[1]
R. A. Ackerman, J. N. Pitts, Jr., R. P. Steer, Journal of Chemical
Physics 1970, 52, 1603-1604 Abstract
[2] 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 Abstract
[3] J. N. Pitts, Jr., W. A. Kummer, R. P. Steer, Environmental Science
and Technology 1971, 5, 1045-1047Abstract
[4] R. A. Ackerman, J. N. Pitts, Jr., R. P. Steer, Chemical Physics Letters
1972, 12, 526- Abstract
[5] J. N. Pitts, Jr., W. A. Kummer, R. P. Steer, B. J. Finlayson, Advances
in Chemistry Series 1972, 113, 246-254 Abstract
[6] D. R. Dice, R. P. Steer, Journal of the Chemical Society, Chemical
Communications 1973, 106-107 Abstract
[7] D. R. Dice, R. P. Steer, Journal of Physical Chemistry 1973, 77, 434438 Abstract
[8] D. R. Dice, R. P. Steer, Journal of the American Chemical Society
1974, 96, 7361-7362 Abstract
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] 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. 2Thiabicyclo[2.2.1]hept-5-ene(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] 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
out-of-plane bending mode of the ground state and terminating in the
lowest vibrational levels of the excited state. [on SciFinder (R)] asdfgf
[11] 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 1,4-biradical 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] 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] 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(1A11A1) 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] 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] D. Phillips, R. P. Steer, Journal of Chemical Physics 1977, 67, 47804782
The fluorescence decay of CSCl2 vapors was studied as a function of
pressure and excitation wavelength by using time-correlated singlephoton 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] 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] 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] 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 isolatedmol. conditions, and radiative and nonradiative decay consts. were
estd. [on SciFinder (R)] asdfgf
[19] 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] 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-2norbornene due to restricted rotation imposed by the ring system. [on
SciFinder (R)] asdfgf
[21] 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 out-of-plane bending coordinate. Good
agreement with expt. was obtained. [on SciFinder (R)] asdfgf
[22] P. G. Mezey, R. P. Steer, A. Kapur, Journal of Photochemistry 1978,
9, 130-131
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] 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] 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] 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. selfquenching 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] 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] 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. outof-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] 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] 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] 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] 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] 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] 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] 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] R. P. Steer, Reviews of Chemical Intermediates 1980, 4, 1-41
A review with .apprx.168 refs. [on SciFinder (R)] asdfgf
[36] 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] 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 gasphase 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] A. Kapur, P. G. Mezey, R. P. Steer, Chemical Physics Letters 1981,
78, 81-84
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-of-plane bending coordinate as a function of out-ofplane angle are compared. [on SciFinder (R)] asdfgf
[39] 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] R. P. Steer, P. G. Mezey, A. Kapur, NATO Advanced Study Institutes
Series, Series C: Mathematical and Physical Sciences 1981, 67, 397402
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. out-of-plane bending mode frequencies were compared for
the 1st triplet state of F2CS. [on SciFinder (R)] asdfgf
[41] 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] 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-ofplane bending coordinates, were detd. Ests. of the 1st triplet state outof-plant vibrational frequencies were calcd. [on SciFinder (R)] asdfgf
[43] A. E. Bruno, R. P. Steer, Journal of Chemical Physics 1983, 78, 66606665
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] 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] 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] 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 Rydbergvalence 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] 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] 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 3-methylpentane 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 S2-S1 electronic energy sepn.
according to the energy gap law indicates that S2-S1 internal
conversion is the major nonradiative decay process in perfluoroalkane
solvents. [on SciFinder (R)] asdfgf
[49] 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] 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] 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] 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] 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
spin-forbidden 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] 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] 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] 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 2,2,3,3tetramethylindanthione, 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] 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] A. Maciejewski, R. P. Steer, Journal of Photochemistry 1986, 35, 5969
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] 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-4thione, 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 3-methylpentane 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] 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] 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] 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] 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] 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 I- showed 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] 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] 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-ofplane angles and barrier heights opposing mol. inversion were estd. as
26 +- 1 Deg and 538 +- 5 cm-1 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] 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] K. J. Falk, R. P. Steer, Canadian Journal of Chemistry 1988, 66, 575577
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] 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 Stern-Volmer 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-1-benzopyran4-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] 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-4thione (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 non-radiative decay consts. for
the triplet are obsd., and no evidence of bi-exponential 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] 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 doublemin. 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. C-Br
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] 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] R. P. Steer, V. Ramamurthy, Accounts of Chemical Research 1988,
21, 380-386
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] 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
3-methylpentane 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. self-quenching, 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 Franck-Condon factors for the T1 -> S0 nonradiative
transitions. A normal D isotope effect is obsd. in xanthione. [on
SciFinder (R)] asdfgf
[75] 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 b-H atoms close to the thiocarbonyl group can undergo net
decay from S2 -> S0, bypassing S1 and T1. [on SciFinder (R)] asdfgf
[76] 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] 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 nC6H14. [on SciFinder (R)] asdfgf
[78] 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] 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-1,2dihydronaphthalene, 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] 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[2,1h]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, chargetransfer 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] 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 triplettriplet, triplet-singlet and singlet-singlet annihilation in fluid systems.
[on SciFinder (R)] asdfgf
[82] K. J. Falk, R. P. Steer, Journal of Physical Chemistry 1990, 94, 57675772
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] 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
purinyl-pyridinium 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] M. Hachey, F. Grein, R. P. Steer, Chemical Physics Letters 1991, 183,
204-208
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] 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] 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] 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] 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. Self-quenching of the ~B state occurs at diffusion-limited
rates. In dil. perfluoroalkane solns. 2 processes dominate the excitedstate 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 n-hexane
and CCl4 the ~B state lifetime and fluorescence quantum yields are
greatly reduced due to solvent-induced radiationless decay. [on
SciFinder (R)] asdfgf
[89] 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 selfquenching process is responsible for the removal of the thione. A
mechanism involving the intermediacy of triplet excimers is proposed.
[on SciFinder (R)] asdfgf
[90] 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
laser-induced 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 thioneadatom interactions are almost exclusively dispersive in the upper
state. [on SciFinder (R)] asdfgf
[91] 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] 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 time-resolved 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] M. Szymanski, R. P. Steer, Journal of Physical Chemistry 1992, 96,
8719-8728
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] 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 short-lived 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] 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] 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
steady-state 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] 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] 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] 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 mol-dm-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] 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] 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] 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] 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] 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
two-photon, 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 2chloroazulene in non-viscous solvents are also reported. [on SciFinder
(R)] asdfgf
[105] 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 two-photon, 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 S1-S0 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] 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 quadraticGuassian 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] 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 perfluoron-alkane complexes with XT and beyond C3 in 1:1 perfluoro-n-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] 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, 2color, 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 2-photon 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] 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] 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 twophoton 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 aNPO, 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] 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 n-hexane 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 n-hexane 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] 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 laserinduced 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] 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/STO3G** 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, CEP-31G,
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] D. Tittelbach-Helmrich, R. P. Steer, Chemical Physics 1995, 197, 99106
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] 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
[116-158]
[116] 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] 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] 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-1,3-dimethylcyclohexane (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] 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] 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 sevenmembered ring and is primarily dispersively bound. In TRN.CO(II),
the CO is located in the plane of the ring and forms a hydrogenbonded 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] 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 1octanol, 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] 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), 2-methylazulene (MAz), and 1,3-dimethylazulene (DMAz),
with the rare gases, Ar, Kr, and Xe, have been measured under jetcooled 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 S2Sn 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 lowfrequency 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] 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,
2-chloro-, 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] 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 energy-resolved fluorescence decays which exhibit
sequential intramol. vibrational redistribution-vibrational predissocn.
[on SciFinder (R)] asdfgf
[125] 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 out-of-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] 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] 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] 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] 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 SmoluchowskiCollins-Kimball (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 2,2,3,3tetramethylindanethione (TMIT) and 9,9-dimethylanthrathione
(DMAT) in inert perfluoroalkane solvents at room temp., were used
for purposes of illustration. The failure of the single-exponential
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 single-exponential
kinetics by using fluorophores of varying unquenched lifetimes also
was explored. [on SciFinder (R)] asdfgf
[130] 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] 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] 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, 477-485
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 BDNA 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 base-pairs
(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] 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 BDNA 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, MDNA behaves as a molecular wire and could be manipulated to
prepare self-assembling electronic circuits. Copyright 1999 Academic
Press. [on SciFinder (R)] asdfgf
[134] 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 hydrogenbonded 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] 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 non-radiative 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] 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] 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- methylsubstituted 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] 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, 1,3difluoroazulene, 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-nhexane. 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, 1,3dichloroazulene 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] 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] 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] 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 5nitrotropolone (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] 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 3methylpentane and perfluoro-2-n-butyltetrahydrofuran, 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 glass-forming 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] V. J. MacKenzie, R. P. Steer, Canadian Journal of Physics 2001, 79,
483-499
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 hydrogenbonding 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] 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] 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 1methyllumichrome, resp., in D2O. [on SciFinder (R)] asdfgf
[146] 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] 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] P. Aich, J. S. Skinner Ryan, D. Wettig Shawn, P. Steer Ronald, S. Lee
Jeremy, Journal of biomolecular structure & dynamics 2002, 20, 9398
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 doubly-labelled duplexes
ranging in length from 20 to 1,000 base pairs. Upon formation of MDNA 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] 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 H-abstracting
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] R. P. Steer, Journal of Photochemistry and Photobiology, A:
Chemistry 2002, 154, 1
asdfgf
[151] 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] 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] E. K. L. Yeow, R. P. Steer, Chemical Physics Letters 2003, 377, 391398
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] 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 S2-S2
energy transfer in our system for designing efficient mol. devices is
discussed. [on SciFinder (R)] asdfgf
[155] 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] 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
time-resolved 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 waterfilled 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 TX-100 micelles. [on SciFinder
(R)] asdfgf
[157] 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 steadystate 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 water-filled 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] 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