer, Chemical Physics Letters 1974, 29,
73-76
Fluorescence was obsd. from the 2nd excited singlet state of thiophosgene
vapor. The emission is excited mainly by transitions originating in the outof-plane bending mode of the ground state and terminating in the lowest
vibrational levels of the excited state. [on SciFinder (R)] asdfgf
[11] Mechanism of the direct photochemical decomposition of thietane
and its derivatives in the vapor phase, in solution, and in glassy matrices
D. R. Dice, R. P. Steer, Canadian Journal of Chemistry 1975, 53, 1744-1755
The direct photolyses of thietane, 3-ethyl-2-propylthietane, and 3methylthietane in the vapor phase, in soln., and in glassy matrices at low
temps. were examd. The effects of varying the photolysis wavelength, the
temp., the pressure and the phase of the substrate, and of adding inert
thermalizers on the nature and yields of the various products were measured.
The results are interpreted in terms of initial C-S cleavage to give a 1,4biradical which may, in the gas phase, decompose or ring close before
complete equilibration of the various rotamers is achieved, or which may be
thermalized in condensed media and trapped in glassy matrices at low temp.
[on SciFinder (R)] asdfgf
[12] Spectroscopic study of fluorescence from the second excited
singlet state of thiophosgene vapor
T. Oka, A. R. Knight, R. P. Steer, Journal of Chemical Physics 1975, 63,
2414-2420
The S2-S0 absorption, emission, and excitation spectra of thiophosgene
vapor were measured and the obsd. single vibronic transitions assigned.
Comparison of the absorption and excitation spectra reveals that emission is
excited only when transitions to the upper state terminate in 3m4n, m or n =
0 or 1, and that the upper state is depopulated by rapid radiationless
processes when higher vibrational levels are populated. The most prominent
bands in the emission spectrum are those in which the transitions terminate
in 1p3q4r with 1?p?10, 1 ?q?5, and 1?r?4. All 3 types of spectra are
consistent with a pyramidal excited state in which the equil. internuclear
distances are substantially greater than those of the ground state. The
implications of this fluorescence anomaly with respect to the nature of
radiationless process in the thiophosgene are discussed. [on SciFinder (R)]
asdfgf
[13] Nanosecond time-resolved emission spectroscopy. The 1La band
excitation of styrenes
R. P. Steer, M. D. Swords, P. M. Crosby, D. Phillips, K. Salisbury, Chemical
Physics Letters 1976, 43, 461-464
Nonexponential decay of fluorescence from low pressure vapors of styrene,
alkylstyrenes and indene, following excitation in the intense 1La(1A1-1A1)
band were obsd. using a conventional single-photon-counting technique.
Time resolved emission spectra, using a frequency doubled, cavity dumped
Ar+ laser (l excitation = 257.25 nm), revealed the presence of two emitting
states. The nature of both the short and long-lived emitting states was
discussed. [on SciFinder (R)] asdfgf
[14] Fluorescence from S2 single vibronic levels in thiophosgene
vapor: Quantum yields and quenching effects
T. Oka, A. R. Knight, R. P. Steer, Journal of Chemical Physics 1977, 66,
699-706
The quantum yields of fluorescence from the 2nd excited singlet state of
thiophosgene vapor were measured as a function of pressure for 8 S2 -> S0
transitions, and parameters for the decay of the levels pumped are calcd. The
results indicate that reassignment of the origin of the S2 -> S0 band system
to 35,112 from 36,007 cm-1 is necessary. Quantum yields of fluorescence
from the lowest levels in S2 are extremely high (0.20-1.0) and are indicative
of a very slow radiationless decay rate. Weakening of fluorescence from
higher S2 vibrational levels is most likely caused by predissocn. [on
SciFinder (R)] asdfgf
[15] Fluorescence decay of thiophosgene vapor excited to its second
excited singlet state
D. Phillips, R. P. Steer, Journal of Chemical Physics 1977, 67, 4780-4782
The fluorescence decay of CSCl2 vapors was studied as a function of
pressure and excitation wavelength by using time-correlated single-photon
counting with up to 6h counting at low pressures. All the decays were
exponential, but did not show oscillatory behavior. The pressure effects were
consistent with collisional self-quenching which was significant at >0.5 torr.
Hard-sphere collision rates at 0.5 torr were 1.6 * 107 s-1. The
nonexponential decay indicates that in the excited S2 state of CSCl2,
reversible radiationless transitions occur. The data support the reassignment
of T. Oka, et al. (1977) of the origin of the S2 -> S0 band system. [on
SciFinder (R)] asdfgf
[16] Sonoluminescence of aqueous solutions
C. Sehgal, R. P. Steer, R. G. Sutherland, R. E. Verrall, Journal of Physical
Chemistry 1977, 81, 2618-2620
The effects of adding efficient H atom and OH radical scavengers on the
spectral distribution and intensity of sonoluminescence from Ar satd. aq.
solns. were investigated. The results indicate that the emissive continuum is
due to a chemiluminescent process, likely H + OH + M -> H2O + M + hn.
[on SciFinder (R)] asdfgf
[17] Photophysics of 1-phenyl-2-butene
R. P. Steer, K. Salisbury, Journal of Photochemistry 1977, 7, 417-420
Illumination of the title compd. at 257.25 nm populates only vibronic levels
in S1; the 2nd excited singlet of the mol. is much higher in energy. The rate
consts. for the excited singlet radiative and nonradiative decay processes are
detd. [on SciFinder (R)] asdfgf
[18] Second excited singlet state fluorescence of thiocarbonyl
chlorofluoride
D. J. Clouthier, A. R. Knight, R. P. Steer, Chemical Physics Letters 1978,
59, 62-65
Gas-phase absorption and fluorescence-emission and -excitation spectra of
the ~B1A' -> ~X1A' (p* -> p) band system of ClCSF at room temp. and at -
78 Deg were measured. Quantum yields of fluorescence from 9 single
vibronic levels of the ~B state were measured under isolated-mol.
conditions, and radiative and nonradiative decay consts. were estd. [on
SciFinder (R)] asdfgf
[19] Mercury photosensitized decomposition of thietane
D. R. Dice, R. P. Steer, Canadian Journal of Chemistry 1978, 56, 114-118
The Hg-photosensitized decompn. of thietane vapor at room temp. has given
ethylene, propylene, cyclopropane, 1,2-dithiocane, and 1,5-hexadiene. At
low pressures quantum yields of thietane decompn. exceed unity. A
mechanism involving thiatrimethylene and 4,5-dithiahexamethylene
biradicals is proposed. [on SciFinder (R)] asdfgf
[20] The relaxation of electronically excited styrenes: a 'twisted'
excited state
K. P. Ghiggino, K. Hara, G. R. Mant, D. Phillips, K. Salisbury, R. P. Steer,
M. D. Swords, Journal of the Chemical Society, Perkin Transactions 2:
Physical Organic Chemistry (1972-1999) 1978, 88-91
Gas phase studies of excited 1La states (at 257.25 nm) of styrene, trans-1phenylpropene, and 1-phenylcyclobutene indicated relaxation to 'twisted'
long-lived singlet states, formed by a change in geometry of the excited state
by a rotation about the olefinic double bond. No long-lived component was
obsd. on excitation of 2-phenyl-2-norbornene due to restricted rotation
imposed by the ring system. [on SciFinder (R)] asdfgf
[21] Ab initio SCF MO calculations of the potential surfaces of
thiocarbonyls. I. ~X1A1 and ~a3A2 electronic and ~a3A2(b1)
vibrational states of carbonothioic difluoride
A. Kapur, R. P. Steer, P. G. Mezey, Journal of Chemical Physics 1978, 69,
968-975
Ab-initio SCF-MO calcns. with an STO-3G basis set were done to obtain the
potential-energy surfaces of the ground ~X1A1 and lowest triplet ~a3A2
states of F2CS. The calcd. equil. geometry of the ground state agrees with
the exptl. geometry. By using a suitable analytic approxn. to the triplet-state
potential surface, energies were calcd. for the stack of triplet-state inversion
vibrational levels assocd. with a double-min. potential curve along the outof-plane bending coordinate. Good agreement with expt. was obtained. [on
SciFinder (R)] asdfgf
[22] A non-empirical SCF MO study on the first triplet state potential
energy surfaces and vibrational frequencies of simple thiocarbonyls
P. G. Mezey, R. P. Steer, A. Kapur, Journal of Photochemistry 1978, 9, 130131
The 1st triplet state vibrational potentials of small model mols. contg. the
thiocarbonyl functional group were studied with the aid of ab initio MO SCF
theory. Large portions of the ground state singlet and 1st triplet state
potential energy surfaces were calcd. and theor. ests. were obtained for the
excited state vibrational frequencies using a perturbation theor. approach.
The theor. results on the triplet state vibrational frequencies show good
agreement with the available exptl. information obtained from the T1 -> S0
absorption spectra. The calcd. triplet state double min. potentials for the
pyramidal inversions were studied in some detail using an analytic approxn.
to the ab initio potential. The calcd. inversion barrier heights are of the order
of .apprx.6-8 kcal/mol (7.74 and 5.80 kcal/mol for F2C:S and FClC:S,
resp.). [on SciFinder (R)] asdfgf
[23] Vibrational relaxation in first electronically excited singlet state of
fluorobenzenes
R. P. Steer, M. D. Swords, D. Phillips, Chemical Physics 1978, 34, 95-102
The 1B2u state of p-F2C6H4 is apparently strongly self-quenched, with rateconst. of the order of 5.8 * 1011 L mol-1 s-1 for the zero point level.
Possible causes are discussed. Vibrational relaxation in this manifold (after
excitation of members of the n1 progression in the 1B2u state) was studied
by fluorescence quantum yields and decay times. A stepladder model does
not reproduce the latter data as well as found previously for the related mol.
p-FC6H4Me. It is suggested that the d. of initially populated levels should
be high in order for such models to be appropriate. P-F2C6H4 (1Ag) was a
much more efficient relaxer of p-F2C6H4 (1B2u) than CHF3. The
involvement of resonant V-V energy transfer through the sym. C-F
stretching mode which has almost identical frequency in 1Ag and 1B2u
states can thus be discounted. An alternative mech. is suggested. [on
SciFinder (R)] asdfgf
[24] Optical spectroscopy and second excited state photophysics of
small thiocarbonyls
R. P. Steer, A. R. Knight, D. J. Clouthier, D. C. Moule, Journal of
Photochemistry 1978, 9, 157-158
UV-visible absorption spectra with resoln. of .apprx.100,000 were obtained
for portions of the S2 -> S0 (p* -> p) band systems of Cl2CS and ClFCS
vapors at 298 and 196 K. Two excited state frequencies expected to be
prominent on Franck-Condon grounds were identified. For the out-of-plane
bending mode in S2, small inversion doublet splittings (<1 cm-1) were
calcd. for both ClFCS and Cl2CS. Both compds. exhibit remarkably high
quantum yields of anomalous S2 -> S0 fluorescence in the gas phase at low
pressure. fluorescence quantum yields from single vibronic levels near the
origin are unity within exptl. error for Cl2CS and exceed 0.55 for ClFCS.
Fluorescence spectra of these compds. are broad and relatively structureless,
although a progression of 10 members in the C-S stretching mode can be
identified in Cl2CS; quite well-resolved fluorescence excitation spectra can
be obtained, however. Present data suggest that ClFCS and Cl2CS in their
S2 states are examples of \"intermediate case\" mols. [on SciFinder (R)]
asdfgf
[25] Laser excitation study of the second excited singlet state
photophysics of carbonothioic chloride fluoride
D. J. Clouthier, A. R. Knight, R. P. Steer, P. A. Hackett, Journal of
Chemical Physics 1979, 71, 5022-5029
A tunable dye laser was used to excite selected single vibronic levels of the
~B(1A') state of ClFCS in the gas phase. Fluorescence (~B -> ~X) excitation
spectra, quantum yields, and excited state lifetimes were measured.
Radiative and nonradiative decay consts. and bimol. self-quenching consts.
were calcd. for each selected state. A model in which a single dissociative
vibrational mode provides the sole nonradiative decay mechanism is
proposed. [on SciFinder (R)] asdfgf
[26] Ab initio SCF MO calculations of the potential surfaces of
thiocarbonyls. III. Ground state and first excited triplet state of
thiourea, (NH2)2CS
A. Kapur, R. P. Steer, P. G. Mezey, Journal of Chemical Physics 1979, 71,
588-592
Equil. geometries for the ground and 1st excited triplet states of thiourea (I)
were calcd. using ab initio SCF MO methods. Apart from the pyramidality
of the NH2 groups, the calcns. for I indicate a planar ground state analogous
to simple tetraat. thiocarbonyls. The 1st triplet state out-of-plane bending
mode frequencies were calcd. by a perturbational method. The rotation of
the NH2 moiety about the C-N bound was studied for the ground and 1st
excited triplet states, and the barriers to these internal rotations were calcd.
[on SciFinder (R)] asdfgf
[27] Ab initio SCF MO calculations of the potential surfaces of
thiocarbonyls. II. Methanethial, thioformyl fluoride, carbonothioic
chloride fluoride, and carbonothioic dichloride
A. Kapur, R. P. Steer, P. G. Mezey, Journal of Chemical Physics 1979, 70,
745-748
Potential energy surfaces and equil. geometries for the ground and 1st
excited triplet states of H2CS, HFCS, ClFCS, and Cl2CS were calcd. using
ab initio SCF MO methods. The triplet state out-of-plane bending mode
frequencies were calcd. using a perturbation theor. treatment and a suitable
analytic approxn. to the potential along the out-of-plane bending coordinate.
Where exptl. results are available, good agreement exists between calcd. and
exptl. data. The equil. out-of-plane angle in the lowest triplet state is
correlated with the decrease in the p electron d. on the C atom
accompanying the change from the planar to the equil. out-of-plane
conformation. [on SciFinder (R)] asdfgf
[28] Sonoluminescence of argon saturated alkali metal salt solutions as
a probe of acoustic cavitation
C. Sehgal, R. P. Steer, R. G. Sutherland, R. E. Verrall, Journal of Chemical
Physics 1979, 70, 2242-2248
Emission from Ar satd. aq. alkali metal salts solns. was obsd. during
insonation at 460 kHz. It arises from deexcitation of excited alkali metal
atoms formed by free radical redn. processes. In addn. to the emission
resonance lines, diffuse bands were obsd. at .apprx.554 and .apprx.740 nm
for Na and K, resp. The latter are due to emission from alkali metal-Ar
exciplexes and are known to occur when mixts. of alkali metal vapor and Ar
are rapidly compressed. An est. of the cavitational temps. and pressures was
obtained by comparison of exptl. emission band parameters with those
derived theor.. [on SciFinder (R)] asdfgf
[29] The ~B(1A') -> ~X(1A') spectrum of carbonothioic chloride
fluoride
D. J. Clouthier, A. R. Knight, R. P. Steer, R. H. Judge, D. C. Moule, Journal
of Molecular Spectroscopy 1980, 83, 148-160
The vibrational structure of the first strong electronic transition(.hivin.B1A'>.hivin.X1A') of CIFCS was analyzed. Four excited state frequencies were
identified. The excited state is nonplanar and has a barrier to inversion of
.gtorsim.2000 cm-1 and a C-S bond which is ~0.5 .ANG. longer than the
ground state. [on SciFinder (R)] asdfgf
[30] Second excited singlet state emission spectroscopy of
thiocarbonyls. II. Thiocarbonyl chlorofluoride
D. J. Clouthier, A. R. Knight, R. P. Steer, P. A. Hackett, Chemical Physics
1980, 48, 13-20
Single vibronic level emission spectra were measured for 4 excitation
wavelengths pumping the 2nd excited singlet state of ClFCS. Only
resonance fluorescence is obsd. Progressions involving Dv''1 = 0,1,2,...,
Dv''6 = 0, 2, 4... dominate. Anal. of the spectra indicates that neither intra- or
intermol. processes lead to odd Dv'6 changes and that collisions in the pure
gas lead to electronic quenching of mols. in the excited state. [on SciFinder
(R)] asdfgf
[31] Second excited singlet state emission spectroscopy of
thiocarbonyls. I. Thiophosgene
D. J. Clouthier, A. R. Knight, R. P. Steer, P. A. Hackett, Chemical Physics
1980, 48, 1-11
Well resolved emission spectra were recorded after dye laser excitation of
low-lying vibronic levels in the ~B(1A1) state of Cl2CS. The effects of
substrate pressure, added SF6 and excitation wavelength on the spectra are
reported. A Franck-Condon anal. suggests that the excited state C-S bond
length is 0.5 .ANG. longer than in the ground state. [on SciFinder (R)]
asdfgf
[32] Sequential two-photon absorption and singlet-singlet energy
pooling in thiophosgene vapor
D. J. Clouthier, A. R. Knight, R. P. Steer, P. A. Hackett, Journal of
Chemical Physics 1980, 72, 1560-1568
Emission from the 2nd excited singlet state of Cl2CS vapor was monitored
following excitation with a tightly focused pulsed dye laser in the region of
the ground state to 1st excited singlet and triplet absorption systems.
Excitation in the triplet bands produces prompt fluorescence from S2 as a
result of sequential 2-proton absorption, S0 -> T1 -> S2. Excitation in the
bulk of the singlet bands produces no prompt fluorescence, but does produce
delayed fluorescence as a result of a bimol. singlet-singlet energy pooling
process, 2S1 -> S2 + S0. [on SciFinder (R)] asdfgf
[33] Photophysics of the ~B1A1 state of thiophosgene
D. J. Clouthier, A. R. Knight, R. P. Steer, P. A. Hacket, Chemical Physics
Letters 1980, 70, 89-92
The results of fluorescence lifetime measurements using a dye laser
excitation source are combined with other recently published data to present
a consistent picture of the photophysics of the B1A1 state of Cl2CS. [on
SciFinder (R)] asdfgf
[34] Do aromatic C-nitroso compounds fluoresce from their second
excited singlet states?
D. A. Condirston, A. R. Knight, R. P. Steer, Journal of Photochemistry
1980, 14, 257-260
Absorption, emission, and emission excitation spectra of nitrosobenzene, p(dimethylamino)nitrosobenzene, and p-(diethylamino)nitrosobenzene in
hydrocarbon and EtOH solvents were obtained and indicated that
fluorescence of significant intensity does not occur from the 2nd excited
singlet state of these compds. The fluorescence excitation spectrum bears no
resemblance to the absorption spectrum, particularly at the long-wavelength
end. Previously reported emission attributed by other workers to
fluorescence from 2nd excited singlet states of these compds. can be
considered as due to impurities. [on SciFinder (R)] asdfgf
[35] Structure and decay dynamics of electronic excited states of
thiocarbonyl compounds
R. P. Steer, Reviews of Chemical Intermediates 1980, 4, 1-41
A review with .apprx.168 refs. [on SciFinder (R)] asdfgf
[36] Rydberg and valence-shell transitions in the quartz ultraviolet
spectra of aliphatic thiones
A. E. Bruno, D. J. Clouthier, P. G. Mezey, R. P. Steer, Canadian Journal of
Chemistry 1981, 59, 952-956
The transitions obsd. in the gas-phase electronic absorption spectrum of
Me2CRCSMe (R = H, Me) were assigned using ab initio and semiempirical
calcns. Four transitions are obsd. in the quartz-UV region. A strong, broad,
structureless band is assigned to the p -> p* transition, and 3 weaker,
narrower bands are assigned to Rydberg n -> 4s, n -> 4pv and n -> 4pz
transitions on S. [on SciFinder (R)] asdfgf
[37] Structures and decay dynamics of thiocarbonyl excited states
D. J. Clouthier, P. A. Hackett, A. R. Knight, R. P. Steer, Journal of
Photochemistry 1981, 17, 319-326
Gas-phase absorption, emission and emission-excitation spectra provided
detailed ground- and excited-state structural information for several
representative small thiocarbonyls. The ground states are planar but the
excited states are generally nonplanar and exhibit a large variation in
inversion-barrier heights and equil. out-of-plane angles. The C-S bond is
elongated in excited states, by as much as 0.5 .ANG. in the S2 1(p,p*) states
of Cl2CS and ClCSF. Tetraat. thiocarbonyls decay solely by radiative means
from bound levels of their S1 1(n,p*) states under collision-free conditions;
radiative decay of the corresponding T1 states was not obsd. Excitation to
low-lying vibrational levels of the S2 states of ClCSF and Cl2CS in the gas
phase results in S2 -> S0 emission with high quantum yields (jf -> 1.0). At
higher vibrational energies, non-radiative decay (probably leading to
dissocn.) dominates. The observation of intense emission from S2 was
exploited to probe the mechanisms of the 2-photon absorption in gas-phase
Cl2CS. Sequential 2-photon absorption via T1 (S0 -> T1 -> S2) and bimol.
singlet-singlet energy pooling (S1 + S1 -> S2 + S0v, were obsd. [on
SciFinder (R)] asdfgf
[38] Calculation of the out-of-plane bending coordinates of tetraatomic
molecules by the G-matrix method
A. Kapur, P. G. Mezey, R. P. Steer, Chemical Physics Letters 1981, 78, 8184
A general expression for the calcn. of the G matrix element assocd. with the
out-of-plane bending coordinate of tetraat. mols. having C2v or Cs equil.
symmetry is presented. Three methods of calcg. the mass weighted out-ofplane bending coordinate as a function of out-of-plane angle are compared.
[on SciFinder (R)] asdfgf
[39] Oxygen quenching of the lowest triplet state of Nmethylthioacridone
A. Safarzadeh-Amiri, D. A. Condirston, R. E. Verrall, R. P. Steer, Chemical
Physics Letters 1981, 77, 99-102
Emission, assigned as ~a3A2 -> ~X1A1 phosphorescence, was observes
when N-methylthioacridone is excited in any of its visible or near UV
absorption bands in hydrocarbon soln. The O quenching of the thione triplet
(ET = 14,900 cm-1) proceeds with kq ~ 0.5 kd. Possible mechanisms of O
quenching are discussed. [on SciFinder (R)] asdfgf
[40] Out-of-plane bending coordinates for tetraatomic molecules
R. P. Steer, P. G. Mezey, A. Kapur, NATO Advanced Study Institutes Series,
Series C: Mathematical and Physical Sciences 1981, 67, 397-402
Different formulas are discussed for the transformation of the out-of-plane
angle to a mass weighted coordinate for tetraat. mols. Some of the problems
assocd. with such coordinate transformations are discussed. A general
formula for Cs as well as C2v tetraat. mols. is given. Calcd. and exptl. outof-plane bending mode frequencies were compared for the 1st triplet state of
F2CS. [on SciFinder (R)] asdfgf
[41] Pressure effects in the ~A-~X laser-excited fluorescence spectrum
of thiophosgene
D. A. Condirston, A. R. Knight, R. P. Steer, Chemical Physics Letters 1982,
86, 254-258
The intensities of bands in the ~A-~X fluorescence spectrum of
thiophosgene, laser-excited at 514.5 nm, were measured as a function of
pressure. Weak, nonresonance bands grow in with increasing pressure,
demonstrating the existence of collisional vibrational relaxation. Differential
electronic quenching of ~A states of different vibronic symmetry is
indicated. [on SciFinder (R)] asdfgf
[42] Ab initio SCF-MO calculations of features of the lowest triplet
state potential surfaces of several tetraatomic carbonyl compounds
A. Kapur, R. P. Steer, P. G. Mezey, Canadian Journal of Chemistry 1982,
60, 100-105
Non-empirical SCF-MO calcns. were carried out for RR1CO (R, R1 = H, H;
H, F; Cl, Cl; Cl, F; F, F). Portions of the ground singlet and 1st triplet state
potential energy surfaces, in particular those along the out-of-plane bending
coordinates, were detd. Ests. of the 1st triplet state out-of-plant vibrational
frequencies were calcd. [on SciFinder (R)] asdfgf
[43] Photophysics of laser-excited thioformaldehyde ~A(1A2)
A. E. Bruno, R. P. Steer, Journal of Chemical Physics 1983, 78, 6660-6665
Fluorescence was excited in the rotational subband heads of low lying
vibrational levels in the ~A(1A2) state of thioformaldehyde. Single
exponential fluorescence decays measured as a function of pressure yield
linear Stern-Volmer plots from which gas kinetic quenching rate and
collision-free lifetimes of between 14 (000 band) and 4 ms (402 band) were
obtained. The results are interpreted in terms of solely radiative decay of the
vibronic states investigated under isolated mol. conditions. [on SciFinder
(R)] asdfgf
[44] The thioketone-enethiol tautomerism of aliphatic thiocarbonyls:
an ab initio study
A. E. Bruno, R. P. Steer, P. G. Mezey, Journal of Computational Chemistry
1983, 4, 104-109
Approx. min. energy reaction paths have been calcd. for two thioketoneenethiol tautomeric systems using an ab initio SCF-MO method. The calcns.
indicate nearly equal stabilities of the isolated tautomers in both systems and
an energy barrier of ca. 85 kcal/mol for their interconversion. This barrier is
expected to be significantly lower in soln. as a result of solvent-solute
interactions. [on SciFinder (R)] asdfgf
[45] Excitation pulse-shape mimic technique for improving
picosecond-laser-excited time-correlated single-photon counting
deconvolutions
D. R. James, D. R. M. Demmer, R. E. Verrall, R. P. Steer, Review of
Scientific Instruments 1983, 54, 1121-1130
The detn. of accurate subnanosecond fluorescence lifetimes by the timecorrelated single-photon counting technique is often limited by difficulties in
obtaining the correct instrumental response function f(l,t) to the excitation
pulse. These difficulties are increased when a grating monochromator is
used to disperse the emission due to the introduction of an addnl. temporal
broadening of f(l,t). A technique for detg. the correct f(l,t) at the wavelength
of sample emission, lem, is described. The technique consists of using a very
short-lived fluorophore to mimic the excitation pulse shape at lem, yielding
the convoluted mimic decay function Cm(lem,*), and then to
computationally ext. f(lem,t) from Cm(lem,t). The technique is exptl. and
computationally simple and yields the desired instrumental response
function at lem which eliminates problems due to the sensitivity of f(l,t) to l
(color shift artifact). The photomultiplier tube receives spatially equal
illumination when both the sample decay and Cm(lem,t) are detd. because
both the mimic and sample emission are spectrally broad, eliminating the
problem of the variation of f(l,t) with position of incident light on the
photomultiplier tube photocathode (targeting artifact). Artifacts due to
monochromator temporal broadening are eliminated since both the mimic
and sample emission are monitored at const. wavelength. [on SciFinder (R)]
asdfgf
[46] Thioketone spectroscopy: an analysis of the lower electronic
transitions in thioacetone and thioacetaldehyde
R. H. Judge, D. C. Moule, A. E. Bruno, R. P. Steer, Chemical Physics
Letters 1983, 102, 385-389
Visible and UV spectra of the unstable species, thioacetaldehyde, CH3CHS,
thioacetone, (CH3)2CS, and thioacetone-d6 were studied in the gas phase.
The valence n -> p* excitations, ~A -> ~X and ~a -> ~X, were identified.
Rydberg n -> 4s, 4py and 4pz and p -> p* valence excitations were found in
thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the
n,4s and p,p* states which leads to a broad absorption of mixed character
between 200 and 220 nm. [on SciFinder (R)] asdfgf
[47] Effect of solvent on the subnanosecond decay of the second excited
singlet state of tetramethylindanethione
A. Maciejewski, R. P. Steer, Chemical Physics Letters 1983, 100, 540-545
The S2 -> S0 fluorescence spectra and quantum yields and the S2 lifetimes
of 2,2,3,3-tetramethylindanethione (TMIT) were measured in several
solvents by using a synchronously pumped picosecond dye laser excitation
system. The S2 nonradiative decay rate is markedly solvent dependent. In
inert perfluoroalkane solvents, remarkably large S2-S0 fluorescence
quantum yields (ff = 0.14) and long S2 lifetimes (t = 880 ps) were measured.
Hydrocarbons are efficient excited-state quenchers. [on SciFinder (R)]
asdfgf
[48] Photophysics of the second excited singlet states of xanthione and
related thiones in perfluoroalkane solvents
A. Maciejewski, R. P. Steer, Journal of the American Chemical Society
1983, 105, 6738-6740
S2 -> S0 fluorescence quantum yields and subnanosecond lifetimes of the
second excited singlet states of xanthione, 4H-1-benzopyran-4-thione and
4H-pyran-4-thione were measured in perfluoroalkane and common
hydrocarbon solvents. Both quantities are markedly solvent dependent and
indicate, together with rate consts. for xanthione S2 quenching by 3methylpentane and benzene (6.5 * 109 M-1 s-1 and 1.1 * 1010 M-1 s-1,
resp.), that exclusively photophys. properties of thione S2 decay cannot be
measured in hydrocarbon solvents. Variation of knr for the thiones with S2S1 electronic energy sepn. according to the energy gap law indicates that S2S1 internal conversion is the major nonradiative decay process in
perfluoroalkane solvents. [on SciFinder (R)] asdfgf
[49] Decay dynamics of aromatic thione triplet states in fluid solution
A. Safarzadeh-Amiri, R. E. Verrall, R. P. Steer, Canadian Journal of
Chemistry 1983, 61, 894-900
Phosphorescence emission and excitation spectra, quantum yields, and
lifetimes of arom. thiones are detd. in liq. soln. at room temp. Excitation to
any of the excited electronic states accessible in the near-UV and visible
regions of the spectrum results in population of the lowest triplet state with
near unit quantum yield. Triplets of relatively inflexible thiones decay
nonradiatively with rate consts. of the order of 105 s-1; thiones capable of
higher amplitude flexion or group rotation exhibit nonradiative decay consts.
>=107 s-1 at infinite diln. in aprotic solvents. Self-quenching occurs at
diffusion-controlled rates. [on SciFinder (R)] asdfgf
[50] Concerning the lifetime of the second excited singlet state of
adamantanethione
K. J. Falk, A. R. Knight, A. Maciejewski, R. P. Steer, Journal of the
American Chemical Society 1984, 106, 8292-8293
No S2 -> S0 fluorescence (ff < 5 * 10-6) is obsd. when adamantanethione is
illuminated in its elec. dipole allowed, S2 -> S0 absorption band in alkane or
perfluoroalkane solns. at room temp. On the basis of this observation and a
calcn. of its intrinsic radiative lifetime (1.6 ns), it is demonstrated that the S2
state must have a lifetime <<1 ps. The S2 state therefore cannot be the 250
ps reactive transient thought to be responsible for initiating the photochem.
of adamantanethione when excited to S2 in the presence of cycloalkanes and
olefins. Several possible alternate identities of the 250-ps transient are
discussed. [on SciFinder (R)] asdfgf
[51] Radiationless decay of the second excited singlet states of
aromatic thiones: experimental verification of the energy gap law
A. Maciejewski, A. Safarzadeh-Amiri, R. E. Verrall, R. P. Steer, Chemical
Physics 1984, 87, 295-303
The S2 -> S0 fluorescence quantum yields and S2 lifetimes of thiones I, II (
x = O, S, CMe2), III (R = H, X = O; R = Me, X = S), IV and (4MeOC6H4)2CS in inert perfluoroalkane solns. at room temp. have been
measured using picosecond laser techniques. Photostable, structurally rigid
thiones undergo S2 -> S1 internal conversion at rates consistent with the
energy-gap law of radiationless transitions. An av. electronic coupling
matrix element of 1.9 * 102 cm-1 is found. [on SciFinder (R)] asdfgf
[52] Bimolecular quenching of the second excited singlet state of
tetramethylindanthione
A. Maciejewski, R. P. Steer, Journal of Photochemistry 1984, 24, 303-307
Rate consts. for the title quenching of 2,2,3,3-tetramethylindanthione by O2,
MeCN, EtOH, and cyclohexane were detd. [on SciFinder (R)] asdfgf
[53] Vibronic analyses of the Rydberg and lower intravalence
electronic transitions in thioacetone
S. Paone, D. C. Moule, A. E. Bruno, R. P. Steer, Journal of Molecular
Spectroscopy 1984, 107, 1-11
The absorption spectrum of thioacetone, was measured at 190-700 nm, and 5
distinct absorptions were identified. These are the spin-allowed and spinforbidden n -> p*, ~A(1A2) -> ~X(1A1), and ~a(3A2) -> ~X(1A1)
transitions; the orbitally allowed p -> p*, ~B(1A1) -> ~X(1A1) transition;
and the Rydberg transitions n -> 4s, ~C(1B2) -> ~X(1A1), n -> 4pz,
~D(1B2) -> ~X(1A1), and n -> 4py, ~E(1A1) -> ~X(1A1). In addn., the
laser phosphorescence excitation spectrum was obtained for the ~a -> ~X
transition. Partial vibrational assignments were obtained for all of the excited
electronic states. The analyses showed that the ~a, ~A, and ~C states are all
planar or pseudoplanar. Of interest is the vibrational activity of the Me
torsional modes. The presence of torsional progressions in the visible
absorption spectrum led to the conclusion that the Me groups are rotated in
the ~a state by 60 Deg from the ~X state. Observation of torsional sequence
transitions rather than progressions in the ~C -> ~X spectrum indicates that
the Me groups do not change their configuration on excitation into the ~C
state. [on SciFinder (R)] asdfgf
[54] Simple, inexpensive autocorrelator for use with cw mode-locked
lasers
D. R. M. Demmer, A. Leyland, M. Szymanski, R. P. Steer, Review of
Scientific Instruments 1985, 56, 331-332
An improved method of obtaining variable pulse spacing for use in a
continuous-wave (CW) 2nd harmonic generation autocorrelation
interferometer is described. [on SciFinder (R)] asdfgf
[55] Fluorescence lifetime quenching and anisotropy studies of
ribonuclease T1
D. R. James, D. R. Demmer, R. P. Steer, R. E. Verrall, Biochemistry 1985,
24, 5517-5526
The time-resolved fluorescence of the lone tryptophanyl residue of RNase
T1 was investigated by using a mode-locked, frequency-double picosecond
dye laser. The fluorescence decay could be characterized by a single
exponential function with a lifetime of 3.9 ns. The fluorescence was readily
quenched by uncharged solutes but was unaffected by iodide. These
observations were interpreted in terms of the electrostatic properties of the
amino acid residues at the active site of the protein, which would appear to
restrict the access of solute species to the tryptophanyl residue. The temp.
dependence of the fluorescence lifetime and anisotropy decay time could be
rationalized in terms of a model which postulates a significant ordering of
the solvent layer immediately surrounding the surface of the protein. [on
SciFinder (R)] asdfgf
[56] Relaxation of the second excited singlet states of aromatic thiones:
the role of specific solute-solvent interactions
A. Maciejewski, D. R. Demmer, D. R. James, A. Safarzadeh-Amiri, R. E.
Verrall, R. P. Steer, Journal of the American Chemical Society 1985, 107,
2831-2837
Second excited singlet state (S2) fluorescence quantum yields and lifetimes
of 6 arom. thiones were measured in soln. at room temp. Intramol. S2 -> S1
internal conversion dominates S2 decay in inert perfluoroalkane solvents,
but intermol. photochem. and photophys. processes dominate in more
strongly interacting solvents. Stern-Volmer quenching is obsd. when
perfluoroalkane solns. of 2,2,3,3-tetramethylindanthione, a model thione, are
spiked with additives at concns. <.apprx.0.3 M. Measurements of the rate
consts. for the bimol. quenching of S2 fluorescence by 26 different additives
reveal that the thione S2 state is highly reactive, and that the initial
intermol.-interaction path divides into phys. and chem. branches, the
branching ratio being a function of the nature of the quencher. [on SciFinder
(R)] asdfgf
[57] Vibronic analysis of the ~A(1A2) -> ~X(1A1) spectrum of
carbonothioic dibromide (Br2CS)
B. Simard, R. P. Steer, Canadian Journal of Chemistry 1985, 63, 1418-1423
~A(1A2)->~X(1A1) Fluorescence excitation and emission spectra of Br2CS
were recorded. A vibronic anal. shows that the mol. is nonplanar in the ~A
state with an equil. out-of-plane angle of 13 Deg and a barrier to inversion of
164 cm-1. Upper state vibrational frequencies of modes 1, 2, 3, 4, and 6
were obtained. Comparisons with other tetraat. thiocarbonyls support the
assignments. A ground state C-Br dissocn. energy of 52.3 kcal-mol-1 was
obtained from the onset of diffuseness in the ~A-~X spectra. A search for
phosphorescence proved unsuccessful. [on SciFinder (R)] asdfgf
[58] Spectral and photophysical properties of 9,10-diphenylanthracene
in perfluoro-n-hexane: the influence of solute-solvent interactions
A. Maciejewski, R. P. Steer, Journal of Photochemistry 1986, 35, 59-69
The photophys. properties of 9,10-diphenylanthracene (DPA) were unusual
in perfluoro-n-hexane (PFH) compared with other solvents. Low values of
the fluorescence quantum yield, fF = 0.58, and S1 lifetime, tS1 = 6.7 ns, and
relatively large shifts in the absorption and emission spectra in PFH are
attributable to changes in the electronic energies of DPA which permit an
enhancement of the rate of S1 -> T2 relaxation in this solvent. A large value
of the intersystem crossing quantum yield in PFH, fISC = 0.35, supports this
conclusion. [on SciFinder (R)] asdfgf
[59] Thermally activated delayed S1 fluorescence of aromatic thiones
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of Physical Chemistry
1986, 90, 6314-6318
Absorption emission, and emission excitation spectra, emission lifetimes,
and quantum yields were measured for 4 arom. thiones, 4H-pyran-4-thione,
4H-1-benzopyran-4-thione, xanthione (XT), and 2,2,3,3tetramethylindanethione, in several solvents. Emission to the blue of T1
phosphorescence was identified as thermally activated delayed fluorescence
and its temp. dependence in a perfluoroalkane and a hydrocarbon solvent
investigated. The effect of quenching by O2 was studied. For XT in 3methylpentane at 293 K the quantum yield of delayed fluorescence is estd.
as 1.1 * 10-3, and the rate const. of T1 -> S1 intersystem crossing is 5.6 *
109 s-1. [on SciFinder (R)] asdfgf
[60] Time-resolved LIF detection of silylene in the IR MPD of
ethylsilane
D. M. Rayner, R. P. Steer, P. A. Hackett, C. L. Wilson, P. John, NBS Special
Publication (United States) 1986, 716, 577-588
Formation of SiH2 in the collisionless multiphoton IR dissocn. of
CH3CH2SiH3 resulted in secondary photolysis of the primary photolysis
product CH3CH2SiH. The mechanism involved reactions (1) CH3CH2SiH3
+ nhn -> CH3CH2SiH + H2 and (2) CH3CH2SiH + nhn -> C2H4 + SiH2
and was consistent with known shock-induced dissocn. of CH3CH2SiH3.
[on SciFinder (R)] asdfgf
[61] Time-resolved LIF detection of silylene in the IR MPD of
ethylsilane
D. M. Rayner, R. P. Steer, P. A. Hackett, C. L. Wilson, P. John, Chemical
Physics Letters 1986, 123, 449-452
Silylene was obsd. in the collisionless IR MPD (multiphoton dissocn.) of
ethylsilane via LIF (laser-induced fluorescence). A mechanism for the
unimol. dissocn. of ethylsilane which is consistent with known shockinduced dissocn. and IR MPD data is advanced. [on SciFinder (R)] asdfgf
[62] Potential energy surfaces for the molecular and free radical
dissociations of thioformaldehyde, thiocarbonyl difluoride, and
thiocarbonye dichloride: an ab initio SCF MO study
B. Simard, A. E. Bruno, P. G. Mezey, R. P. Steer, Chemical Physics 1986,
103, 75-83
Energies along the planar sym. (C2v) and planar asym. (Cs) paths to mol.
dissocn. of the ground-state thiocarbonyl halides F2CS and Cl2CS, together
with their transition-state geometries, were calcd. by ab-initio SCF-MO
methods using the STO-3G and 4-31G basis sets. For comparison, results on
H2CS at similar levels of calcn. were also existing. In addn., the 4-31G**
basis set was employed to predict the geometries of the ground-state species
and the endothermicities of their free-radical dissocns. The results of
existing expts., in which the lowest excited singlet states of these mols. were
photoexcited, were interpreted in light of these calcns. Thermodn. data for
both mol. and free-radical dissocns. were evaluated. [on SciFinder (R)]
asdfgf
[63] Unusual deuterium isotope effect on the radiationless decay of
xanthione S2
S. R. Abrams, M. Green, R. P. Steer, M. Szymanski, Chemical Physics
Letters 1987, 139, 182-186
S2 fluorescence lifetimes and quantum yields were measured for xanthione
and its d4, d6 and d8 substituted analogs in perfluoroalkane and hydrocarbon
solvents. A large D/H isotope effect on the nonradiative S2 decay rate was
found and was identified with deuteration in the 1 and 8 positions. The
results are ascribed to the existence of parallel S2 ->S1 and S2 ->S0
processes, the latter resulting from motion of the C(1)-H or C(8)-H atoms
towards the thiocarbonyl group, consistent with the early stages of a path
leading the intramol. H transfer. The effects are correlated with known S2
photochem. of similar mols. [on SciFinder (R)] asdfgf
[64] Three exponential fluorescence decay of horse liver alcohol
dehydrogenase revealed by iodide quenching
D. R. Demmer, D. R. James, R. P. Steer, R. E. Verrall, Photochemistry and
Photobiology 1987, 45, 39-48
The time-resolved fluoroscence decay of horse liver alc. dehydrogenase was
measured using a frequency doubled picosecond dye laser and timecorrelated single-photon counting detection. A flow-cell technique was
described which eliminated the photodegrdn. artifacts which commonly
occur with laser excitation. A procedure was introduced which used
fluorescence quenching to reveal minor fluorescence lifetime components.
The decay of the unquenched tryptophanyl fluorescence could be described
by a double exponential decay law, but expts. conducted in the presence of Ishowed that the fluorescence decay must be more complex than this. A
model is presented in which the fluorescence decay consists of 3 exponential
components, only 2 of which are susceptible to quenching by I-. Several
possibilities were presented for the origin of this minor decay component,
the most reasonable of which was that it arose from conformational
heterogeneity in the solvent-exposed tryptophanyl residue. [on SciFinder
(R)] asdfgf
[65] Thiocarbonyl spectroscopy: methyl torsional vibrations and
internal rotational barriers of thioacetaldehyde in its ~a 3A\" and ~X
1A' states
R. H. Judge, D. C. Moule, A. E. Bruno, R. P. Steer, Journal of Chemical
Physics 1987, 87, 60-67
MeCHS was prepd. by the vacuum flash pyrolysis of 1,3,5-trimethyl-strithiane and was obsd. spectroscopically in a rapidly flowing system. The
614 nm absorption system of MeCHS/MeCDS/CD3CHS/CD3CDS was
recorded photog. at path lengths of 72 m. Phosphorescence excitation and
emission spectra of MeCHS were obsd. with a continuous-wave dye laser.
The assignment of the 1st absorption system was made to the spin forbidden
transition ~a 3A\"->~X 1A'. The bands obsd. in the region of the origin had
a complex structure, and were assigned to torsional transitions. The
observation of several hot band progressions which could be fitted to levels
derived from microwave data gave the parameters (in cm-1) V3\" = 534.3,
F\" = 6.9950, and n0 = 16293.8 for MeCHS. The fit to the excited state
torsional levels was less satisfactory and gave V3' = 94.18, F' = 5.3608 (all
in cm-1). An anal. of the data for the other isotopic species showed that the
torsional motion in the upper state was coupled to a large amplitude wagging
of the aldehydic H. The obsd. phase shift in the torsional angle on electronic
excitation was attributed to the effects of hyperconjugation between the Me
H's and the p (CS) orbitals. [on SciFinder (R)] asdfgf
[66] ~A-~X laser excitation spectroscopy of bromochlorothiocarbonyl
and dibromothiocarbonyl at room temperature and in cold supersonic
jets
B. Simard, P. A. Hackett, R. P. Steer, Journal of Molecular Spectroscopy
1987, 126, 307-328
~A-~X room-temp. and jet-cooled fluorescence excitation spectra of BrClCS
and Br2CS were recorded. Vibrational analyses place the electronic origins
of BrClCS and Br2CS at 18,363 and 17,992 cm-1, resp. In both mols., all 6
~A state vibrational frequencies were obtained. Some ground state
vibrational frequencies of BrClCS were reevaluated. The excited mols.
exhibit nonplanar structures in their ~A states. By fitting quadratic-Gaussian
and quartic-quadratic double-min. potentials to the obsd. levels of the ~A
state out-of-plane manifolds, the equil. out-of-plane angles and barrier
heights opposing mol. inversion were estd. as 26 +- 1 Deg and 538 +- 5 cm1 for BrClCS and 19 +- 1 Deg and 465 +- 30 cm-1 for Br2CS. Comparisons
with other tetraat. thiocarbonyls support the given assignments. [on
SciFinder (R)] asdfgf
[67] The phosphorescence of 4H-pyran-4-thione: large quantum yields
from room-temperature fluid solutions
M. Szymanski, R. P. Steer, A. Maciejewski, Chemical Physics Letters 1987,
135, 243-248
The phosphorescence spectra, phosphorescence quantum yields and triplet
lifetimes of 4H-pyran-4-thione (PT) in fluid soln. at room temp. were
measured. In inert perfluoroalkane solvents at 293 K, the phosphorescence
quantum yield of PT is 0.33 on excitation to S2 and 0.47 on direct excitation
to T1. The reasons for these extremely large radiative yields are discussed.
[on SciFinder (R)] asdfgf
[68] Rydberg states in adamantanethione, thiofenchone, and
thiocamphor
K. J. Falk, R. P. Steer, Canadian Journal of Chemistry 1988, 66, 575-577
The absorption spectra of adamantanethione (I), thiofenchone (II), and
thiocamphor (III) were measured in the quartz UV region in both the vapor
phase and in perfluoroalkane solns. Rydberg as well as p -> p* transitions
are identified in the gas phase spectra, and are discussed in terms of the
known photochem. of these compds. [on SciFinder (R)] asdfgf
[69] Photochemistry and photophysics of thione triplets in fluid
solution
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of Physical Chemistry
1988, 92, 6939-6944
The photochem. and photophys. behavior was studied of the lowest triplet
states of 9 arom. thiones. The phosphorescence quantum yields, and the
triplet lifetimes were measured in fluid soln. at room temp. as a function of
the nature of the solvent, excitation wavelength (encompassing the S0 -> T1,
S0 -> S1, and S0 -> S2 absorption systems), and thione concn. Linear SternVolmer plots over at least 3 decades of thione concn. were obsd. The
variations of the obsd. nonradiative decay consts. (obtained from quantum
yield and lifetime data at infinite diln.) with thione structure, triplet energy,
and solvent were analyzed. Three thiones, 4H-pyran-4-thione, 4H-1benzopyran-4-thione, and xanthione, were subjected to photochem. study in
solns. sufficiently dil. to permit anal. of the rates of their intramol. and
excited solute-solvent reactions in the absence of excited solute-solute
interactions. Intermol. self-quenching interactions dominated thione triplet
decay in room temp. solns. even at thione concns. as low as 1 * 10-6 M.
Triplet thione-solvent interactions were responsible for almost all of the
photochem. degrdn. of the thione at infinite diln., and for substantial
decreases in lifetime in all solvents except inert perfluoroalkanes. [on
SciFinder (R)] asdfgf
[70] T1-T2 inversion in aromatic thiones
A. Maciejewski, M. Szymanski, R. P. Steer, Chemical Physics Letters 1988,
143, 559-564
The emission spectra and phosphorescence lifetimes and quantum yields of
xanthione (XT), 4H-pyran-4-thione (PT) and 4H-1-benzopyran-4-thione
(BPT) were measured in 5 solvents of varying polarities at room temp. A
dramatic change in the T1 -> S0 phosphorescence spectrum is obsd., and this
is interpreted in terms of inversion of the 3(n,p*) and 3(p,p*) states with
increasing solvent polarity. Only modest variations in the radiative and nonradiative decay consts. for the triplet are obsd., and no evidence of biexponential decay is found. Strong coupling between T1-T2 and between the
singlet and triplet manifolds is indicated. Comparisons are drawn with
ketones of similar structure. [on SciFinder (R)] asdfgf
[71] Vibrational analysis of the low resolution ~a -> ~X absorption
spectra of carbonothioic bromide chloride and carbonothioic dibromide
B. Simard, R. P. Steer, R. H. Judge, D. C. Moule, Canadian Journal of
Chemistry 1988, 66, 359-366
The .hivin.a -> .hivin.X absorption spectra of BrClCS and Br2CS were
photographed under low resoln. at room temp. The electronic origins of
BrClCS and Br2CS were placed at 17,116 and 16,859 cm-1, resp. Vibronic
analyses show that the mols. are non-planar in their .hivin.a states. By fitting
quadratic-Gaussian and quadratic-quartic double-min. potentials to the obsd.
vibrational levels of the out-of-plane manifolds, the equil. out-of-plane
angles and the barrier heights to mol. inversion were estd. to be 25 +- 1 Deg
and 541 +- 10 cm-1 for BrClCS, and 17.5 +- 1 Deg and 524 +- 10 cm-1 for
Br2CS. In the case of BrClCS, all 6 .hivin.a state vibrational frequencies
were obtained. In the case of Br2CS, only modes 1 (C-S stretch), 2 (sym. CBr stretch), 3 (in-plane Br-C-Br scissor), and 4 (out-of-plane bend) are active
in the spectrum. Comparisons with other tetraat. thiocarbonyls support the
given assignments. [on SciFinder (R)] asdfgf
[72] Photophysical properties of fluorescent N-purin-6-ylpyridinium
chloride
B. Skalski, R. P. Steer, R. E. Verrall, Journal of the American Chemical
Society 1988, 110, 2055-2061
The absorption and fluorescence spectra and the excited-state lifetimes of Npurin-6-ylpyridinium chloride were studied in MeCN, MeOH, and EtOH,
and, as a function of pH, in H2O. The ground state exhibits a prototropic
equil. between the cation and zwitterion with pKa = 6.7 in H2O. At pH 3-6
in H2O, excitation of the ground-state cation results in proton transfer to
give only the excited zwitterion, on a time scale that is short compared to the
lifetime of the excited species. A large Stokes shift in the emission results.
The fluorescence quantum yield and lifetime of the excited zwitterion are
0.08 and 1.6 ns, resp., in H2O and remain const. over the entire pH 3-9
range. Diabatic proton transfer to the excited zwitterion at pH <3 results in
quenching of the emission with a rate const. of 2 * 109 M-1 s-1. Proton
quenching does not dominate the excited zwitterion decay, however, so that
the apparent pKa* values of 0.4 +- 0.3 from the Forster cycle and 0.6 +- 0.1
from fluorescence titrn. give good approxns. to the true excited state pKa*.
The effects of solvent and temp. were also examd. The potential application
of pyridinium salts of purines as fluorescence probes for biomols. is
discussed. [on SciFinder (R)] asdfgf
[73] Photophysics and intramolecular photochemistry of thiones in
solution
R. P. Steer, V. Ramamurthy, Accounts of Chemical Research 1988, 21, 380386
A review with 73 refs. Covered topics include: electronic transitions,
excited-state structures, dynamics and mechanisms of excited-state decay,
and photochem. [on SciFinder (R)] asdfgf
[74] Photophysics of thione triplets in solution: factors controlling the
rates of radiationless decay
M. Szymanski, A. Maciejewski, R. P. Steer, Chemical Physics 1988, 124,
143-154
Several spectroscopic, photochem., and/or photophys. properties of the
lowest triplet states of 11 mols. contg. the CS group were measured in 3methylpentane and perfluoro-1,3-dimethylcyclohexane solvents at room
temp. Values of the phosphorescence lifetimes, phosphorescence quantum
yields, and quantum yields of net photochem. consumption of thione at
infinite diln. were detd. A quant. anal. of the intramol. relaxation processes
of the triplet thiones in the absence of complicating intermol. selfquenching, and solvent-excited solute interactions is possible, using data
obtained with inert perfluoroalkane solns. Photochem. processes contribute
negligibly to the rates of triplet decay. With 1 exception, the radiative rate
consts. fall within .apprx.(3-11) * 103 s-1. These are factor of 105 larger
than those of the triplet hydrocarbons, and are indicative of strong mixing of
the triplet and singlet manifolds. The nonradiative consts. of the rigid thiones
are consonant with the energy gap law and are governed by the FranckCondon factors for the T1 -> S0 nonradiative transitions. A normal D
isotope effect is obsd. in xanthione. [on SciFinder (R)] asdfgf
[75] Radiationless decay of aromatic thiones in solution selectively
excited to their S3, S2, S1, and T1 states
M. Szymanski, A. Maciejewski, R. P. Steer, Journal of Physical Chemistry
1988, 92, 2485-2489
Seven arom. thiones, e.g., xanthione, (4-MeOC6H4)2CS, having a variety of
electronic energy gaps and structures were subjected to spectroscopic and
photophys. examn. Accurate measurements of the relative T1 -> S0
phosphorescence quantum yields obtained following direct excitation into
the S3, S2, S1, and T1 states were made in perfluoroalkane and hydrocarbon
solvents. Together with S2 lifetime and fluorescence quantum yield
measurements, these data were used to obtain quant. information about the
excited-state relaxation paths. S3 -> S2 internal conversion is quant.
Intermol. processes dominate the decay of S2 in 3-methylpentane and other
common solvents, whereas intramol. processes are revealed in inert
perfluoroalkanes. The majority of S2 species decay via S1 to T1. However,
the thermally activated T1 -> S1 -> S0 decay process accounts for a
significant fraction of decay events at room temp. In addn., mols. having bH atoms close to the thiocarbonyl group can undergo net decay from S2 ->
S0, bypassing S1 and T1. [on SciFinder (R)] asdfgf
[76] Decay dynamics of the lowest triplet and lowest excited singlet
states of thioacetaldehyde and thioacetone
A. E. Bruno, D. C. Moule, R. P. Steer, Journal of Photochemistry and
Photobiology, A: Chemistry 1989, 46, 169-180
The lifetimes of the emission obsd. when thioacetaldehyde and
thioacetoneare excited in their ~a-~X and ~A-~X absorption systems were
measured as a function of pressure. The lifetimes of the ~a states under
collision-free conditions are in the 10 ms region, slightly varying with
vibrational energy content and, in the case of thioacetone, on deuteriation. A
collision-induced intersystem crossing was identified as a likely mechanism
of radiation-free decay of the ~a states, and was also implicated in the decay
of the ~A states in both mols. [on SciFinder (R)] asdfgf
[77] Dynamics of the second excited singlet state of thiophosgene in
solution
K. J. Falk, S. E. Sveinson, R. P. Steer, Chemical Physics Letters 1989, 163,
490-494
The dynamic behavior of the 2nd excited singlet state of Cl2CS was
investigated in liq. solns. In perfluoroalkane solvents strong ~B-~X
fluorescence (ff0 ?0.077) was obsd. when the mol. is excited to bound
vibrational states. ~B lifetimes of ?2.8 ns were obtained. Dissocn., either
directly (in competition with vibrational relaxation) at high vibrational
energies or indirectly by solvent activation when low vibrational levels are
initially populated, apparently is the main radiationless decay path.
Fluorescence is weaker in other solvents. A rate const. of 9.5 * 109 M-1 s-1
was measured for quenching by n-C6H14. [on SciFinder (R)] asdfgf
[78] Photophysics and intramolecular photochemistry of
adamantanethione, thiocamphor, and thiofenchone excited to their
second excited singlet states: evidence for subpicosecond photoprocesses
K. J. Falk, R. P. Steer, Journal of the American Chemical Society 1989, 111,
6518-6524
The absorption, emission, and excitation spectra of adamantanethione (I),
perdeuterioadamantanethione, thiofenchone (II), and thiocamphor (III) were
measured in a variety of solvents including inert perfluoroalkanes. Weak S2
-> S0 fluorescence was obsd. for the first time in these compds., and its
quantum yields were measured. Quantum yields of phosphorescence
resulting from S2 -> S0 excitation and the quantum yields of net thione
consumption in dil. solns. where photodimerization is unimportant were also
measured. The S2 states have lifetimes <1 ps and therefore, contrary to
previous reports, are not the immediate precursors responsible for the
intermol. photochem. reactions of these compds. Evidence is presented that
an intermediate, X, derived from S2 is involved, and that the most probable
decay path is S2 -> X ->T2 .dblarw. T1 -> S0. Rydberg states lie higher in
energy than S2 and are not involved. In II and III, intramol. photochem.
reaction to give the previously obsd. homothioenol product occurs via X. [on
SciFinder (R)] asdfgf
[79] Photophysics of the second excited singlet states of cyclic
enethiones
V. P. Rao, R. P. Steer, Journal of Photochemistry and Photobiology, A:
Chemistry 1989, 47, 277-286
The spectroscopic and photophys. parameters of the 2nd excited singlet
states of several enethiones were measured in inert perfluoroalkane solvents.
Two rigid, related enethiones, 1,1-dimethyl-2-thioxo-1,2dihydronaphthalene and thiocoumarin, behave in accordance with the energy
gap law and decay predominately via S2 -> S1 -> T1 radiationless transition,
1,1,3-trimethyl-2-thioxo-1,2-dihydronaphthalene, with a Me group at an sp2
carbon a to the thiocarbonyl moiety, exhibits a much faster non-radiative
decay rate. Thioisophorone and 4H-pyran-4-thione are structurally flexible
and decay non-radiatively from S2 at rates much faster than the rigid
bicyclic enethione, thioverbenone. The results are rationalized in terms of
the theory of radiationless transitions. [on SciFinder (R)] asdfgf
[80] Photophysical studies of luminarosine. A new, highly fluorescent
ribonucleoside with pteridine-like betaine as the aglycone
B. Skalski, S. Paszyc, R. W. Adamiak, R. P. Steer, R. E. Verrall, Journal of
the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry
(1972-1999) 1989, 1691-1696
The solvent and pH dependence of the absorption and fluorescence spectra,
fluorescence quantum yields and lifetimes, and fluorescence quenching
behavior of luminarosine (I, R = H), its triacetate (I, R = Ac), a-anomer of I
(R = Ac) and the aglycon 4-aminopyrido[2,1-h]pteridin-11-ium-6-olate
(luminarine) are reported. Spectroscopic properties of these compds. in
solvents of different polarities are characterized by evidence of the
occurrence of a high dipolar, charge-transfer excited state within the
heterocyclic betaine system. The pH dependence of the absorption spectra of
luminarosine and luminarine infer the occurrence of ground state prototropic
equil. in soln. involving protonation of the neg. charged oxygen shown
followed by tautomerism to a lactam. The excited state prototropic equil., as
shown by fluorescence properties, seem to be more complex and suggest the
possible occurrence of proton-transfer and/or phototautomerization
reactions. [on SciFinder (R)] asdfgf
[81] Electronic energy-pooling processes in polyatomic molecules
R. P. Steer, Research on Chemical Intermediates 1989, 12, 81-101
A review with 77 refs. is given on electronic energy pooling processes in
polyat. mols. with closed-shell ground state, with emphasis on triplet-triplet,
triplet-singlet and singlet-singlet annihilation in fluid systems. [on SciFinder
(R)] asdfgf
[82] Thermally activated emission of adamantanethione, thiofenchone,
and thiocamphor
K. J. Falk, R. P. Steer, Journal of Physical Chemistry 1990, 94, 5767-5772
Measurements of the emission, absorption, and excitation spectra and triplet
lifetimes of adamantanethione (I), thiofenchone, and thiocamphor as a
function of temp. and solvent polarity have established that their lowest
triplets are of p,p* configuration. In all 3 mols. thermally activated
phosphorescence is obsd. from a second, higher energy triplet of n,p*
character which remains in thermal equil. with the lowest triplet at temps.
>77 K. Triplet lifetimes of 101 ms and 43.5 ms are obtained for I in frozen
perfluoro-1,3-dimethylcyclohexane(II) at 77 K and in infinitely dil. fluid II
at 295 K, resp. The photophys. processes assocd. with the decay of the
lowest triplet states are discussed. [on SciFinder (R)] asdfgf
[83] Photophysical properties of pyridinium salts derived from purine
bases
B. Skalski, S. Paszyc, R. W. Adamiak, R. P. Steer, R. E. Verrall, Canadian
Journal of Chemistry 1990, 68, 2164-2170
The absorption spectra, fluorescence spectra, fluorescence lifetimes,
fluorescence quenching, phosphorescence spectra, phosphorescence
lifetimes, and picosecond transient absorption spectra of several purinylpyridinium salts have been measured in several solvents at room temp. and
in ethanol glasses at 77 K. It is concluded that the previously obsd.
photochem. transformation of pyridinium chloride I into the highly
fluorescent tri-O-acetylluminarosine (II) occurs via an excited triplet state. It
is also shown that intersystem crossing in purinylpyridinium salts is induced
by an intermol. charge transfer interaction with the counter ions. [on
SciFinder (R)] asdfgf
[84] The 1A1 electronic excited states of thioformaldehyde (H2CS): an
ab initio MRD-CI study
M. Hachey, F. Grein, R. P. Steer, Chemical Physics Letters 1991, 183, 204208
Ab-initio CI studies have been performed to det. the energies of the lowest
five 1A1 states of thioformaldehyde, H2CS, as a function of C-S distance in
C2v symmetry. In addn. to the well-known Rydberg and 1(p, p*) states, a
doubly excited state of n0, p*2 configuration has been found, which, at
ground-state equil. geometry, lies very close to the p, p* state. The chem.
and spectroscopic implications of this state are discussed. [on SciFinder (R)]
asdfgf
[85] The ~B-~X laser-induced fluorescence excitation spectrum of jetcooled thiophosgene: origin location and partial vibronic analysis
M. Ludwiczak, D. R. Latimer, R. P. Steer, Journal of Molecular
Spectroscopy 1991, 147, 414-430
The laser-induced ~B-~X fluorescence excitation spectrum of Cl2CS
(thiophosgene) was measured for the first time in a supersonic jet. A partial
vibronic anal. together with measured Cl isotope splittings places the origin
at 34,275 cm-1 in agreement with the anal. of the high-resoln. photog.
absorption spectrum, but at odds with an anal. of the optical-optical double
resonance spectrum. Reasons for the discrepancy are discussed, including
the possibility that a doubly excited state of 1A1 symmetry is located
nearby. Progressions in n1' (C-S stretch), 2 addnl. upper state modes, and
n4'' are observable. The onset of predissocn. is fixed at 35,725 +- 50 cm-1,
1450 +- 50 cm-1 above the zero point of the ~B state. [on SciFinder (R)]
asdfgf
[86] Photochemistry of N-[9-(2',3',5'-tri-O-acetyl-b-Dribofuranosyl)purin-6-yl]-pyridinium chloride in aqueous solutions.
Mechanism of the formation of tri-O-acetylluminarosine
B. Skalski, R. P. Steer, R. E. Verrall, Journal of the American Chemical
Society 1991, 113, 1756-1762
The photochem. of N-[9-(2',3',5'-tri-O-acetyl-b-D-ribofuranosyl)purin-6yl]pyridinium chloride (I) was investigated in aq. solns. in the pH range 6-8
under aerobic and anaerobic conditions. A multistep mechanism of the
photochem. transformation of I into the highly fluorescent nucleoside
2',3',5'-tri-O-acetylluminarosine (II) is proposed in which, as the first step, I
undergoes light-induced, hydrolytic ring opening in the imidazole portion of
the purine ring to form N-[5-formamido-6-[(2',3',5'-tri-O-acetyl-b-Dribofuranosyl)amino]pyrimidin-4-yl]pyridinium chloride (III). At pH >7, III
exists partly in the elec. neutral, zwitterionic form, which undergoes
electron-transfer-induced ring closure to give II. This process is sensitized
by the excited triplet state of I, which serves as an electron acceptor. Under
anaerobic conditions, the resulting pyridinyl radicals undergo dimerization.
[on SciFinder (R)] asdfgf
[87] Concerning apparent intersystem crossing efficiencies in
molecules with small S1-T1 energy gaps
M. Szymanski, A. Maciejewski, R. P. Steer, Journal of Photochemistry and
Photobiology, A: Chemistry 1991, 57, 405-418
Accurate values of the apparent quantum yields jST of S1 -> T1 intersystem
crossing were detd. for several arom. thiones in fluid solns. at room temp.
All values of jST are significantly <1 at 295 K; this can be compared with a
value of unity based on limited data previously obtained at 77 K, which has
been assumed to hold at room temp. The importance of triplet decay via
back intersystem crossing to S1 followed by radiative and nonradiative
decay to S0 is assessed. The possible involvement of T2 in the
subpicosecond decay of vibrationally excited S1 is discussed. The potential
importance of back intersystem crossing in other mols. with small S1-T1
gaps is briefly addressed. [on SciFinder (R)] asdfgf
[88] Quenching and photodissociation of the ~B state of thiophosgene
in solution
M. Szymanski, S. E. Sveinson, R. P. Steer, Journal of Physical Chemistry
1991, 95, 5159-5163
The lifetimes and quantum yields of fluorescence of the second excited
singlet (~B(1A1)) state of Cl2CS were measured in perfluoroalkane solvent
as a function of laser excitation wavelength, temp., and Cl2CS concn. Selfquenching of the ~B state occurs at diffusion-limited rates. In dil.
perfluoroalkane solns. 2 processes dominate the excited-state decay:
fluorescence to the ground state and photodissocn. (likely via a predissocn.
mechanism). The activation energy for the photodissocn. process is 1430 +80 cm-1, ~ to the vibrational energy difference between the predissocn.
threshold and the zero point in the ~B state detd. spectroscopically. The rate
const. for dissocn. exhibits a mild dependence on the initial vibrational
energy content of the ~B state but shows no discontinuity at the
spectroscopic predissocn. threshold. The latter indicates that the rate of
vibrational relaxation is faster than that of photodissocn. when the ~B state
is excited to vibrational levels at or a few hundred cm-1 above the
predissocn. energy. In other more strongly interacting solvents such as nhexane and CCl4 the ~B state lifetime and fluorescence quantum yields are
greatly reduced due to solvent-induced radiationless decay. [on SciFinder
(R)] asdfgf
[89] Photochemistry of aromatic thiones in their S2 and T1 states: role
of triplet self-quenching and excimer formation
J. Kozlowski, A. Maciejewski, M. Szymanski, R. P. Steer, Journal of the
Chemical Society, Faraday Transactions 1992, 88, 557-562
The quantum yields of photodecompn. of xanthione (XT), benzopyran-4thione (BPT) and pyran-4-thione (PT) after excitation to their S2 and S1
states were measured as a function of the thione concn. in inert
perfluoroalkane solvents. The quantum yield of intramol. photodecompn. of
these thiones in S2 is quite small. Intermol. processes dominate the
photochem. removal of the triplet states even at very low (<10-5 mol dm-3)
thione concns. In inert perfluoroalkane solvents, a photochem. channel in the
diffusion-controlled triplet self-quenching process is responsible for the
removal of the thione. A mechanism involving the intermediacy of triplet
excimers is proposed. [on SciFinder (R)] asdfgf
[90] Van der Waals complexes of xanthione and benzopyranthione
with rare gases. S2-S0 fluorescence excitation spectroscopy and
microscopic solvation effects
M. Ludwiczak, H. K. Sinha, R. P. Steer, Chemical Physics Letters 1992,
194, 196-202
The 1:1 and 2:1 van der Waals complexes of xanthione and
benzopyranthione with He, Ne, Ar, Kr and Xe were obsd. by S2-S0 laserinduced fluorescence excitation spectroscopy in a supersonic jet. The
adatom is located over the pyranthione ring in the 1:1 complex, and the 2:1
complex has a sym. sandwich structure. An anal. of the microscopic solvent
shifts indicates that the thiones have substantially smaller elec. dipole
moments in the S2 state than in the ground state, in qual. agreement with
previous semi-empirical calcns. The dipole moment of xanthione S2 is estd.
to be only 2.0 D, so that thione-adatom interactions are almost exclusively
dispersive in the upper state. [on SciFinder (R)] asdfgf
[91] A dark, excited, electronic state of thiophosgene in the near
ultraviolet
A. Maciejewski, M. Szymanski, R. P. Steer, Chemical Physics 1992, 165,
101-112
The absorption, emission and excitation spectra of thiophosgene were
measured in inert perfluoroalkane solvents. [on SciFinder (R)] asdfgf
[92] Transient effects in the fluorescence quenching of xanthione in
perfluoro-1,3-dimethylcyclohexane
A. Maciejewski, M. Sikorski, M. Szymanski, W. Augustyniak, R. P. Steer,
Radiation Physics and Chemistry 1992, 39, 155-158
The quenching by benzene, 3-methylpentane, n-decane and n-hexadecane of
the second excited singlet state of xanthione (ts2 = 160 ps) in inert, nonpolar
perfluoro-1,3-dimethylcyclohexane was studied using picosecond timeresolved fluorescence decay methods. This system is used to test the
application of the Smoluchowski-Collins-Kimball (SCK) model for
diffusion-limited, time-dependent quenching. The quenching mechanism
involves short-range interactions, and SCK model provides a satisfactory
theor. basis for describing the process. [on SciFinder (R)] asdfgf
[93] Vibrational activation in the radiationless decay of the S2, S1, T1,
and T2 states of aromatic thiones in solution: red edge effects
M. Szymanski, R. P. Steer, Journal of Physical Chemistry 1992, 96, 87198728
The quantum yields of phosphorescence, jp, of six arom. thiones (xanthione,
xanthione-d6, benzopyranthione, pyranthione, tetramethylindanthione, and
4-bromotetramethylindanthione) have been measured accurately as a
function of excitation wavelength and temp. in two solvents (3methylpentane and perfluoro-1,3-dimethylcyclohexane). The values of jp
decrease on the red edge of the S2 -> S0 absorption band, particularly for
pyranthione which has the shortest S2 lifetime. This red edge effect is
attributed to competition between fast S2v=0 -> S0 and the finite rate of
vibrational activation required before S2 can decay nonradiatively via S2v > S1v. The quantum yield of phosphorescence gradually increases with
increasing excitation wavelength over the S1 -> S0 absorption region as the
fraction of triplet formed directly by excitation in the underlying T1 -> S0
system increases. Excitation on the red edge of the T1 -> S0 absorption
results in a dramatic increase in jp. This second red edge effect is interpreted
in terms of the nonequil. dynamic behavior of the coupled T1-S1-T2 system.
Finite triplet sublevel spin depolarization rates may play an important role in
detg. the difference in the dynamics of the system when excited initially to
T1 compared with S1. [on SciFinder (R)] asdfgf
[94] Photophysics of the S1 and T1 states of thiophosgene in solution
M. Szymanski, A. Maciejewski, R. P. Steer, Journal of Physical Chemistry
1992, 96, 7857-7863
The absorption, emission, and excitation spectra and emission lifetimes of
thiophosgene, Cl2CS, in its S1 and T1 states were measured as a function of
concn. in a perfluoroalkane solvent at room temp. Significant dimerization
occurs in soln. at concns. >10-4 M, and this must be taken into account
when interpreting the spectra. Fluorescence from the monomer in its first
excited singlet state is obsd. (Ff0 = 5.5 * 10-6) and constitutes the shortlived component (t = 40 ps) of a biexponential emission decay. The quantum
yield of intersystem crossing from S1 to T1 is estd. to be <0.4.
Phosphorescence from the monomer (Fp0 = 1.9 * 10-5) measured under
excitation to S1 was obsd. for the first time. The lifetime of T1 is 90 ns at
infinite diln. in perfluoroalkane solns. at room temp. T1 decays almost
exclusively by direct intersystem crossing to S0 (kISCT = 1.1 * 107 s-1).
Unlike the larger arom. thiones, equil. between T1 and S1 is not established
despite Cl2CS's relatively small S1-T1 electronic energy gap of 1124 cm-1.
Quenching of T1 by ground-state n-mers occurs at diffusion limited rates in
soln. [on SciFinder (R)] asdfgf
[95] Radiationless decay of the S2 states of azulene and related
compounds: solvent dependence and the energy gap law
B. D. Wagner, D. Tittelbach-Helmrich, R. P. Steer, Journal of Physical
Chemistry 1992, 96, 7904-7908
The UV-visible absorption spectra, S2 lifetimes, and S2 -> S0 fluorescence
quantum yields of azulene, azulene-d8, 1,3-dichloroazulene, 1,3dibromoazulene, 4,6,8-trimethylazulene, and 1,4-dimethyl-7isopropylazulene (guaiazulene) have each been measured accurately in six
solvents. The S2-S1 electronic energy spacings of each solute vary by ca.
500 cm-1 in these solvents. The variations in the S2 nonradiative relaxation
rates with electronic energy spacing are interpreted within the framework of
the energy gap law of radiationless transition theory. S2 -> S1 internal
conversion dominates the nonradiative decay in azulene and azulene-d8, but
intersystem crossing (likely S2 -> T1) is important in the halogenated derivs.
The alkyl-substituted compds. exhibit anomalous behavior and demonstrate
that factors other than the electronic energy spacing are involved in detg. the
rates of their radiationless relaxation. Previous energy gap law correlations
based on data from a series of structurally different compds. must be
reinterpreted. [on SciFinder (R)] asdfgf
[96] Transient effect in fluorescence quenching of S2-xanthione by 3,3diethylpentane in perfluoroalkane solvent: a steady-state and dynamic
approach
W. Augustyniak, J. Koput, A. Maciejewski, M. Sikorski, R. P. Steer, M.
Szymanski, Polish Journal of Chemistry 1993, 67, 1409-1423
The transient quenching phenomenon has been studied for the S2xanthione/3,3-diethylpentane/perfluoro-1,3-dimethylcyclohexane (as a
donor/quencher/solvent, resp.) system. Two kinds of expts. have been
successfully performed: time dependent fluorescence decay anal. and steadystate fluorescence quenching measurements, in the quencher concn. range
from 0 to about 0.5 M and for a low concn. of xanthione (.apprx.10-5 M). A
time-correlated single photon counting system, with a proximity type
microchannel plate photomultiplier tube was used in dynamic studies of a
strong time dependence of the transient quenching of S2-xanthione
fluorescence. For the system under examn., the time dependent component
of fluorescence quenching was slightly bigger than the time independent
one. A satisfactory description of the data from dynamic measurements
within the Smoluchowski-Collins-Kimball model has been obtained,
permitting the detn. of realistic values of the encounter radius and the sum of
diffusion coeffs. Less satisfactory, but still reasonable data on the intrinsic
rate coeff. have been calcd. Numerical anal. of the data from steady-state
emission intensity measurements with the \"all-time\" function on the basis
of the above mentioned model also provided a satisfactory detn. of mol.
parameters characterizing the phenomenons studied. Both the steady-state
and dynamic, results have been discussed in terms of a Stern-Volmer
dependence of the relative emission intensity and the effective \"lifetime\".
This approach permitted an evaluation of the role of ground-state complex
formation (pseudoquenching). [on SciFinder (R)] asdfgf
[97] The structure and vertical excitation energies of the
chlorothioformyl radical, ClCS: implications for the
photofragmentation of thiophosgene
M. Hachey, F. Grein, R. P. Steer, Canadian Journal of Chemistry 1993, 71,
112-117
Ab initio CI studies are performed to det. the geometry of the ground and
first electronically excited states of .bul.C(S)Cl (I) and the vertical excitation
energies of its 10 lowest doublet states and 2 lowest quartet states. The
results are used to construct a more complete energy level correlation
diagram for the photofragmentation of Cl2CS (II). The lowest excited
electronic state of I lies only 0.79 eV (adiabatic) above the ground state. Its
discovery indicates that the results of previous photofragmentation expts. of
II may need to be reinterpreted. [on SciFinder (R)] asdfgf
[98] Thiophosgene monomer-dimer equilibria in perfluoroalkane
solutions: spectroscopic and photophysical implications of selfassociation in the ground state
A. Maciejewski, M. Szymanski, R. P. Steer, Canadian Journal of Chemistry
1993, 71, 1548-1555
The S2-S0 absorption, emission, and emission excitation spectra of
thiophosgene in a perfluoroalkane solvent have been measured as a function
of the anal. concn., C, of the solute. Strong evidence of the formation of van
der Waals dimers at 10-4 M < C < 10-1 M and of higher oligomers at C >
10-1 M has been found. Variations in the absorption spectra at C < 10-1 M
have been analyzed successfully in terms of a single 2Cl2CS .dblarw.
(Cl2CS)2 equil., for which the value of K is (5.0 +- 0.4) * 102 M-1 at 295 K.
The mole fractions of monomer and dimer have been calcd. at each C and
have been used to construct the absorption spectrum of the dimer. Both the
monomer and dimer have similar S0 -> S1 and S0 -> S2 absorption spectra,
but the dimer has an oscillator strength that is about twice that of the
monomer. The effects of self-aggregation on the S2-S0 emission and
excitation spectra have been analyzed. Absorption by the dimer in the UV
also leads to emission, but with a different quantum efficiency compared
with direct excitation of the monomer. The implications of these
observations on the interpretation of the spectroscopy, photophysics, and
photochem. of thiophosgene in soln. are assessed. [on SciFinder (R)] asdfgf
[99] Photophysics of UV-excited thiophosgene in a perfluoroalkane
solvent
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of the Chemical
Society, Faraday Transactions 1993, 89, 3251-3256
The quantum yields of fluorescence from UV-excited Cl2CS in inert
perfluoroalkane solns. were measured as a function of the anal. concn. of the
solute in the range 5 * 10-5 ? C/mol-dm-3 ? 1, and as a function of excitation
energy in the range 245 ? lex/nm ? 330. The excited-state decay rates were
measured at 272 ? lex/nm ? 300 over the same concn. range. A quant.
description of the photophysics of the system was achieved, which accounts
for significant self-aggregation of the ground state solute at C > 10-4 moldm-3, absorption by a nearby dark excited state of the monomer in the 285 ?
lex/nm ? 330 range, different lifetimes and rate consts. of concn. quenching
of the excited monomer and dimer, and the excitation wavelength
dependence of fluorescence-weakening non-radiative relaxation processes in
each excited species. [on SciFinder (R)] asdfgf
[100] The photophysics, physical photochemistry, and related
spectroscopy of thiocarbonyls
A. Maciejewski, R. P. Steer, Chemical Reviews (Washington, DC, United
States) 1993, 93, 67-98
A review with 254 refs. [on SciFinder (R)] asdfgf
[101] S2-S0 fluorescence excitation spectroscopy of van der Waals
complexes of xanthione and benzopyranthione with N2 and CO
H. K. Sinha, R. P. Steer, Chemical Physics Letters 1993, 211, 397-402
The S2-S0 fluorescence excitation spectra of xanthione (XT) and
benzopyranthione (BPT) and their 1:1 and 1:2 complexes with N2 and CO
have been measured in a supersonic expansion. Remarkably long, harmonic
progressions in van der Waals modes are obsd. and are assigned on the basis
of the structural and dipole moment changes in the chromophores. The
microscopic solvation shifts are rationalized in terms of dipolar and
dispersive contributions to the binding energies. [on SciFinder (R)] asdfgf
[102] Ground and excited state dipole moments of pyranthione and
xanthione by the electro-optic method
H. K. Sinha, O. K. Abou-Zied, R. P. Steer, Chemical Physics Letters 1993,
201, 433-436
Elec. field-induced changes in the absorption spectrum (electrochromism)
are used to obtain the ground (S0) and second excited (S2) state dipole
moments of heteroarom., pyranthione and xanthione, which have C2v
symmetry. The magnitude of the change in the dipole moment [(Dm =
|m(S2)-m(S0)|]) was nearly 2 D for both compds., which is consistent with
results reported previously based on microscopic solvent shifts. The
transition dipole moment for the S0-S2 transition is parallel to the direction
of the ground state dipole moment (mg), i.e. along the C2 axis which
contains the C:S bond. For comparison the ground state dipole moments of
both compds. are also calcd. using the AM1 method with full geometry
optimization. [on SciFinder (R)] asdfgf
[103] Photophysics of van der Waals dimers of thiophosgene (Cl2CS) in
perfluoroalkane solvents
M. Szymanski, A. Maciejewski, R. P. Steer, Chemical Physics 1993, 175,
413-425
The photophysics of thiophosgene in perfluoro-1,3-dimethylcyclohexane
solns. of widely varying concns., C, has been investigated. For 10-3 M < C <
10-1 M, the dominant absorbing species is the van der Waals dimer,
(Cl2CS)2, the S2 -> S0 fluorescence quantum yield and S2 lifetime of which
have been detd. as a function of C and excitation wavelength. The data have
been analyzed to yield values of the radiative and nonradiative decay consts.
and concn. quenching rate const. of the excited dimer. Excitation in the
visible produces both S1 -> S0 fluorescence of the monomer and dimer and
T1 -> S0 phosphorescence of the monomer. The concn. dependence of the
fluorescence and phosphorescence lifetimes and quantum yields have been
analyzed in order to ext. the radiative and nonradiative decay parameters of
the dimer in its S1 state. At C >10-1 M, the spectroscopy and photophysics
of the system are dominated by higher oligomers of thiophosgene. [on
SciFinder (R)] asdfgf
[104] The effect of solvent viscosity on the population relaxation times
of the S1 states of azulene and related compounds
D. Tittelbach-Helmrich, B. D. Wagner, R. P. Steer, Chemical Physics
Letters 1993, 209, 464-468
The S1 population decay times of azulene and 4,6,8-trimethylazulene have
been measured in solvents with a wide range of viscosities using a twophoton, two-color, pump-probe method. The S1 lifetimes vary as expected
on the basis of the energy gap law. No effect of solvent viscosity is obsd.,
contrary to a previous suggestion. Data for 2-chloroazulene in non-viscous
solvents are also reported. [on SciFinder (R)] asdfgf
[105] Subpicosecond pump-probe measurements of the electronic
relaxation rates of the S1 states of azulene and related compounds in
polar and nonpolar solvents
B. D. Wagner, M. Szymanski, R. P. Steer, Journal of Chemical Physics
1993, 98, 301-307
The lifetimes of the S1 states of azulene, azulene-d8, 4,6,8-trimethylazulene,
and guaiazulene (1,4-dimethyl-7-isopropylazulene) have been measured in
three nonviscous solvents of different polarity and structure using a twophoton, two-color, pump-probe method with subpicosecond time resoln. A
significant solvent effect is measured. The rate consts. for S1-S0 internal
conversion in all four compds. in all three solvents exhibit one common S1S0 energy gap law correlation, indicating that variations in the electronic
relaxation rates are governed exclusively by changes in the Franck-Condon
factors for the transition. No effect is obsd. when the exciting wavelength is
changed, indicating that vibrational relaxation is occurring on a time scale
which is faster than that of electronic relaxation in these systems. No
significant deuterium isotope effect is measured in azulene, indicating that
high frequency C-H(D) stretching vibrations do not act as significant
accepting modes in the radiationless transition. [on SciFinder (R)] asdfgf
[106] Optical-optical double resonance study of the ~B(1A1) and
~a(3A2) states of thiophosgene
B. Simard, V. J. MacKenzie, P. A. Hackett, R. P. Steer, Canadian Journal of
Chemistry 1994, 72, 745-757
The ~a(3A2) and ~B(1A1) states of jet-cooled thiophosgene (Cl2CS) have
been studied by optical-optical double resonance (OODR) spectroscopy.
Two OODR schemes have been used to probe the ~B(1A1) state. One
scheme uses selected vibronic levels of the ~A(1A2) state as the
intermediate state, while the other uses the vibrationless and 2131 levels of
the ~a(3A2) state. All of the vibronic levels in the 33,980 - 35,600 cm-1
region can be rationalized with the following origin band and fundamentals:
00 = 34,277 cm-1, n1 = 505 cm-1, n2 = 470 cm-1, n3 = 213 cm-1, n6 = 249
cm-1, 42 = 341 cm-1, 44 = 627 cm-1. The discrepancies among the various
studies on the ~B(1A1) state will be discussed and reconciled. It is
conjectured that the ~B(1A1) state potential along the C-S coordinate
exhibits an asym. double-min. potential resulting from the interaction of the
1A1 states arising from the (p1CS n2S pCS*1) and (pCS2 nS- pCS*2)
configurations. The min. corresponding to the (pCS1 nS2 pCS*1)
configuration lies higher in energy and the principal decay mechanism for
mols. pumped to its first few vibronic levels is fluorescence. On the other
hand, mols. pumped to the min. corresponding to the nominal (pCS2 nS0
pCS*2) configuration decay nonradiatively. The barrier height to inversion
and the out-of-plane bending angle along the out-of-plane bending
coordinate, n4, have been detd. to be 945 cm-1 and 25 Deg, resp., by fitting
quartic-quadratic and quadratic-Guassian double-min. potentials to the obsd.
energy levels. The ~a(3A2) state has been studied by a novel OODR scheme
which uses the fluorescent vibrationless levels of the ~B(1A1) state to
monitor ~a(3A2)-~X(1A1) transitions. A vibronic anal. has been carried out
the following origin band and fundamentals derived for the ~a(3A2) state:
010 = 17 499 cm-1, n1 = 923 cm-1, n2 = 474 cm-1, n3 = 247 cm-1, 42 = 297
cm-1, 44 = 560 cm-1, 46 = 741 cm-1. With the exception of a few
corrections and addns., the results confirm the findings of previous studies,
notably regarding the bent geometry and barrier height to inversion. An
overall comparison of the data suggests that the wavenumber of n6 in the
~A(1A2) state is 279 cm-1 instead of 189 cm-1. [on SciFinder (R)] asdfgf
[107] Origins of the differences in solvation by alkanes and
perfluoroalkanes. Evidence from the S2-S0 spectra of jet-cooled van der
Waals complexes of xanthione and azulene
H. K. Sinha, O. K. Abou-Zied, M. Ludwiczak, A. Maciejewski, R. P. Steer,
Chemical Physics Letters 1994, 230, 547-554
The S2-S0 fluorescence excitation spectra of xanthione (XT) and azulene
(AZ) complexed with 1 or 2 mols. of the C1 to C10 n-alkanes and the C1 to
C6 perfluoro-n-alkanes have been measured. The 1:1 complexes exhibit
microscopic solvent shifts, Dn, which are larger for the alkanes than the
corresponding perfluoroalkanes, despite the larger mol. polarizability of the
latter. The values of Dn increase monotonically with carbon no. of the
adduct to C10 in 1:1 n-alkane complexes with XT and to C5 in 1:1 n-alkane
complexes with AZ. However, Dn exhibits no further increase beyond C2 in
1:1 perfluoro-n-alkane complexes with XT and beyond C3 in 1:1 perfluoron-alkane complexes with AZ. The results are interpreted in terms of a model
in which the n-alkanes stretch out along the long axis of the chromophore
and 'wet' its surface whereas the perfluoro-n-alkanes with carbon nos. >=3
are 'nonwetting'. [on SciFinder (R)] asdfgf
[108] Perturbative treatments of pump-probe laser-molecule
interactions with applications to azulene and trimethylazulene
P. Tran, W. J. Meath, B. D. Wagner, R. P. Steer, Journal of Chemical
Physics 1994, 100, 4165-4170
Semianalytic perturbative approaches for investigating the spectroscopy, and
the underlying dynamics, assocd. with fast time-resolved, 2-photon, 2-color,
pump-probe, low intensity laser-mol. interactions are developed and
discussed. The perturbation theory is developed with emphasis on mol.
models assocd. with pump-probe expts. on the S0->S1->S2 2-photon
transitions in azulene and trimethylazulene. The expts. are discussed and the
theory is used to det. the lifetimes of the intermediates S1 states by
analyzing the exptl. 2-photon fluorescence signals from the S2 states as a
function of the time-delay between the pump and probe lasers. The
advantages of using this approach relative to the traditional methods for
detg. the lifetime of the S1 states are discussed. The dependence of the 2photon fluorescence signals on the angle between the polarization vectors of
the pump and probe lasers, for fixed time delay between the lasers, is
considered. [on SciFinder (R)] asdfgf
[109] The application of picosecond laser spectroscopy in investigation
of initial stages of fluorescence quenching
W. Augustyniak, A. Maciejewski, M. Sikorski, R. P. Steer, M. Szymanski,
Proceedings of SPIE-The International Society for Optical Engineering
1995, 2202, 416-421
The initial stage of fluorescence quenching (the transient quenching) was
studied for the S2-xantanione/3,3-diethylpentane/perfluoro-1,3dimethylcyclohexane (as a donor/quencher/solvent, resp.) system. A
picosecond time-correlated single photon counting system, with a proximity
type microchannel plate photomultiplier tube was used in dynamic studies of
a strong time dependence of the transient quenching of S2-xanthione
fluorescence. A satisfactory description of the data within the
Smoluchowski-Collins-Kimball model was obtained, permitting the detn. of
realistic values of the encounter radius, the sum of diffusion coeffs. and the
intrinsic rate coeff. [on SciFinder (R)] asdfgf
[110] Perturbation treatment of pump-probe laser-molecule
interactions: an application to the fluorescence from the S1 state of aNPO
B. N. Jagatap, W. J. Meath, D. Tittelbach-Helmrich, R. P. Steer, Journal of
Chemical Physics 1995, 103, 121-135
Time-dependent perturbation theory, together with a (minimal) mol. model
consisting of three energy levels (S0,S1,S3), was used to investigate the
spectroscopy and the dynamics of fast time resolved, two-photon, two-color,
pump-probe expts., involving the direct S0->S1 two-photon excitation of 2(1-naphthalenyl)-5-phenyl-oxazole (a-NPO). In particular the theory is used
to examine the Q-dependence of the fluorescence signal from the S1 state,
where Q is the angle between the polarization vectors of the pump and probe
lasers, for fixed (zero) time delay between the laser pulses. It is predicted, in
contradistinction to the cos2 Q dependence of the fluorescence signal from
the S2 state of azulene arising from the sequential two-photon S0->S1->S2
transition, that the signal from the S1 state of a-NPO can vary between pure
cos2 Q and pure cos4 Q dependencies and that secondary maxima in the
signal, as a function of Q, can occur for certain laser intensities. Also
reported is a new series of expts. for a-NPO, motivated by the theory, that
yields results for the fluorescence intensity of the S1 state, as a function of Q
and laser intensity, in agreement with the theor. predictions. Comparison of
expt. and theory is used to est. the relative orientations of the relevant
transition and permanent dipole moments, and the transition moment
between the S1 and S3 states of a-NPO. The important role played by the
permanent dipoles of the S0 and S1 states, and the importance of including
avs. over the relative laser phase, the jitter in the time-delay, and the
orientations of the absorbing mols., is emphasized in the theor. anal. of the
problem. [on SciFinder (R)] asdfgf
[111] Evidence of a solvent-mediated barrier to radiationless decay in
the ~B(1A1) state of thiophosgene in solution
J. Li, R. P. Steer, Canadian Journal of Chemistry 1995, 73, 131-138
Thiophosgene, Cl2CS, was excited in the near-UV and the resulting
quantum yields of ~B-~X fluorescence and of Cl2CS consumption were
measured as a function of solvent compn. (perfluoro-n-hexane, n-hexane,
CCl4), temp., and excitation wavelength. In agreement with previous work
(i) nonradiative processes dominate the decay of the ~B state in soln., and
(ii) perfluoroalkane solvents act as inert heat baths. The process by which
the .simeq.b's decay involves activation and crossing a barrier, the height of
which is a function of the nature and compn. of the solvent. CCl4 and nhexane do not quench the excited state, but instead accelerate its rate of
relaxation by lowering the barrier between the bound, radiative portion of
the surface and a dark, unbound region. There is evidence that CCl4 or nhexane form clusters around a Cl2CS solute mol. in mixed perfluoroalkane-
CCl4 or n-hexane solns. Barrier crossing leads to photodecompn. via at least
two parallel paths, one free radical (Cl + ClCS) and the other likely mol.
(Cl2 + CS), the relative contributions of which are a function of excitation
wavelength. [on SciFinder (R)] asdfgf
[112] Proton tunneling in the tropolone-N2 van der Waals complex
H. K. Sinha, R. P. Steer, Chemical Physics Letters 1995, 241, 328-334
Proton tunneling splitting has been measured for the S0->S1 origin band of
the tropolone: N2 van der Waals complex by recording its laser-induced
fluorescence excitation spectrum in a supersonic jet. A large decrease in the
magnitude of the tunneling splitting, from 19.4 cm-1 in bare tropolone to 6.0
cm-1 in the 1 : 1 complex, has been obsd., indicating that nitrogen perturbs
the reaction coordinate by significantly increasing the barrier height in the
excited state. Strong coupling of the tunneling mode to the in-plane N2
torsional mode is the likely source of this effect. [on SciFinder (R)] asdfgf
[113] Vibrational spectra of xanthione and xanthone
H. K. Sinha, L. Chantranupong, R. P. Steer, Journal of Molecular
Spectroscopy 1995, 169, 302-314
The fundamental vibrational frequencies of xanthione (XT) and xanthone
(XN), two structurally similar heteroarom. mols., have been obtained exptl.
using FTIR and Raman spectroscopy. Ab initio RHF/STO-3G** and
RHF/CEP-31G and semiempirical RHF/AM1 and RHF/PM3 MO calcns.
have been performed to obtain the vibrational frequencies of these mols.
from their force fields. A comparison of the exptl. and calcd. frequencies
reveals that all four theor. methods overestimate the vibrational frequencies
and that the rms deviations increase in the order CEP31G < STO-3G** <
Pm3< Am1. The frequencies obtained by the effective core potential, CEP31G, method are in excellent agreement with expt. when the calcd.
frequencies are scaled uniformly by a factor of 0.8953. [on SciFinder (R)]
asdfgf
[114] Measurements of the subpicosecond relaxation rates of the first
excited singlet states of some pseudoazulenes in solution
D. Tittelbach-Helmrich, R. P. Steer, Chemical Physics 1995, 197, 99-106
The subpicosecond population decay times of the S1 states of a no. of
pseudoazulenes have been measured in solvents of different polarity, using a
two-photon, two-color, pump-probe method. The nonradiative decay rates of
the S1 states vary exponentially with the S1-S0 energy gap, and the results
are interpreted in terms of the energy gap law formalism of radiationless
transition theory. The behavior of the excited states of the pseudoazulenes is
compared with that of azulene and its derivs. [on SciFinder (R)] asdfgf
[115] Sub picosecond vibrational relaxation of the S1 states of azulene
and guaiazulene in solution
D. Tittelbach-Helmrich, B. D. Wagner, R. P. Steer, Canadian Journal of
Chemistry 1995, 73, 303-306
The S1 population decay times of azulene and 1,4-dimethyl-7isopropylazulene (guaiazulene) in soln. have been detd. as a function of their
initial vibrational energy content, Evibi, using a pump-probe expt. with sub
picosecond time resoln. The S1 lifetime of azulene does not depend on Evibi
for energies up to 1760 cm-1, whereas the lifetime of the shorter-lived S1
state of guaiazulene is independent of Evibi only for energies up to
.apprx.1000 cm-1. At higher energies, the lifetime decreases with increasing
Evibi, and exhibits the same behavior in two structurally different solvents.
It is suggested that internal conversion to the electronic ground state from a
vibrationally unrelaxed S1 state is responsible for the effects obsd. in
guaiazulene, and that intramol. vibrational redistribution occurs with a time
const. of several hundred femtoseconds. [on SciFinder (R)] asdfgf
[116] S2-S0 Spectroscopy of the van der Waals Complexes of Azulene
with Rare Gases
O. K. Abou-Zied, H. K. Sinha, R. P. Steer, Journal of Physical Chemistry
1996, 100, 4375-4381
The S2-S0 fluorescence emission and excitation spectra of jet-cooled
azulene and its complexes with the rare gases, Ne, Ar, Kr, and Xe, were
measured. Features due to AZ.RGn (n = 1-4 when RG = Ar, Kr, Xe and n =
1 when RG = Ne) are obsd. in the S2-S0 fluorescence excitation spectra
when azulene is coexpanded with the rare gases. The microscopic solvent
shifts, d.hivin.n, of the origin bands for each complex scale linearly with the
polarizability of the adatom(s), indicating that binding is dominated by
dispersive interactions. This conclusion was confirmed in calcns. of the sep.
contributions of dispersion and induction to d.hivin.n. Mildly anharmonic
progressions in very low frequency excited-state intermol. (van der Waals)
modes are attached to the origin bands of each AZ.Krn and AZ.Xen
complex. The dominant progression is assigned to single quantum changes
in that excited-state bending mode which involves motion of the adatom(s)
in the plane perpendicular to the azulene ring contg. the long (x) axis of the
mol. This assignment was confirmed by calcg. the frequencies of the
bending and stretching vibrations in the ground state by using 1-dimensional
Morse and Taylor's series potential functions. The most stable geometries of
the n = 1-4 complexes in their ground states were calcd. by using a
summation of pairwise atom-atom Lennard-Jones 6-12 potentials. The
potential min. of the 1:1 complexes are located over the 7-membered ring;
no 2nd min. is found over the 5-membered ring. The most stable 1:2 species
appears to be the sym. (1 + 1) complex in which 1 adatom is bound on each
side of the azulene surface. Some evidence of the asym. (2 + 0) isomer is
found. [on SciFinder (R)] asdfgf
[117] Photophysical triplet state processes of 4H-1-benzopyran-4-thione
in a perfluoroalkane. Part 2. Delayed S2 -> S0 fluorescence due to
triplet-triplet annihilation
H. Eisenberger, B. Nickel, A. A. Ruth, R. P. Steer, Journal of the Chemical
Society, Faraday Transactions 1996, 92, 741-746
With 4H-1-benzopyran-4-thione (BPT) in perfluoro-1,3dimethylcyclohexane a delayed monomer fluorescence S2 -> S0 due to
triplet-triplet annihilation (TTA) is observable. The probability of generating
an S2 state by the annihilation of two triplet states is ca. 0.03. The generation
of S2 by TTA can be interpreted as a special case of Foerster energy
transfer. There is no indication of a delayed fluorescence from an upper
excimer state. [on SciFinder (R)] asdfgf
[118] The photophysics of thioflavone in solution
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of Photochemistry and
Photobiology, A: Chemistry 1996, 100, 43-52
The absorption, emission and emission excitation spectra, S2->S0
fluorescence and T1->S0 phosphorescence quantum yields and S2 and T1
excited state lifetimes of thioflavone (TF) were measured in perfluoro-1,3dimethylcyclohexane (PFDMCH) and 3-methylpentane (3-MP) at room
temp. The results were analyzed to provide a quant. description of the decay
processes of excited TF, with emphasis on the effect of the Ph rotor on the
radiationless decay rates. In the inert perfluoroalkane solvent, both S2 and
T1 relax intramolecularly via an S2->S1->T1->S0 path. S2->S0
fluorescence and T1->S0 phosphorescence also account for a small fraction
of the excited state decay events, but no thermally activated delayed
fluorescence, S1->S0+hndf, is obsd. In 3-MP, the decay mechanism is
dominated by intermol. interactions between excited TF and the solvent. The
rates of intramol. radiationless decay are larger in TF than in rigid thiones
having the same electronic energy gaps owing to the influence of the
torsional motion of the Ph group. By comparison of the obsd. rate consts. for
intramol. radiationless decay of excited TF with those of a hypothetical rigid
thione having the same electronic energy spacings, contributions to the
radiationless decay rates of S2 and T1 due to torsional motion of the Ph
group were quantified. The second-order rate consts. for quenching of triplet
thione by mol. oxygen and by ground state thione were also measured.
Excitation of TF to singlet excited states higher in energy than S2 results in
decay to S1 and T1 which partially bypasses S2. [on SciFinder (R)] asdfgf
[119] Photophysical triplet state processes of 4H-1-benzopyrane-4thione in a perfluoroalkane. Part 3. Excitation-wavelength dependence
of luminescence quantum yields
B. Nickel, H. Eisenberger, M. T. Wick, R. P. Steer, Journal of the Chemical
Society, Faraday Transactions 1996, 92, 1101-1104
The phosphorescence excitation spectrum of 4H-1-benzopyrane-4-thione
(BPT) in perfluoro-1,3-dimethylcyclohexane coincides with the absorption
spectrum in the range of the transitions S0 -> T1 and S0 -> S1. Hence the
quantum yield jP of the phosphorescence from BPT is independent of the
primary excited state, T1 or S1, in contrast to a reported lower value of jP
for excitation to S1. The ratio jPF2/jP of the quantum yields jPF2 of the
prompt S2 -> S0 fluorescence and jP of the phosphorescence decreases with
decreasing excitation wavelength. [on SciFinder (R)] asdfgf
[120] Laser-induced fluorescence excitation spectroscopy of jet-cooled
tropolone-carbon monoxide van der Waals complexes
H. K. Sinha, V. J. MacKenzie, R. P. Steer, Chemical Physics 1996, 213,
397-411
The 1:1 and 1:2 van der Waals complexes of tropolone (TRN) and
tropolone-OD with CO [TRN.(CO)n (n = 1, 2)] have been synthesized in a
supersonic expansion, and their S1-S0 laser-induced fluorescence excitation
spectra have been measured. Two distinct isomers of the 1 : 1 complex,
TRN.CO(I) and TRN.CO(II), with their origins displaced 54.5 and 79.5 cm-
1 resp. to the red of the origin of bare tropolone, have been identified. In
TRN.CO(I), the CO is located over the seven-membered ring and is
primarily dispersively bound. In TRN.CO(II), the CO is located in the plane
of the ring and forms a hydrogen-bonded complex with the hydroxyl group
of the chromophore. TRN.(CO)2 has a spectral shift which is exactly twice
that of the 1 : 1 TRN.CO(I) complex, indicating that the 1 : 2 complex has a
sym. sandwich structure with one CO residing on each side of the
chromophore. A large decrease in the proton tunneling splitting, from 19.4
cm-1 in bare tropolone to 3.5 cm-1 in TRN.CO(I) is obsd., indicating that
CO strongly perturbs the reaction coordinate by significantly increasing the
barrier height in the excited state. Strong coupling of the tunneling mode to
the in-plane CO torsional mode is suggested to be the likely source of this
effect. No tunneling doublets are obsd. in TRN.CO(II), suggesting that
hydrogen bonding quenches excited state proton transfer. The results of
parallel studies on tropolone-OD are completely consistent with this model.
Empirical calcns. of the geometries and binding energies of the complexes
using Lennard-Jones 6-12 atom-atom pair potentials support the
interpretations. [on SciFinder (R)] asdfgf
[121] Subpicosecond population decay time of the first excited singlet
state of thioxanthione in fluid solution
D. Tittelbach-Helmrich, R. P. Steer, Chemical Physics Letters 1996, 262,
369-373
The population decay time of the first excited singlet state of an arom.
thione, thioxanthione, has been measured for the first time in fluid soln. at
room temp., using a laser pump-probe method. The av. value of t(S1) is
0.53+-0.12 ps in three solvents, acetonitrile, acetone and 1-octanol, giving a
nonradiative S1 decay const. of (1.9+-0.4)*1012 s-1. Using these data and a
calcd. value of the radiative lifetime of S1, a quantum yield of S1 -> S0
fluorescence, jf<5*10-8, is estd., a value which is consistent with the failure
to observe prompt fluorescence in these compds. The short S1 lifetime of
thioxanthione is attributed to fast S1(1A2)-Tn(3A1) intersystem crossing,
which can occur by a first order spin-orbit coupling mechanism because
Tn(3A1) (n = 1 or 2) has a lower energy than S1(1A2) in the polar solvents
employed. [on SciFinder (R)] asdfgf
[122] Van der Waals complexes of 2-chloro-, 2-methyl-, and 1,3dimethylazulene with rare gases: microscopic solvent shifts, structures,
and binding energies
O. K. Abou-Zied, H. K. Sinha, R. P. Steer, Journal of Physical Chemistry A
1997, 101, 7989-7997
The S2-S0(1La) fluorescence excitation and emission spectra of the van der
Waals complexes of three azulene (Az) derivs., 2-chloroazulene (ClAz), 2methylazulene (MAz), and 1,3-dimethylazulene (DMAz), with the rare
gases, Ar, Kr, and Xe, have been measured under jet-cooled conditions. The
microscopic solvent shifts, d.hivin.n, of the origin bands in the S0-S2 spectra
assocd. with complexation of the chromophores with one and two rare gas
atoms increase with increasing polarizability of the adatom(s), consistent
with the dominance of dispersion in the binding. Although there are
substantial variations in the relative values of d.hivin.n among the Az derivs.
examd., all of the d.hivin.n values are relatively small and are similar to
those of the 1Lb(S0-S1) transitions in the rare gas complexes of naphthalene
and its methyl-substituted derivs. The theory of microscopic solvent shifts of
Jortner et al. has been used to analyze the solvent shift data. Comparisons of
the sources of the oscillator strengths and van der Waals binding interactions
in the azulene- and naphthalene-rare gas systems are revealing and suggest
that the variations in d.hivin.n with substitution pattern are primarily
electronic in their origin and arise from variations in excited state
configuration interactions, the magnitude of which depend on the S2-Sn
energy spacings. These spacings can be varied by placing substituents either
along the long axis (2-position) or parallel to the short axis( 1,3-positions) so
that they selectively perturb, resp., the long axis polarized and the short axis
polarized transitions. The structures and binding energies of the complexes
of these derivs. have also been modeled using Lennard-Jones type calcns.
and have been compared with those of Az itself. The obsd. progressions in
the low-frequency intermol. vibrations in each case are assigned to that
excited state bending mode which is parallel to the long axis of the
chromophore, in agreement with model calcns. using one-dimensional
Morse and Taylor's series potential functions. [on SciFinder (R)] asdfgf
[123] S2-S0 Spectroscopy of azulene and its 2-chloro, 2-methyl, and 1,3dimethyl derivatives
O. K. Abou-Zied, H. K. Sinha, R. P. Steer, Journal of Molecular
Spectroscopy 1997, 183, 42-56
The laser-induced S2-S0 fluorescence excitation and dispersed emission
spectra of azulene and three azulene derivs. of C2u skeletal symmetry, 2chloro-, 2-methyl-, and 1,3-dimethylazulene, were measured under ultracold,
isolated conditions in a supersonic expansion. The FTIR spectra of the
derivs. were measured, and the ground state vibrational frequencies were
calcd., with scaling, from the force fields by an ab initio method at the
RHF/CEP-31G level for all four compds. to aid in the assignments of these
spectra. To achieve global consistency, the vibrational assignments of the
ground and 2nd excited singlet states of azulene have required refinement;
the reassignments have implications for the vibronic and vibrational
coupling models used in existing interpretations of its S2-S0 spectra and the
S2 photophysics. Probably small changes of the energy gap between the S2
and S1 states which result from substitutions in the ring produce large
changes in both the S2-S0 fluorescence excitation spectra and the
photophysics of azulene. Substitution at the 2-position accelerates the rate of
radiationless relaxation, whereas this rate remains nearly unaffected by
substitution at the 1,3-positions when compared with azulene itself. The
threshold for intramol. vibrational redistribution is significantly reduced the
Me-substituted azulenes. consistent with an increase in the d. of coupled
states. [on SciFinder (R)] asdfgf
[124] Picosecond dynamics of the S2 excited state of azulene and its van
der Waals complexes with Ar and Xe
O. K. Abou-Zied, D. R. M. Demmer, S. C. Wallace, R. P. Steer, Chemical
Physics Letters 1997, 266, 75-85
The dynamics of the 2nd excited singlet (S2) states of jet-cooled azulene
(Az) and its van der Waals complexes with one and two atoms of Ar and Xe
were studied. Quantum interference effects due to restricted intramol.
vibrational redistribution in azulene were reinterpreted. Lifetime shortening
in the 1:1 and 1:2 complexes with Xe is attributed to an enhancement of the
rate of S2-T1 intersystem crossing by the external heavy atom effect.
Excitation of the complexes to states with Evib>Eb produces energyresolved fluorescence decays which exhibit sequential intramol. vibrational
redistribution-vibrational predissocn. [on SciFinder (R)] asdfgf
[125] S2-S0 spectra of jet-cooled xanthione and benzopyranthione:
microscopic solvation by rare gases
H. K. Shinha, R. P. Steer, Journal of Molecular Spectroscopy 1997, 181,
194-206
The laser-induced S2-S0 fluorescence excitation and dispersed emission
spectra of 2 arom. thiones, xanthione (XT) and benzopyranthione (BPT),
were measured under jet-cooled conditions. These emission spectra, together
with a corresponding FTIR and FT-Raman spectra and ab initio CEP-31G
calcns., were used to achieve a complete vibrational anal. of BPT in its
ground state, complementing that of XT, previously published. The 1:1 and
1:2 van der Waals complexes of XT and BPT with He, Ne, Ar, Kr, and Xe
were synthesized and characterized using their microscopic solvation shifts
and the frequencies of their intermol. (van der Waals) vibrations. In all cases
the adatom preferentially occupies a position over the pyranthione ring in the
1:1 complex, and the 1:2 complex has a sym. sandwich (1|1) structure. No
(2|0) isomer is obsd. An anal. of the microscopic solvent shifts of the origin
bands of the complexes indicates that dipole-induced dipole interactions
contribute significantly to the binding energies. The binding energies and
geometries of the 1:1 and 1:2 van der Waals complexes of XT, BPT, and
pyanthione (PT) were calcd. using pairwise atom-atom Lennard-Jones 6-12
potentials and are consistent with the structures derived from expt. The outof-plane intermol. stretching vibrations can be clearly obsd. in the excitation
spectra of most complexes, and their assignments were confirmed using
model vibrational potentials. Ab initio SCF calcns. including 2nd order
Moller-Plesset correlation are also reported for the PT.rare gas systems, and
show nice agreement with the Lennard-Jones calcns. and expt. [on SciFinder
(R)] asdfgf
[126] Electronic coupling matrix element for electronic excitation
energy transfer: S1+S1 -> S2+S0 excited state annihilation
R. D. Harcourt, K. P. Ghiggino, G. D. Scholes, R. P. Steer, Journal of
Chemical Physics 1998, 109, 1310-1314
For the singlet-singlet annihilation process S1+S1 -> S2+S0, an expression
is derived for the electronic coupling matrix element involved in electronic
excitation energy transfer (EET) between two equiv. chromophores.
Allowance is made for vibronic coupling to occur between the S1 and S2
states of the chromophores, for each of which the S1 .rharw. S0 and S2
.rharw. S0 excitations are, resp., elec.-dipole forbidden and allowed.
Attention is also given to aspects of the theory for singlet-singlet
(pDpD*)(nAnA) -> (pDpD)(nApA*) EET with vibronic coupling. The
theory for S1+S1 -> S2+S0 is illustrated via the results of some model
calcns. for two C:S chromophores. For each chromophore, the calcns. treat
explicitly the valence-shell pp AOs and a sulfur lone-pair AO. The results of
the calcns. highlight the nature of the primary contributors to the vibronic
coupling component of the electronic EET matrix element, namely
Coulombic-type terms. The theory also shows that a Coulombic term is the
primary contributor to the vibronic coupling component for singlet-singlet
(pDpD*)(nAnA) (pDpD)(nApA*) EET. [on SciFinder (R)] asdfgf
[127] The electronic spectroscopy and photophysics of tropolone and its
van der Waals complexes
V. J. Mackenzie, R. P. Steer, Research on Chemical Intermediates 1998, 24,
813-829
Studies of the electronic spectroscopy of tropolone in a variety of media are
reviewed. Attempts to understand the effects of the surrounding medium on
tropolone in its ground and first excited singlet states by studying the spectra
and dynamics of its van der Waals complexes are described. The van der
Waals complexes studied to date fall into two groups. Those which are
primarily dispersively bound exhibit red microscopic solvent shifts, have
observable tunneling doublet splittings and have structures in which the
solvent species are bound above and below the plane of the chromophore in
the 1:1 and 1:2 clusters. Those which are primarily hydrogen-bonded exhibit
blue microscopic solvent shifts and exhibit no observable tunneling
doublets. 40 Refs. [on SciFinder (R)] asdfgf
[128] Efficiency of singlet oxygen generation by alloxazines and
isoalloxazines
E. Sikorska, M. Sikorski, R. P. Steer, F. Wilkinson, D. R. Worrall, Journal
of the Chemical Society, Faraday Transactions 1998, 94, 2347-2353
Some spectroscopic properties of alloxazines and the lifetimes of their
singlet and triplet states in acetonitrile are reported. In addn., the efficiencies
of singlet oxygen prodn. by energy transfer from the excited states of a range
of substituted alloxazines and lumiflavine have been detd. in aerated
acetonitrile soln. Triplet state quantum yields have also been measured and it
has been shown that the efficiency of singlet oxygen prodn. from the triplet
state is equal to unity within exptl. error in all cases. [on SciFinder (R)]
asdfgf
[129] Numerical simulations of thione S2 -> S0 fluorescence decay: an
aid for modeling fast quenching kinetics
M. Sikorski, E. Krystkowiak, R. P. Steer, Journal of Fluorescence 1998, 8,
273-280
Numerical simulations designed to illuminate the opportunities for and
limitations on recovering the parameters in the Smoluchowski-CollinsKimball (SCK) model of time-dependent fluorescence quenching from the
data of time-correlated single-photon counting expts. were carried out. Two
donors with lifetimes of 880 and 410 ps, corresponding to the unquenched
S2 lifetimes of 2,2,3,3-tetramethylindanethione (TMIT) and 9,9dimethylanthrathione (DMAT) in inert perfluoroalkane solvents at room
temp., were used for purposes of illustration. The failure of the singleexponential decay model due to the transient effect can be detected at lower
quencher concns. for longer-lived fluorophores. While good values of the
diffusion coeff., effective encounter distance, and scaling parameters can be
recovered at sufficiently high quencher concns., the simulations show that
values of the specific rate const., K, can be expected to vary by up to two
orders of magnitude when it is recovered from typical time-correlated single
photon counting data even if the data can be reliably described by the SCK
model. The ability to differentiate between SCK kinetics and singleexponential kinetics by using fluorophores of varying unquenched lifetimes
also was explored. [on SciFinder (R)] asdfgf
[130] The kinetics of fast fluorescence quenching processes
M. Sikorski, E. Krystkowiak, R. P. Steer, Journal of Photochemistry and
Photobiology, A: Chemistry 1998, 117, 1-16
The results of fluorescence quenching expts. which bear on the kinetics of
fast bimol. processes are reviewed and analyzed critically in light of ongoing
developments of the theory of these processes. A set of recommendations for
'best' exptl. procedures and data anal. methods is proposed. 146 Refs. [on
SciFinder (R)] asdfgf
[131] Photophysics of aromatic thioketones supported on cellulose
M. Sikorski, F. Wilkinson, R. P. Steer, Journal of the Chemical Society,
Faraday Transactions 1998, 94, 1855-1859
The photophys. and spectroscopic properties of four thioketones, 4H-1benzopyran-4-thione, xanthione, thioflavone and Michler's thione, adsorbed
on cellulose have been measured and have been compared with the
properties of the same compds. in homogeneous solns. of varying polarities
and in b-cyclodextrin (b-CD) inclusion complexes. The phosphorescence
spectra, solvent shifts and triplet lifetimes all suggest that the thioketones are
bound at polar sites on cellulose, that the lowest triplets are of p,p*
character. Anal. of the triplet state decays using dispersion kinetics reveals
that the thioketones exhibit a wider distribution of decay consts. on cellulose
than in b-CD as a consequence of their larger no. of possible orientations
and binding locations on the open-chain polysaccharide. [on SciFinder (R)]
asdfgf
[132] M-DNA: A Complex Between Divalent Metal Ions and DNA
which Behaves as a Molecular Wire
P. Aich, S. L. Labiuk, L. W. Tari, L. J. T. Delbaere, W. J. Roesler, K. J.
Falk, R. P. Steer, J. S. Lee, Journal of Molecular Biology 1999, 294, 477485
M-DNA is a complex of DNA with divalent metal ions (Zn2+, Co2+, or
Ni2+) which forms at pH conditions above 8. Upon addn. of these metal
ions to B-DNA at pH 8.5, the pH decreases such that one proton is released
per base-pair per metal ion. Together with previous NMR data, this result
demonstrated that the imino proton in each base-pair of the duplex was
substituted by a metal ion and that M-DNA might possess unusual
conductive properties. Duplexes of 20 base-pairs were constructed with
fluorescein (donor) at one end and rhodamine (acceptor) at the other. Upon
formation of M-DNA (with Zn2+) the fluorescence of the donor was 95 %
quenched. Fluorescence lifetime measurements showed the presence of a
very fast component in the decay kinetics with t?10 ps. The fast component
was absent in B-DNA and in M-DNA lacking an acceptor chromophore; a
result which is only consistent with electron transfer. Efficient signal
transduction was also obsd. between the two fluorophores sepd. by 54 basepairs (over 150 .ANG.) in an M-DNA duplex. The addn. of a sequencespecific DNA-binding protein prevented the flow of electrons and this was
reversed by protease digestion. Therefore, M-DNA behaves as a mol. wire
and could be manipulated to prep. self-assembling electronic circuits. (c)
1999 Academic Press. [on SciFinder (R)] asdfgf
[133] M-DNA: A complex between divalent metal ions and DNA which
behaves as a molecular wire
P. Aich, S. L. Labiuk, L. W. Tari, L. J. Delbaere, W. J. Roesler, K. J. Falk,
R. P. Steer, J. S. Lee, Journal of molecular biology 1999, 294, 477-485
M-DNA is a complex of DNA with divalent metal ions (Zn(2+), Co(2+), or
Ni(2+)) which forms at pH conditions above 8. Upon addition of these metal
ions to B-DNA at pH 8.5, the pH decreases such that one proton is released
per base-pair per metal ion. Together with previous NMR data, this result
demonstrated that the imino proton in each base-pair of the duplex was
substituted by a metal ion and that M-DNA might possess unusual
conductive properties. Duplexes of 20 base-pairs were constructed with
fluorescein (donor) at one end and rhodamine (acceptor) at the other. Upon
formation of M-DNA (with Zn(2+)) the fluorescence of the donor was 95 %
quenched. Fluorescence lifetime measurements showed the presence of a
very fast component in the decay kinetics with tau</=10 ps. The fast
component was absent in B-DNA and in M-DNA lacking an acceptor
chromophore; a result which is only consistent with electron transfer.
Efficient signal transduction was also observed between the two
fluorophores separated by 54 base-pairs (over 150 A) in an M-DNA duplex.
The addition of a sequence-specific DNA-binding protein prevented the flow
of electrons and this was reversed by protease digestion. Therefore, M-DNA
behaves as a molecular wire and could be manipulated to prepare selfassembling electronic circuits. Copyright 1999 Academic Press. [on
SciFinder (R)] asdfgf
[134] Van der Waals Complexes of Tropolone with Carbon Dioxide
V. J. MacKenzie, M. Z. Zgierski, R. P. Steer, Journal of Physical Chemistry
A 1999, 103, 8389-8395
Van der Waals complexes of tropolone (TRN) with carbon dioxide have
been synthesized by expanding mixts. of TRN and CO2 in He in a
supersonic free-jet expansion, and have been examd. by laser-induced
fluorescence excitation spectroscopy and by ab initio structural calcns. At
higher partial pressures of CO2 large clusters of TRN(CO2)n are formed in
which TRN excited to S1 remains fluorescent. At sufficiently low partial
pressures of CO2, well-resolved blue-shifted features in the spectra due to
TRN(CO2) and TRN(CO2)2 may be identified. Their microscopic solvent
shifts and the effects of solvation on the proton tunneling doublets and
vibrational frequencies of the chromophore have been measured. The exptl.
results and calcns. all suggest that the CO2 binds to TRN in the 1:1 complex
in a fashion which is similar to common hydrogen-bonding addends. The
CO2 lies in the plane of the TRN ring and interacts with the keto oxygen and
the hydroxyl hydrogen in such a way that the intramol. hydrogen bond of
TRN is partially disrupted, lengthening the RO-H distance and opening the
H-O-C angle. Consistent with this interaction, the proton tunneling rate is
reduced so that the tunneling doublets can no longer be resolved, and the
frequency of the out-of-plane wagging vibration of the two oxygen atoms in
the chromophore increases. The microscopic solvatochromic shifts of these
and other hydrogen-bonded complexes are well-correlated with the calcd.
binding energies of the addends to the chromophore and with their proton
affinities. The latter correlation suggests that CO2 acts as a weak proton
acceptor in a hydrogen-bonding-like interaction with TRN. [on SciFinder
(R)] asdfgf
[135] Photophysics of tropolone
V. J. MacKenzie, H. K. Sinha, S. C. Wallace, R. P. Steer, Chemical Physics
Letters 1999, 305, 1-7
The excited state fluorescence lifetimes of isolated tropolone in a supersonic
expansion have been measured as a function of vibrational energy in the
Evib = 0-293 cm-1 region. A fluorescence quantum yield of 0.06 is estd. for
the zero-point level of S1; the lifetimes decrease rapidly with increasing
vibrational energy. Semi-empirical calcns. of the energies of nearby excited
states suggest that intersystem crossing to T3 may dominate the nonradiative decay of vibrationless S1, whereas the decrease of lifetime with
increasing Evib is due to an increasing rate of internal conversion via the
'proximity effect'. [on SciFinder (R)] asdfgf
[136] Triplet states of aromatic thioketones supported on cellulose
M. Sikorski, F. Wilkinson, J. L. Bourdelande, R. Gonzalez Moreno, R. P.
Steer, Physical Chemistry Chemical Physics 1999, 1, 3639-3645
Diffuse-reflectance and emission and transient diffuse-reflectance
measurements of the spectra and decay kinetics of several arom. thiones
supported on cellulose provided information concerning the 2nd excited
singlet state, S2, the lowest triplet state, T1, and transient chem.
intermediates formed when these materials are excited in the near UV.
Thermally activated delayed fluorescence from the lowest excited singlet
state, S1, is a minor component of the emission at room temp., but not at 77
K because back-intersystem crossing is eliminated, resulting in a substantial
lengthening of the triplet lifetime. At room temp., the triplet states have
lifetimes of the order of ms and the chem. intermediates have lifetimes of
hundreds of ms. Comparisons of the triplet spectra in polar and nonpolar
media with those on cellulose show that T1 is of p,p* configuration on the
solid support, as expected if the thione is adsorbed in a polar
microenvironment. The triplet decay times, but not the spectra, indicate that
the thiones are in different microenvironments when they are adsorbed from
MeCN compared with MeOH. The latter have decay times characteristic of
single mols.; the former may indicate the thiones are adsorbed as aggregates.
Singlet mol. oxygen, O2(1Dg), obsd. directly by transient emission at 1270
nm, is formed with near unit efficiency from the thione triplets in fluid
MeCN soln., but O2 has no measurable effect on the triplet spectra and
decay times when the thione is supported on cellulose. [on SciFinder (R)]
asdfgf
[137] Studies of the photophysics and spectroscopy of alloxazine and
related compounds in solution and in the solid state
M. Sikorski, E. Sikorska, F. Wilkinson, R. P. Steer, Canadian Journal of
Chemistry 1999, 77, 472-480
The absorption and emission spectra and the fluorescence lifetimes and
quantum yields of alloxazine and several of its N- and C- methyl-substituted
derivs., including two isoalloxazines, have been measured in a nonpolar
solvent (1,2-dichloroethane), a polar aprotic solvent (acetonitrile), and a
polar protic solvent (ethanol). The excited state decays are all single
exponential, suggesting that only one emitting species is present in all cases.
The spectroscopic data show that the emitting species is the p,p* electronic
excited state corresponding to the ground state of the absorbing mol.; there is
no evidence of excited state proton transfer for any of these solutes in any of
the three solvents. The longer lifetimes and larger fluorescence quantum
yields of the isoalloxazines compared with the alloxazines can be attributed
exclusively to significantly slower rates of radiationless relaxation in the
excited isoalloxazines. The remission and emission spectra of these same
compds. adsorbed on cellulose are similar to the corresponding absorption
and emission spectra in homogeneous soln., suggesting that the emitting
species are the same in the adsorbed state as they are in homogeneous soln.
However, the adsorbed excited species exhibit nonexponential temporal
decay, attributed to inhomogeneities in the binding of the chromophore to
cellulose. The emission spectra of polycryst. samples of alloxazine and
lumichrome suggest the possible occurrence of double intermol. excited
state proton transfer, but this interpretation is not supported by the further
photophys. data obtained here. [on SciFinder (R)] asdfgf
[138] Control of the photophysical properties of polyatomic molecules
by substitution and solvation: the second excited singlet state of azulene
N. Tetreault, R. S. Muthyala, R. S. H. Liu, R. P. Steer, Journal of Physical
Chemistry A 1999, 103, 2524-2531
The UV-visible-near-IR absorption spectra, S2 -> S0 fluorescence quantum
yields and S2 fluorescence lifetimes of 1-fluoroazulene, 1,3-difluoroazulene,
and several of their alkyl-substituted derivs. have been measured at room
temp. in up to six solvents, benzene, dichloromethane, ethanol, acetonitrile,
n-hexane, and perfluoro-n-hexane. The quantum yields (up to 0.2) and
lifetimes (up to 9.5 ns) of the S2 state of 1,3-difluoroazulene are
exceptionally large-the largest ever reported for an upper excited singlet
state of a polyat. mol. with a closed-shell ground state. The nonradiative rate
consts. for the decay of the S2 states of these mols. in these solvents and of
azulene, 1,3-dichloroazulene and 1,3-dibromoazulene, detd. previously, have
been analyzed in terms of the weak coupling case of radiationless transition
theory. The data show that the nonradiative rate consts. for the S2 states of
azulene, 1-fluoroazulene, and 1,3-difluoroazulene in the nonpolar solvents
follow the log-linear relationship expected of the energy gap law, provided
that S2-S1 internal conversion is assumed to dominate the decay mechanism.
The same linear correlation is obtained, irresp. of whether DE(S2-S1) is
varied by solvatochromism or fluorine substitution. Substitution by alkyl
groups increases the nonradiative decay rates by increasing the effective no.
of coupled states while the electronic coupling matrix element remains
const. Substitution at the 6-position by an iso-Pr group increases the rate
const. by a const. factor of 2.9; however, multiple substitution does not have
a multiplicative effect. Substitution by chlorine or bromine increases the S2
decay rates by enhancing the rate of intersystem crossing to the triplet
manifold. The rate enhancement is semiquant. modeled by considering the
effects of spin-orbit coupling of the halogen atoms. [on SciFinder (R)]
asdfgf
[139] Electronic spectroscopy and structures of the van der Waals
complexes of a,w-dihaloalkanes with anthracene
J. C. Hlady, R. P. Steer, Chemical Physics 2000, 260, 249-260
Anthracene has been coexpanded in helium with the a,w-dichloroalkanes of
carbon chain length n=1-6,8 and with the a,w-dibromoalkanes of chain
length n=3-5 in a supersonic jet. Van der Waals complexes of 1:1 and 1:2
stoichiometry are obsd. and their microscopic solvent shifts and excited state
van der Waals vibrational progressions have been measured by laser induced
fluorescence spectroscopy. The exptl. results, together with Lennard-Jones
calcns. of the ground state potential energy surfaces, suggest that the 1:1
complexes exhibit structures and dynamic behavior which are different when
n is even compared with when n is odd. The most stable 1:1 complexes have
structures in which the odd n dihaloalkanes are aligned parallel to the long
axis of the substrate, whereas those of n=2,4,6 lie across the ring and those
with n=2,4 have C2 symmetry. Isomers are found in the 1:2 complexes; only
1,2-dichloroethane forms a sym. (1|1) complex; the other dihaloalkanes form
(2|0) complexes preferentially. Evidence of intermol. electronic quenching
was found in the dibromoalkane complexes, and this was confirmed by
steady-state fluorescence quenching measurements in soln. [on SciFinder
(R)] asdfgf
[140] Synthesis and photophysical properties of fluorophore-labeled
abscisic acid
X. Hou, S. R. Abrams, J. J. Balsevich, N. Irvine, T. Norstrom, M. Sikorski,
H. K. Sinha, R. P. Steer, Canadian Journal of Chemistry 2000, 78, 963-974
The 8'-benzophenone, 8'-dansylhydrazone, 3'-(S)-(2-ethyldansylamide), and
3'-(S)-acetamidofluorescein derivs. of the plant hormone abscisic acid
(ABA) were synthesized for use in photoaffinity labeling (benzophenone
deriv.) or fluorescence probe expts. The 3'-tethered fluorescein deriv.
exhibits spectroscopic and photophys. properties which indicate that it could
be an excellent fluorescent probe of ABA interactions in vivo. The 3'tethered fluorescein and ABA moieties do not interact strongly, so that the
fluorescence properties of the fluorescein-labeled hormone are very similar
to those of fluorescein itself. Measurements of the absorption, emission, and
fluorescence excitation, fluorescence quantum yields, and fluorescence
decay parameters of this deriv. as a function of pH indicate that the
photophysics are dominated by ground- and excited state prototropic equil.
involving only the fluorescein moiety. The fluorescein dianion is the only
significant absorber and emitter at pH > 6.7, whereas only the cation absorbs
and emits at pH < 0. In the intervening pH range, strong emission from the
monoanion and weak emission from two neutral species, tentatively
assigned to the zwitterion and the lactone of the fluorescein moiety, is obsd.
[on SciFinder (R)] asdfgf
[141] Spectroscopy and photophysics of 5-nitrotropolone in condensed
media
L. C. T. Shoute, K. J. Falk, R. P. Steer, Chemical Physics Letters 2000, 332,
359-366
The absorption, fluorescence and fluorescence excitation spectra,
fluorescence quantum yields and fluorescence lifetimes of 5-nitrotropolone
(5NT) have been measured in water at 0<pH<8, and in several glass-forming
liqs. of varying polarity at temps. between 295 and 77 K. With pKa=3.3 and
pKa*>=-4.3, 5NT is much more acidic than tropolone itself and exists as the
anion over a wide range of pH in aq. media. The photophys. and kinetic
parameters describing the decay of both the excited anion and the
undissociated neutral mol. have been detd. in various condensed media at
temps. between 77 and 298 K. [on SciFinder (R)] asdfgf
[142] Spectroscopy and photophysics of tropolone in condensed media
L. C. T. Shoute, V. J. MacKenzie, K. J. Falk, H. K. Sinha, A. Warsylewicz,
R. P. Steer, Physical Chemistry Chemical Physics 2000, 2, 1-9
A systemic study of the spectroscopy and photophysics of tropolone in
condensed media has been undertaken by measuring its UV-visible
absorption, emission and emission-excitation spectra in a no. of solvents of
varying structure, at temps. between 77 K and 295 K, and by measuring its
quantum yields of emission, fem, and time-resolved emission decays as a
function of temp., T, and excitation wavelength, lex, in each solvent. In
weakly interacting solvents, such as 3-methylpentane and perfluoro-2-nbutyltetrahydrofuran, the only species yielding significant fluorescence is the
S1, 1(p,p*) state of the intramolecularly hydrogen-bonded neutral mol. In
acidic aq. soln. at pH <3, the emitting state is also 1(p,p*) S1, but the
intramol. hydrogen bond has been disrupted by the solvent. In basic aq. soln.
at pH >8 the emitting species is the corresponding excited anion. In some
solvents, emission from photochem. products prevents reliable
measurements from being made at room temp. Contrary to previous reports,
no emission from solvent-stabilized 1(n,p*) excited states is obsd. In glassforming media the fluorescence quantum yields and the lifetimes of the
emitting species increase in a sigmoidal fashion as the temp. decreases, and
are a factor of more than 100 greater at 77 K than at room temp. The
fluorescence quantum yields also decrease with increasing excitation energy
in the UV, an effect which is most pronounced in more weakly interacting
solvents at temps. near room temp. A mechanism is proposed. Both the
spectra and the excited state temporal decays indicate that emission from
short-lived, vibrationally unrelaxed S1 constitutes the majority of the
fluorescence at room temp., a small fraction of the total emission at 77 K,
and an increasing fraction of the total emission with decreasing excitation
wavelength. The variations of fem and lifetime with temp. are attributed to
the effects of temp.-dependent solvent relaxation dynamics on the nonradiative decay, based on qual. correlations between the obsd. parameters
describing the effects in different media and known characteristics of the
media, including their glass transition temps. and polarity. [on SciFinder
(R)] asdfgf
[143] Intermolecular interactions between tropolone and
fluoromethanes
V. J. MacKenzie, R. P. Steer, Canadian Journal of Physics 2001, 79, 483499
Van der Waals complexes of tropolone (TRN) with CF4, CFH3, CF2H2,
and CF3H have been synthesized by expanding mixts. of TRN and the
fluorinated methane (FM) in a supersonic free-jet and have been examd.
using laser induced fluorescence excitation spectroscopy. The sign and
magnitude of the microscopic solvent shifts and the magnitude of the
tunneling doublet splittings of the origin bands of each distinct complex
have been detd. from the LIFE spectra. These data, together with both
empirical and ab initio calcns. of the potential energy surfaces of the 1:1
complexes, have been used to assign the structures of the complexes and det.
their approx. binding energies. Expansion of TRN with CF4 produces one
identifiable 1:1 complex in which the solvent is primarily dispersively bound
and lies above the TRN ring in a sym. three-legged stool conformation.
Expansion of TRN with CFH3 produces two 1:1 complexes, both primarily
dispersively bound, in which the solvent mol. lies above the sevenmembered ring of TRN in a three-legged stool conformation but which
differ in the conformational orientation of the CFH3 species on the TRN
surface. Expansion of TRN with CF2H2 produces one 1:1 complex in which
the solvent mol. lies above the plane of the TRN ring, but is considerably
displaced from its center of mass and in which binding is primarily
electrostatic rather than dispersive. All three partially fluorinated methane
mols. produce 1:1 complexes with TRN in which the solvent is bound in the
TRN plane by intermol. hydrogen-bonding. Such structures partially disrupt
the intramol. hydrogen bond of the chromophore and consequently exhibit
LIFE spectra characterized by intense, strongly blue-shifted origin bands in
which the proton tunneling doublets are unresolvable because of a large
decrease in the intramol. proton tunneling rate. The existence of good
correlations between the solute-solvent binding energy and the microscopic
solvent shift and between the binding energy and the proton affinities of the
solvent for the entire group of hydrogen-bonding solvents, including the
partially fluorinated methanes, suggests that C-F ... H-O and F-C-H ... O:C
interactions result in weak hydrogen bonds which are not qual. different
from those of more traditional hydrogen-bonding species. [on SciFinder (R)]
asdfgf
[144] Vibronically selective intersystem crossing in van der Waals
complexes of 9-cyanoanthracene with xenon
L. C. T. Shoute, S. Hirayama, R. P. Steer, Chemical Physics Letters 2001,
338, 123-129
Supersonically expanded 9-cyanoanthracene (9CNA) and its van der Waals
complexes with Xe were examd. by laser-induced fluorescence excitation
spectroscopy. Previously unassigned vibronic features in the spectrum of the
bare mol. were assigned. Measurements of the relative intensities of
corresponding vibronic features in the cor. spectra of 9CNA and its 1:1
complex with Xe were used to calc. the quantum yields of fluorescence
arising from single vibronic levels at energies in the range 0<Ev<800 cm-1.
Variations are attributed to mode selectivity in S1-T1 intersystem crossing.
[on SciFinder (R)] asdfgf
[145] Photophysical properties of lumichromes in water
M. Sikorski, E. Sikorska, A. Koziolowa, R. Gonzalez Moreno, J. L.
Bourdelande, R. P. Steer, F. Wilkinson, Journal of Photochemistry and
Photobiology, B: Biology 2001, 60, 114-119
The photophysics of lumichrome, 1-methyllumichrome, and lumiflavin in
water solns. have been investigated. Fluorescence lifetimes of 2.7 and 2.2 ns
were obsd. for lumichrome and 1-methyllumichrome, resp., the
corresponding triplet state lifetimes of 17 and 18 ms have been obtained
from the transient absorption spectra. Evidence for long lived species with
absorption maxima near 450 nm and lifetimes of ca. 400 ms has been found
in the transient absorption spectra of both lumichromes. Quantum yields for
the sensitized prodn. of singlet oxygen, jD, are 0.36 and 0.41 for lumichrome
and 1-methyllumichrome, resp., in D2O. [on SciFinder (R)] asdfgf
[146] The photophysics of Lissamine rhodamine-B sulfonyl chloride in
aqueous solution: implications for fluorescent protein-dye conjugates
S. N. Smith, R. P. Steer, Journal of Photochemistry and Photobiology, A:
Chemistry 2001, 139, 151-156
The absorption, emission and fluorescence excitation spectra, fluorescence
quantum yields and fluorescence lifetimes of Lissamine rhodamine-B
sulfonyl chloride (LRSC) dye have been measured as a function of pH and
concn. in air-satd. and degassed aq. solns. and in acetone, and the spectra
measured at both 295 and 80 K. A single chem. species, the zwitterion, is
responsible for both absorption and emission at all pH's examd. With
excitation in the dye's strong S1-S0 absorption system in the visible, the
quantum yield of emission in aq. media is 0.33, independent of excitation
wavelength, and the excited state decays monoexponentially with a lifetime
of 1.63 ns. Oxygen in air-satd. solns., and dye concn. up to 0.1 mM have no
effect on the photophysics, and no phosphorescence from degassed frozen
solns. is obsd. These results and literature data are used to det. the
stoichiometry of Ig-LRSC dye conjugation using capillary electrophoresis
and a fiber optic-based fluorescence detector. [on SciFinder (R)] asdfgf
[147] Long range molecular wire behaviour in a metal complex of DNA
P. Aich, R. J. S. Skinner, S. D. Wettig, R. P. Steer, J. S. Lee, Journal of
Biomolecular Structure & Dynamics 2002, 20, 93-98
M-DNA is a complex of metal ions such as Zn2+ with duplex DNA.
Previous results showed that the fluorescence of a donor fluorophore was
quenched when an acceptor fluorophore was placed at the opposite end of a
short M-DNA duplex. In order to investigate further the mol. wire behavior
of M-DNA, 30-mer duplexes were constructed with fluorescein as donor and
rhodamine, pyrene and the cyanine dyes, Cy5 and Cy5.5 as acceptors. Good
quenching was obsd. in all cases even though the efficiency of resonance
energy transfer was calcd. to be < 5%. The distance dependence of
quenching was investigated by prepg. doubly-labeled duplexes ranging in
length from 20 to 1,000 base pairs. Upon formation of M-DNA significant
quenching of the fluorescence of the donor fluorophore was obsd. in
duplexes up to 500 base pairs in length. The amt. of quenching decreased
with increasing length of the duplexes with a shallow distance dependence.
The results are consistent with an electron transfer mechanism in which the
electron hops between metal centers. This process can occur efficiently over
long distances. [on SciFinder (R)] asdfgf
[148] Long range molecular wire behaviour in a metal complex of DNA
P. Aich, J. S. Skinner Ryan, D. Wettig Shawn, P. Steer Ronald, S. Lee
Jeremy, Journal of biomolecular structure & dynamics 2002, 20, 93-98
M-DNA is a complex of metal ions such as Zn(2+) with duplex DNA.
Previous results showed that the fluorescence of a donor fluorophore was
quenched when an acceptor fluorophore was placed at the opposite end of a
short M-DNA duplex. In order to investigate further the molecular wire
behaviour of M-DNA, 30-mer duplexes were constructed with fluorescein as
donor and rhodamine, pyrene and the cyanine dyes, Cy5 and Cy5.5 as
acceptors. Good quenching was observed in all cases even though the
efficiency of resonance energy transfer was calculated to be < 5%. The
distance dependence of quenching was investigated by preparing doublylabelled duplexes ranging in length from 20 to 1,000 base pairs. Upon
formation of M-DNA significant quenching of the fluorescence of the donor
fluorophore was observed in duplexes up to 500 base pairs in length. The
amount of quenching decreased with increasing length of the duplexes with
a shallow distance dependence. The results are consistent with an electron
transfer mechanism in which the electron hops between metal centers. This
process can occur efficiently over long distances. [on SciFinder (R)] asdfgf
[149] The relaxation of electronically excited molecules: effects of alkyl
substitution on the S2 and T1 states of benzopyranthione in solution
J. Kubicki, A. Maciejewski, M. Milewski, T. Wrozowa, R. P. Steer, Physical
Chemistry Chemical Physics 2002, 4, 173-179
The effects of n-alkyl group substitution on the photophysics, photochem.
and spectroscopy of benzopyranthione (BPT), a rigid model arom. thione,
were studied. The absorption, emission and excitation spectra of BPT and its
2-substituted Bu (I) and n-C10H21 (II) derivs., their S2 -> S0 fluorescence
and T1 -> S0 phosphorescence quantum yields and their S2 and T1 lifetimes
were measured in several solvents, in the presence and absence of O2.
Perfluorinated solvents were used to minimize the effects of solute-solvent
interactions on the excited state dynamics. The rate consts. of radiative and
nonradiative decay of the S2 and T1 states and of their bimol. quenching by
O2 and by ground state thione were obtained. The spectra of I and II are
nearly identical in every respect, except that the S2 -> S0 absorption
spectrum of II exhibits a greater absorbance on its red edge, attributable to
its larger d. of low-frequency torsional states. The expected effect of
intramol. interaction of the longer alkyl chain with the strongly Habstracting C:S group was not obsd. in the S2 states of II compared with I,
and this lack of effect was confirmed by showing that the excited state
lifetimes were unaffected by a 40-fold change in the viscosity of the
perfluoroalkane solvents. By contrast, a significantly shorter lifetime of the
triplet state of II compared with I suggests that intramol. interaction between
the hydrocarbon tail of II and the excited C:S group may induce nonradiative
relaxation in the longer-lived triplets. [on SciFinder (R)] asdfgf
[150] Guest editorial
R. P. Steer, Journal of Photochemistry and Photobiology, A: Chemistry
2002, 154, 1
asdfgf
[151] Dynamics of the B~ excited state of thiophosgene
A. M. Warsylewicz, K. J. Falk, R. P. Steer, Chemical Physics Letters 2002,
352, 48-56
Thiophosgene, Cl2CS, was excited with ps laser pulses in bulbs and a
continuous supersonic expansion to nominal single vibronic levels in the
B~(1A1) state, and its emission spectra, temporal fluorescence decay
profiles and quenching consts. measured. Bound bright vibronic B~ states
decay exponentially by primarily radiative means. Further evidence is found
of nonradiative passage of a subset of bright B~ states to a dark portion of
the B~ state hypersurface. [on SciFinder (R)] asdfgf
[152] Kinetics of disaggregation of a non-covalent zinc
tetraphenylporphyrin dimer in solution
Y. Li, R. P. Steer, Chemical Physics Letters 2003, 373, 94-99
Zinc tetraphenylporphyrin forms J-aggregates, consisting mainly of dimers
at moderate concns., in freshly-prepd. dry acetonitrile soln. at room temp.
The kinetics of disaggregation of these dimers may be followed by
monitoring their absorption and emission spectra and their fluorescence
lifetimes as a function of time and temp. following diln. The activation
energy of the disaggregation process is 20.0 kJ mol-1. [on SciFinder (R)]
asdfgf
[153] Energy transfer involving higher electronic states: a new direction
for molecular logic gates
E. K. L. Yeow, R. P. Steer, Chemical Physics Letters 2003, 377, 391-398
The design of a mol. logic circuit based on sequential forward S2-S2 energy
transfer and back S1-S1 energy transfer (cyclic energy transfer) for a system
comprising of an azulene and a zinc porphyrin is discussed. Using a
computational dynamics simulation, the effect of varying the orientation
factor via the viscosity of the medium on the cyclic energy transfer
dynamics is investigated and is incorporated into the logic circuit. [on
SciFinder (R)] asdfgf
[154] Dynamics of electronic energy transfer from the S2 state of
azulene to the S2 state of zinc porphyrin
E. K. L. Yeow, R. P. Steer, Physical Chemistry Chemical Physics 2003, 5,
97-105
Electronic energy transfer between the S2 state of azulene as donor and the
S2 state of zinc porphyrin as acceptor in dichloromethane and CTAB
micelles has been investigated. In dichloromethane high S2-S2 energy
transfer efficiency, which cannot be explained using the Foerster theory, is
obsd. An inhomogeneous distribution of acceptors surrounding the donor,
leading to short-range exchange interaction and higher multipole interaction
is proposed. In CTAB micelles, Foerster's mechanism is found to agree well
with the obsd. energy transfer efficiency when a surface-uniform distance
distribution between donor and acceptor is assumed. The implications of S2S2 energy transfer in our system for designing efficient mol. devices is
discussed. [on SciFinder (R)] asdfgf
[155] Photophysical Studies of Porphyrins and Metalloporphyrins:
Accurate Measurements of Fluorescence Spectra and Fluorescence
Quantum Yields for Soret Band Excitation of Zinc
Tetraphenylporphyrin
J. Karolczak, D. Kowalska, A. Lukaszewicz, A. Maciejewski, R. P. Steer,
Journal of Physical Chemistry A 2004, 108, 4570-4575
For the test system zinc tetraphenylporphyrin in ethanol, the S2-S0 and S1S0 absorption and emission spectra and fluorescence quantum yields have
been measured as a function of excitation wavelength within the Soret and
Q-bands under conditions where self-absorption of emission and solute
aggregation are either eliminated or properly compensated. Under these
conditions, the smallest S2-S0 Stokes shift yet measured, 115 cm-1, and the
largest S2-S0 abs. fluorescence quantum yield yet measured, 1.84 * 10-3, are
obtained. Accurate measurements of the relative quantum yields of S2-S0 to
S1-S0 emission as a function of excitation wavelength reveal that a fast
radiationless process that bypasses S1 is operative among states accessed at
excitation energies that span the Soret band. The data can be interpreted
using the evidence of Yu, Baskin, and Zewail that a second excited state,
S2', contributes a small fraction of the Soret band's integrated molar
absorptivity and is responsible for an increasing fraction of photon
absorption on both the red edge of the Soret band at lex > 430 nm and the
blue at lex < 409 nm. The lifetimes of the S2 and S1 states have also been
measured under similar conditions; the values obtained confirm previous
measurements. [on SciFinder (R)] asdfgf
[156] Fluorescence quenching of the S1 and S2 states of zinc mesotetrakis(4-sulfonatophenyl)porphyrin by halide ions
T. P. Lebold, E. K. L. Yeow, R. P. Steer, Photochemical & Photobiological
Sciences 2004, 3, 160-166
The excited-state quenching of zinc meso-tetrakis(4sulfonatophenyl)porphyrin (ZnTPPS) by halide ions has been studied in
water and various aq. micellar solns. using both steady-state and timeresolved fluorescence techniques. The quenching efficiencies of the S1 state
of ZnTPPS (t ~ 1.93 ns) by halide ions in homogeneous aq. soln. increase in
the order Cl- < Br- < I-; both dynamic and static quenching processes are
involved. The mechanisms which may possibly be operating (i.e. electron
transfer, heavy atom effect and Watkins mechanism) are discussed.
Quenching of the short-lived S2 state (t ~ 1.3 ps) of ZnTPPS is only possible
using I-; it is suggested that the quenching mechanism proceeds via electron
transfer between the fluorophore and I- within the quenching sphere of
action. The results are consistent with the Gibbs free energy change (DG
Deg) involved in the charge-sepn. process. Finally, the fluorescence
quenching of ZnTPPS by bromide ions in both cationic (DTAB and CTAC)
and neutral (TX-100) micelles has been examd. The quenching obsd. in the
cationic micelles is rationalized using a water-filled channel model, whereby
Br- ions diffuse down these channels to achieve close proximity to the
fluorophore. No emission quenching was obsd. in the case of the neutral TX100 micelles. [on SciFinder (R)] asdfgf
[157] Fluorescence quenching of the S1 and S2 states of zinc mesotetrakis(4-sulfonatophenyl)porphyrin by halide ions
P. Lebold Terry, K. L. Yeow Edwin, P. Steer Ronald, Photochemical &
photobiological sciences : Official journal of the European Photochemistry
Association and the European Society for Photobiology 2004, 3, 160-166
The excited-state quenching of zinc meso-tetrakis(4sulfonatophenyl)porphyrin (ZnTPPS) by halide ions has been studied in
water and various aqueous micellar solutions using both steady-state and
time-resolved fluorescence techniques. The quenching efficiencies of the
S(1) state of ZnTPPS ([small tau][approximate] 1.93 ns) by halide ions in
homogeneous aqueous solution increase in the order Cl(-) < Br(-) < I(-);
both dynamic and static quenching processes are involved. The mechanisms
which may possibly be operating (i.e. electron transfer, heavy atom effect
and Watkins mechanism) are discussed. Quenching of the short-lived S(2)
state ([small tau][approximate] 1.3 ps) of ZnTPPS is only possible using I(-);
it is suggested that the quenching mechanism proceeds via electron transfer
between the fluorophore and I(-) within the quenching sphere of action. The
results are consistent with the Gibbs free energy change ([capital
Delta]G[degree]) involved in the charge-separation process. Finally, the
fluorescence quenching of ZnTPPS by bromide ions in both cationic (DTAB
and CTAC) and neutral (TX-100) micelles has been examined. The
quenching observed in the cationic micelles is rationalised using a waterfilled channel model, whereby Br(-) ions diffuse down these channels to
achieve close proximity to the fluorophore. No emission quenching was
observed in the case of the neutral TX-100 micelles. [on SciFinder (R)]
asdfgf
[158] Sequential Forward S2-S2 and Back S1-S1 (Cyclic) Energy
Transfer in a Novel Azulene-Zinc Porphyrin Dyad
E. K. L. Yeow, M. Ziolek, J. Karolczak, S. V. Shevyakov, A. E. Asato, A.
Maciejewski, R. P. Steer, Journal of Physical Chemistry A 2004, 108,
10980-10988
A covalently tethered dyad contg. the azulene (Az) and zinc
tetraphenylporphyrin (ZnP) chromophores has been synthesized and its
excited-state dynamics investigated, using the tether-substituted
monochromophoric species as ref. compds. One photon excitation of the
dyad at 270 nm results in selective population of the S2 state of the azulene
moiety, followed by near-quant. electronic relaxation in the cycle S2(Az)S2(ZnP)-S1(ZnP)-S1(Az)-S0. Energy transfer from the S2 state of the
azulene moiety to the S2 state of the ZnP moiety is ultrafast (keet > 2 * 1012
s-1) and quant. The ZnP(S2) moiety subsequently undergoes rapid (kic = 3 *
1011 s-1), quant. internal conversion to its S1 state. Thereafter, the
excitation residing on the S1 state of the ZnP is returned to the Az moiety
via an efficient (ca. 90%) back S1-S1 energy transfer process (keet = 2.8 *
109 s-1). Ultimately the system returns to the electronic ground state via
conical intersection of the azulene S1 and S0 surfaces in ca. The cyclic
interchromophoric energy transfer rates are nearly the same in both
acetonitrile and cyclohexane, suggesting that the conformation of the tether
is similar in both solvents. Forster theory is inadequate in explaining the
efficient energy transfer dynamics, and other processes such as the Dexter
mechanism must be invoked. [on SciFinder (R)] asdfgf