[42-79]
[42] Ab initio SCF-MO calculations of features of the lowest triplet
state potential surfaces of several tetraatomic carbonyl compounds
A. Kapur, R. P. Steer, P. G. Mezey, Canadian Journal of Chemistry 1982,
60, 100-105
Non-empirical SCF-MO calcns. were carried out for RR1CO (R, R1 = H, H;
H, F; Cl, Cl; Cl, F; F, F). Portions of the ground singlet and 1st triplet state
potential energy surfaces, in particular those along the out-of-plane bending
coordinates, were detd. Ests. of the 1st triplet state out-of-plant vibrational
frequencies were calcd. [on SciFinder (R)] asdfgf
[43] Photophysics of laser-excited thioformaldehyde ~A(1A2)
A. E. Bruno, R. P. Steer, Journal of Chemical Physics 1983, 78, 6660-6665
Fluorescence was excited in the rotational subband heads of low lying
vibrational levels in the ~A(1A2) state of thioformaldehyde. Single
exponential fluorescence decays measured as a function of pressure yield
linear Stern-Volmer plots from which gas kinetic quenching rate and
collision-free lifetimes of between 14 (000 band) and 4 ms (402 band) were
obtained. The results are interpreted in terms of solely radiative decay of the
vibronic states investigated under isolated mol. conditions. [on SciFinder
(R)] asdfgf
[44] The thioketone-enethiol tautomerism of aliphatic thiocarbonyls:
an ab initio study
A. E. Bruno, R. P. Steer, P. G. Mezey, Journal of Computational Chemistry
1983, 4, 104-109
Approx. min. energy reaction paths have been calcd. for two thioketoneenethiol tautomeric systems using an ab initio SCF-MO method. The calcns.
indicate nearly equal stabilities of the isolated tautomers in both systems and
an energy barrier of ca. 85 kcal/mol for their interconversion. This barrier is
expected to be significantly lower in soln. as a result of solvent-solute
interactions. [on SciFinder (R)] asdfgf
[45] Excitation pulse-shape mimic technique for improving
picosecond-laser-excited time-correlated single-photon counting
deconvolutions
D. R. James, D. R. M. Demmer, R. E. Verrall, R. P. Steer, Review of
Scientific Instruments 1983, 54, 1121-1130
The detn. of accurate subnanosecond fluorescence lifetimes by the timecorrelated single-photon counting technique is often limited by difficulties in
obtaining the correct instrumental response function f(l,t) to the excitation
pulse. These difficulties are increased when a grating monochromator is
used to disperse the emission due to the introduction of an addnl. temporal
broadening of f(l,t). A technique for detg. the correct f(l,t) at the wavelength
of sample emission, lem, is described. The technique consists of using a very
short-lived fluorophore to mimic the excitation pulse shape at lem, yielding
the convoluted mimic decay function Cm(lem,*), and then to
computationally ext. f(lem,t) from Cm(lem,t). The technique is exptl. and
computationally simple and yields the desired instrumental response
function at lem which eliminates problems due to the sensitivity of f(l,t) to l
(color shift artifact). The photomultiplier tube receives spatially equal
illumination when both the sample decay and Cm(lem,t) are detd. because
both the mimic and sample emission are spectrally broad, eliminating the
problem of the variation of f(l,t) with position of incident light on the
photomultiplier tube photocathode (targeting artifact). Artifacts due to
monochromator temporal broadening are eliminated since both the mimic
and sample emission are monitored at const. wavelength. [on SciFinder (R)]
asdfgf
[46] Thioketone spectroscopy: an analysis of the lower electronic
transitions in thioacetone and thioacetaldehyde
R. H. Judge, D. C. Moule, A. E. Bruno, R. P. Steer, Chemical Physics
Letters 1983, 102, 385-389
Visible and UV spectra of the unstable species, thioacetaldehyde, CH3CHS,
thioacetone, (CH3)2CS, and thioacetone-d6 were studied in the gas phase.
The valence n -> p* excitations, ~A -> ~X and ~a -> ~X, were identified.
Rydberg n -> 4s, 4py and 4pz and p -> p* valence excitations were found in
thioacetone. In thioacetaldehyde a Rydberg-valence interaction mixes the
n,4s and p,p* states which leads to a broad absorption of mixed character
between 200 and 220 nm. [on SciFinder (R)] asdfgf
[47] Effect of solvent on the subnanosecond decay of the second excited
singlet state of tetramethylindanethione
A. Maciejewski, R. P. Steer, Chemical Physics Letters 1983, 100, 540-545
The S2 -> S0 fluorescence spectra and quantum yields and the S2 lifetimes
of 2,2,3,3-tetramethylindanethione (TMIT) were measured in several
solvents by using a synchronously pumped picosecond dye laser excitation
system. The S2 nonradiative decay rate is markedly solvent dependent. In
inert perfluoroalkane solvents, remarkably large S2-S0 fluorescence
quantum yields (ff = 0.14) and long S2 lifetimes (t = 880 ps) were measured.
Hydrocarbons are efficient excited-state quenchers. [on SciFinder (R)]
asdfgf
[48] Photophysics of the second excited singlet states of xanthione and
related thiones in perfluoroalkane solvents
A. Maciejewski, R. P. Steer, Journal of the American Chemical Society
1983, 105, 6738-6740
S2 -> S0 fluorescence quantum yields and subnanosecond lifetimes of the
second excited singlet states of xanthione, 4H-1-benzopyran-4-thione and
4H-pyran-4-thione were measured in perfluoroalkane and common
hydrocarbon solvents. Both quantities are markedly solvent dependent and
indicate, together with rate consts. for xanthione S2 quenching by 3methylpentane and benzene (6.5 * 109 M-1 s-1 and 1.1 * 1010 M-1 s-1,
resp.), that exclusively photophys. properties of thione S2 decay cannot be
measured in hydrocarbon solvents. Variation of knr for the thiones with S2S1 electronic energy sepn. according to the energy gap law indicates that S2S1 internal conversion is the major nonradiative decay process in
perfluoroalkane solvents. [on SciFinder (R)] asdfgf
[49] Decay dynamics of aromatic thione triplet states in fluid solution
A. Safarzadeh-Amiri, R. E. Verrall, R. P. Steer, Canadian Journal of
Chemistry 1983, 61, 894-900
Phosphorescence emission and excitation spectra, quantum yields, and
lifetimes of arom. thiones are detd. in liq. soln. at room temp. Excitation to
any of the excited electronic states accessible in the near-UV and visible
regions of the spectrum results in population of the lowest triplet state with
near unit quantum yield. Triplets of relatively inflexible thiones decay
nonradiatively with rate consts. of the order of 105 s-1; thiones capable of
higher amplitude flexion or group rotation exhibit nonradiative decay consts.
>=107 s-1 at infinite diln. in aprotic solvents. Self-quenching occurs at
diffusion-controlled rates. [on SciFinder (R)] asdfgf
[50] Concerning the lifetime of the second excited singlet state of
adamantanethione
K. J. Falk, A. R. Knight, A. Maciejewski, R. P. Steer, Journal of the
American Chemical Society 1984, 106, 8292-8293
No S2 -> S0 fluorescence (ff < 5 * 10-6) is obsd. when adamantanethione is
illuminated in its elec. dipole allowed, S2 -> S0 absorption band in alkane or
perfluoroalkane solns. at room temp. On the basis of this observation and a
calcn. of its intrinsic radiative lifetime (1.6 ns), it is demonstrated that the S2
state must have a lifetime <<1 ps. The S2 state therefore cannot be the 250
ps reactive transient thought to be responsible for initiating the photochem.
of adamantanethione when excited to S2 in the presence of cycloalkanes and
olefins. Several possible alternate identities of the 250-ps transient are
discussed. [on SciFinder (R)] asdfgf
[51] Radiationless decay of the second excited singlet states of
aromatic thiones: experimental verification of the energy gap law
A. Maciejewski, A. Safarzadeh-Amiri, R. E. Verrall, R. P. Steer, Chemical
Physics 1984, 87, 295-303
The S2 -> S0 fluorescence quantum yields and S2 lifetimes of thiones I, II (
x = O, S, CMe2), III (R = H, X = O; R = Me, X = S), IV and (4MeOC6H4)2CS in inert perfluoroalkane solns. at room temp. have been
measured using picosecond laser techniques. Photostable, structurally rigid
thiones undergo S2 -> S1 internal conversion at rates consistent with the
energy-gap law of radiationless transitions. An av. electronic coupling
matrix element of 1.9 * 102 cm-1 is found. [on SciFinder (R)] asdfgf
[52] Bimolecular quenching of the second excited singlet state of
tetramethylindanthione
A. Maciejewski, R. P. Steer, Journal of Photochemistry 1984, 24, 303-307
Rate consts. for the title quenching of 2,2,3,3-tetramethylindanthione by O2,
MeCN, EtOH, and cyclohexane were detd. [on SciFinder (R)] asdfgf
[53] Vibronic analyses of the Rydberg and lower intravalence
electronic transitions in thioacetone
S. Paone, D. C. Moule, A. E. Bruno, R. P. Steer, Journal of Molecular
Spectroscopy 1984, 107, 1-11
The absorption spectrum of thioacetone, was measured at 190-700 nm, and 5
distinct absorptions were identified. These are the spin-allowed and spinforbidden n -> p*, ~A(1A2) -> ~X(1A1), and ~a(3A2) -> ~X(1A1)
transitions; the orbitally allowed p -> p*, ~B(1A1) -> ~X(1A1) transition;
and the Rydberg transitions n -> 4s, ~C(1B2) -> ~X(1A1), n -> 4pz,
~D(1B2) -> ~X(1A1), and n -> 4py, ~E(1A1) -> ~X(1A1). In addn., the
laser phosphorescence excitation spectrum was obtained for the ~a -> ~X
transition. Partial vibrational assignments were obtained for all of the excited
electronic states. The analyses showed that the ~a, ~A, and ~C states are all
planar or pseudoplanar. Of interest is the vibrational activity of the Me
torsional modes. The presence of torsional progressions in the visible
absorption spectrum led to the conclusion that the Me groups are rotated in
the ~a state by 60 Deg from the ~X state. Observation of torsional sequence
transitions rather than progressions in the ~C -> ~X spectrum indicates that
the Me groups do not change their configuration on excitation into the ~C
state. [on SciFinder (R)] asdfgf
[54] Simple, inexpensive autocorrelator for use with cw mode-locked
lasers
D. R. M. Demmer, A. Leyland, M. Szymanski, R. P. Steer, Review of
Scientific Instruments 1985, 56, 331-332
An improved method of obtaining variable pulse spacing for use in a
continuous-wave (CW) 2nd harmonic generation autocorrelation
interferometer is described. [on SciFinder (R)] asdfgf
[55] Fluorescence lifetime quenching and anisotropy studies of
ribonuclease T1
D. R. James, D. R. Demmer, R. P. Steer, R. E. Verrall, Biochemistry 1985,
24, 5517-5526
The time-resolved fluorescence of the lone tryptophanyl residue of RNase
T1 was investigated by using a mode-locked, frequency-double picosecond
dye laser. The fluorescence decay could be characterized by a single
exponential function with a lifetime of 3.9 ns. The fluorescence was readily
quenched by uncharged solutes but was unaffected by iodide. These
observations were interpreted in terms of the electrostatic properties of the
amino acid residues at the active site of the protein, which would appear to
restrict the access of solute species to the tryptophanyl residue. The temp.
dependence of the fluorescence lifetime and anisotropy decay time could be
rationalized in terms of a model which postulates a significant ordering of
the solvent layer immediately surrounding the surface of the protein. [on
SciFinder (R)] asdfgf
[56] Relaxation of the second excited singlet states of aromatic thiones:
the role of specific solute-solvent interactions
A. Maciejewski, D. R. Demmer, D. R. James, A. Safarzadeh-Amiri, R. E.
Verrall, R. P. Steer, Journal of the American Chemical Society 1985, 107,
2831-2837
Second excited singlet state (S2) fluorescence quantum yields and lifetimes
of 6 arom. thiones were measured in soln. at room temp. Intramol. S2 -> S1
internal conversion dominates S2 decay in inert perfluoroalkane solvents,
but intermol. photochem. and photophys. processes dominate in more
strongly interacting solvents. Stern-Volmer quenching is obsd. when
perfluoroalkane solns. of 2,2,3,3-tetramethylindanthione, a model thione, are
spiked with additives at concns. <.apprx.0.3 M. Measurements of the rate
consts. for the bimol. quenching of S2 fluorescence by 26 different additives
reveal that the thione S2 state is highly reactive, and that the initial
intermol.-interaction path divides into phys. and chem. branches, the
branching ratio being a function of the nature of the quencher. [on SciFinder
(R)] asdfgf
[57] Vibronic analysis of the ~A(1A2) -> ~X(1A1) spectrum of
carbonothioic dibromide (Br2CS)
B. Simard, R. P. Steer, Canadian Journal of Chemistry 1985, 63, 1418-1423
~A(1A2)->~X(1A1) Fluorescence excitation and emission spectra of Br2CS
were recorded. A vibronic anal. shows that the mol. is nonplanar in the ~A
state with an equil. out-of-plane angle of 13 Deg and a barrier to inversion of
164 cm-1. Upper state vibrational frequencies of modes 1, 2, 3, 4, and 6
were obtained. Comparisons with other tetraat. thiocarbonyls support the
assignments. A ground state C-Br dissocn. energy of 52.3 kcal-mol-1 was
obtained from the onset of diffuseness in the ~A-~X spectra. A search for
phosphorescence proved unsuccessful. [on SciFinder (R)] asdfgf
[58] Spectral and photophysical properties of 9,10-diphenylanthracene
in perfluoro-n-hexane: the influence of solute-solvent interactions
A. Maciejewski, R. P. Steer, Journal of Photochemistry 1986, 35, 59-69
The photophys. properties of 9,10-diphenylanthracene (DPA) were unusual
in perfluoro-n-hexane (PFH) compared with other solvents. Low values of
the fluorescence quantum yield, fF = 0.58, and S1 lifetime, tS1 = 6.7 ns, and
relatively large shifts in the absorption and emission spectra in PFH are
attributable to changes in the electronic energies of DPA which permit an
enhancement of the rate of S1 -> T2 relaxation in this solvent. A large value
of the intersystem crossing quantum yield in PFH, fISC = 0.35, supports this
conclusion. [on SciFinder (R)] asdfgf
[59] Thermally activated delayed S1 fluorescence of aromatic thiones
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of Physical Chemistry
1986, 90, 6314-6318
Absorption emission, and emission excitation spectra, emission lifetimes,
and quantum yields were measured for 4 arom. thiones, 4H-pyran-4-thione,
4H-1-benzopyran-4-thione, xanthione (XT), and 2,2,3,3tetramethylindanethione, in several solvents. Emission to the blue of T1
phosphorescence was identified as thermally activated delayed fluorescence
and its temp. dependence in a perfluoroalkane and a hydrocarbon solvent
investigated. The effect of quenching by O2 was studied. For XT in 3methylpentane at 293 K the quantum yield of delayed fluorescence is estd.
as 1.1 * 10-3, and the rate const. of T1 -> S1 intersystem crossing is 5.6 *
109 s-1. [on SciFinder (R)] asdfgf
[60] Time-resolved LIF detection of silylene in the IR MPD of
ethylsilane
D. M. Rayner, R. P. Steer, P. A. Hackett, C. L. Wilson, P. John, NBS Special
Publication (United States) 1986, 716, 577-588
Formation of SiH2 in the collisionless multiphoton IR dissocn. of
CH3CH2SiH3 resulted in secondary photolysis of the primary photolysis
product CH3CH2SiH. The mechanism involved reactions (1) CH3CH2SiH3
+ nhn -> CH3CH2SiH + H2 and (2) CH3CH2SiH + nhn -> C2H4 + SiH2
and was consistent with known shock-induced dissocn. of CH3CH2SiH3.
[on SciFinder (R)] asdfgf
[61] Time-resolved LIF detection of silylene in the IR MPD of
ethylsilane
D. M. Rayner, R. P. Steer, P. A. Hackett, C. L. Wilson, P. John, Chemical
Physics Letters 1986, 123, 449-452
Silylene was obsd. in the collisionless IR MPD (multiphoton dissocn.) of
ethylsilane via LIF (laser-induced fluorescence). A mechanism for the
unimol. dissocn. of ethylsilane which is consistent with known shockinduced dissocn. and IR MPD data is advanced. [on SciFinder (R)] asdfgf
[62] Potential energy surfaces for the molecular and free radical
dissociations of thioformaldehyde, thiocarbonyl difluoride, and
thiocarbonye dichloride: an ab initio SCF MO study
B. Simard, A. E. Bruno, P. G. Mezey, R. P. Steer, Chemical Physics 1986,
103, 75-83
Energies along the planar sym. (C2v) and planar asym. (Cs) paths to mol.
dissocn. of the ground-state thiocarbonyl halides F2CS and Cl2CS, together
with their transition-state geometries, were calcd. by ab-initio SCF-MO
methods using the STO-3G and 4-31G basis sets. For comparison, results on
H2CS at similar levels of calcn. were also existing. In addn., the 4-31G**
basis set was employed to predict the geometries of the ground-state species
and the endothermicities of their free-radical dissocns. The results of
existing expts., in which the lowest excited singlet states of these mols. were
photoexcited, were interpreted in light of these calcns. Thermodn. data for
both mol. and free-radical dissocns. were evaluated. [on SciFinder (R)]
asdfgf
[63] Unusual deuterium isotope effect on the radiationless decay of
xanthione S2
S. R. Abrams, M. Green, R. P. Steer, M. Szymanski, Chemical Physics
Letters 1987, 139, 182-186
S2 fluorescence lifetimes and quantum yields were measured for xanthione
and its d4, d6 and d8 substituted analogs in perfluoroalkane and hydrocarbon
solvents. A large D/H isotope effect on the nonradiative S2 decay rate was
found and was identified with deuteration in the 1 and 8 positions. The
results are ascribed to the existence of parallel S2 ->S1 and S2 ->S0
processes, the latter resulting from motion of the C(1)-H or C(8)-H atoms
towards the thiocarbonyl group, consistent with the early stages of a path
leading the intramol. H transfer. The effects are correlated with known S2
photochem. of similar mols. [on SciFinder (R)] asdfgf
[64] Three exponential fluorescence decay of horse liver alcohol
dehydrogenase revealed by iodide quenching
D. R. Demmer, D. R. James, R. P. Steer, R. E. Verrall, Photochemistry and
Photobiology 1987, 45, 39-48
The time-resolved fluoroscence decay of horse liver alc. dehydrogenase was
measured using a frequency doubled picosecond dye laser and timecorrelated single-photon counting detection. A flow-cell technique was
described which eliminated the photodegrdn. artifacts which commonly
occur with laser excitation. A procedure was introduced which used
fluorescence quenching to reveal minor fluorescence lifetime components.
The decay of the unquenched tryptophanyl fluorescence could be described
by a double exponential decay law, but expts. conducted in the presence of Ishowed that the fluorescence decay must be more complex than this. A
model is presented in which the fluorescence decay consists of 3 exponential
components, only 2 of which are susceptible to quenching by I-. Several
possibilities were presented for the origin of this minor decay component,
the most reasonable of which was that it arose from conformational
heterogeneity in the solvent-exposed tryptophanyl residue. [on SciFinder
(R)] asdfgf
[65] Thiocarbonyl spectroscopy: methyl torsional vibrations and
internal rotational barriers of thioacetaldehyde in its ~a 3A\" and ~X
1A' states
R. H. Judge, D. C. Moule, A. E. Bruno, R. P. Steer, Journal of Chemical
Physics 1987, 87, 60-67
MeCHS was prepd. by the vacuum flash pyrolysis of 1,3,5-trimethyl-strithiane and was obsd. spectroscopically in a rapidly flowing system. The
614 nm absorption system of MeCHS/MeCDS/CD3CHS/CD3CDS was
recorded photog. at path lengths of 72 m. Phosphorescence excitation and
emission spectra of MeCHS were obsd. with a continuous-wave dye laser.
The assignment of the 1st absorption system was made to the spin forbidden
transition ~a 3A\"->~X 1A'. The bands obsd. in the region of the origin had
a complex structure, and were assigned to torsional transitions. The
observation of several hot band progressions which could be fitted to levels
derived from microwave data gave the parameters (in cm-1) V3\" = 534.3,
F\" = 6.9950, and n0 = 16293.8 for MeCHS. The fit to the excited state
torsional levels was less satisfactory and gave V3' = 94.18, F' = 5.3608 (all
in cm-1). An anal. of the data for the other isotopic species showed that the
torsional motion in the upper state was coupled to a large amplitude wagging
of the aldehydic H. The obsd. phase shift in the torsional angle on electronic
excitation was attributed to the effects of hyperconjugation between the Me
H's and the p (CS) orbitals. [on SciFinder (R)] asdfgf
[66] ~A-~X laser excitation spectroscopy of bromochlorothiocarbonyl
and dibromothiocarbonyl at room temperature and in cold supersonic
jets
B. Simard, P. A. Hackett, R. P. Steer, Journal of Molecular Spectroscopy
1987, 126, 307-328
~A-~X room-temp. and jet-cooled fluorescence excitation spectra of BrClCS
and Br2CS were recorded. Vibrational analyses place the electronic origins
of BrClCS and Br2CS at 18,363 and 17,992 cm-1, resp. In both mols., all 6
~A state vibrational frequencies were obtained. Some ground state
vibrational frequencies of BrClCS were reevaluated. The excited mols.
exhibit nonplanar structures in their ~A states. By fitting quadratic-Gaussian
and quartic-quadratic double-min. potentials to the obsd. levels of the ~A
state out-of-plane manifolds, the equil. out-of-plane angles and barrier
heights opposing mol. inversion were estd. as 26 +- 1 Deg and 538 +- 5 cm1 for BrClCS and 19 +- 1 Deg and 465 +- 30 cm-1 for Br2CS. Comparisons
with other tetraat. thiocarbonyls support the given assignments. [on
SciFinder (R)] asdfgf
[67] The phosphorescence of 4H-pyran-4-thione: large quantum yields
from room-temperature fluid solutions
M. Szymanski, R. P. Steer, A. Maciejewski, Chemical Physics Letters 1987,
135, 243-248
The phosphorescence spectra, phosphorescence quantum yields and triplet
lifetimes of 4H-pyran-4-thione (PT) in fluid soln. at room temp. were
measured. In inert perfluoroalkane solvents at 293 K, the phosphorescence
quantum yield of PT is 0.33 on excitation to S2 and 0.47 on direct excitation
to T1. The reasons for these extremely large radiative yields are discussed.
[on SciFinder (R)] asdfgf
[68] Rydberg states in adamantanethione, thiofenchone, and
thiocamphor
K. J. Falk, R. P. Steer, Canadian Journal of Chemistry 1988, 66, 575-577
The absorption spectra of adamantanethione (I), thiofenchone (II), and
thiocamphor (III) were measured in the quartz UV region in both the vapor
phase and in perfluoroalkane solns. Rydberg as well as p -> p* transitions
are identified in the gas phase spectra, and are discussed in terms of the
known photochem. of these compds. [on SciFinder (R)] asdfgf
[69] Photochemistry and photophysics of thione triplets in fluid
solution
A. Maciejewski, M. Szymanski, R. P. Steer, Journal of Physical Chemistry
1988, 92, 6939-6944
The photochem. and photophys. behavior was studied of the lowest triplet
states of 9 arom. thiones. The phosphorescence quantum yields, and the
triplet lifetimes were measured in fluid soln. at room temp. as a function of
the nature of the solvent, excitation wavelength (encompassing the S0 -> T1,
S0 -> S1, and S0 -> S2 absorption systems), and thione concn. Linear SternVolmer plots over at least 3 decades of thione concn. were obsd. The
variations of the obsd. nonradiative decay consts. (obtained from quantum
yield and lifetime data at infinite diln.) with thione structure, triplet energy,
and solvent were analyzed. Three thiones, 4H-pyran-4-thione, 4H-1benzopyran-4-thione, and xanthione, were subjected to photochem. study in
solns. sufficiently dil. to permit anal. of the rates of their intramol. and
excited solute-solvent reactions in the absence of excited solute-solute
interactions. Intermol. self-quenching interactions dominated thione triplet
decay in room temp. solns. even at thione concns. as low as 1 * 10-6 M.
Triplet thione-solvent interactions were responsible for almost all of the
photochem. degrdn. of the thione at infinite diln., and for substantial
decreases in lifetime in all solvents except inert perfluoroalkanes. [on
SciFinder (R)] asdfgf
[70] T1-T2 inversion in aromatic thiones
A. Maciejewski, M. Szymanski, R. P. Steer, Chemical Physics Letters 1988,
143, 559-564
The emission spectra and phosphorescence lifetimes and quantum yields of
xanthione (XT), 4H-pyran-4-thione (PT) and 4H-1-benzopyran-4-thione
(BPT) were measured in 5 solvents of varying polarities at room temp. A
dramatic change in the T1 -> S0 phosphorescence spectrum is obsd., and this
is interpreted in terms of inversion of the 3(n,p*) and 3(p,p*) states with
increasing solvent polarity. Only modest variations in the radiative and nonradiative decay consts. for the triplet are obsd., and no evidence of biexponential decay is found. Strong coupling between T1-T2 and between the
singlet and triplet manifolds is indicated. Comparisons are drawn with
ketones of similar structure. [on SciFinder (R)] asdfgf
[71] Vibrational analysis of the low resolution ~a -> ~X absorption
spectra of carbonothioic bromide chloride and carbonothioic dibromide
B. Simard, R. P. Steer, R. H. Judge, D. C. Moule, Canadian Journal of
Chemistry 1988, 66, 359-366
The .hivin.a -> .hivin.X absorption spectra of BrClCS and Br2CS were
photographed under low resoln. at room temp. The electronic origins of
BrClCS and Br2CS were placed at 17,116 and 16,859 cm-1, resp. Vibronic
analyses show that the mols. are non-planar in their .hivin.a states. By fitting
quadratic-Gaussian and quadratic-quartic double-min. potentials to the obsd.
vibrational levels of the out-of-plane manifolds, the equil. out-of-plane
angles and the barrier heights to mol. inversion were estd. to be 25 +- 1 Deg
and 541 +- 10 cm-1 for BrClCS, and 17.5 +- 1 Deg and 524 +- 10 cm-1 for
Br2CS. In the case of BrClCS, all 6 .hivin.a state vibrational frequencies
were obtained. In the case of Br2CS, only modes 1 (C-S stretch), 2 (sym. CBr stretch), 3 (in-plane Br-C-Br scissor), and 4 (out-of-plane bend) are active
in the spectrum. Comparisons with other tetraat. thiocarbonyls support the
given assignments. [on SciFinder (R)] asdfgf
[72] Photophysical properties of fluorescent N-purin-6-ylpyridinium
chloride
B. Skalski, R. P. Steer, R. E. Verrall, Journal of the American Chemical
Society 1988, 110, 2055-2061
The absorption and fluorescence spectra and the excited-state lifetimes of Npurin-6-ylpyridinium chloride were studied in MeCN, MeOH, and EtOH,
and, as a function of pH, in H2O. The ground state exhibits a prototropic
equil. between the cation and zwitterion with pKa = 6.7 in H2O. At pH 3-6
in H2O, excitation of the ground-state cation results in proton transfer to
give only the excited zwitterion, on a time scale that is short compared to the
lifetime of the excited species. A large Stokes shift in the emission results.
The fluorescence quantum yield and lifetime of the excited zwitterion are
0.08 and 1.6 ns, resp., in H2O and remain const. over the entire pH 3-9
range. Diabatic proton transfer to the excited zwitterion at pH <3 results in
quenching of the emission with a rate const. of 2 * 109 M-1 s-1. Proton
quenching does not dominate the excited zwitterion decay, however, so that
the apparent pKa* values of 0.4 +- 0.3 from the Forster cycle and 0.6 +- 0.1
from fluorescence titrn. give good approxns. to the true excited state pKa*.
The effects of solvent and temp. were also examd. The potential application
of pyridinium salts of purines as fluorescence probes for biomols. is
discussed. [on SciFinder (R)] asdfgf
[73] Photophysics and intramolecular photochemistry of thiones in
solution
R. P. Steer, V. Ramamurthy, Accounts of Chemical Research 1988, 21, 380386
A review with 73 refs. Covered topics include: electronic transitions,
excited-state structures, dynamics and mechanisms of excited-state decay,
and photochem. [on SciFinder (R)] asdfgf
[74] Photophysics of thione triplets in solution: factors controlling the
rates of radiationless decay
M. Szymanski, A. Maciejewski, R. P. Steer, Chemical Physics 1988, 124,
143-154
Several spectroscopic, photochem., and/or photophys. properties of the
lowest triplet states of 11 mols. contg. the CS group were measured in 3methylpentane and perfluoro-1,3-dimethylcyclohexane solvents at room
temp. Values of the phosphorescence lifetimes, phosphorescence quantum
yields, and quantum yields of net photochem. consumption of thione at
infinite diln. were detd. A quant. anal. of the intramol. relaxation processes
of the triplet thiones in the absence of complicating intermol. selfquenching, and solvent-excited solute interactions is possible, using data
obtained with inert perfluoroalkane solns. Photochem. processes contribute
negligibly to the rates of triplet decay. With 1 exception, the radiative rate
consts. fall within .apprx.(3-11) * 103 s-1. These are factor of 105 larger
than those of the triplet hydrocarbons, and are indicative of strong mixing of
the triplet and singlet manifolds. The nonradiative consts. of the rigid thiones
are consonant with the energy gap law and are governed by the FranckCondon factors for the T1 -> S0 nonradiative transitions. A normal D
isotope effect is obsd. in xanthione. [on SciFinder (R)] asdfgf
[75] Radiationless decay of aromatic thiones in solution selectively
excited to their S3, S2, S1, and T1 states
M. Szymanski, A. Maciejewski, R. P. Steer, Journal of Physical Chemistry
1988, 92, 2485-2489
Seven arom. thiones, e.g., xanthione, (4-MeOC6H4)2CS, having a variety of
electronic energy gaps and structures were subjected to spectroscopic and
photophys. examn. Accurate measurements of the relative T1 -> S0
phosphorescence quantum yields obtained following direct excitation into
the S3, S2, S1, and T1 states were made in perfluoroalkane and hydrocarbon
solvents. Together with S2 lifetime and fluorescence quantum yield
measurements, these data were used to obtain quant. information about the
excited-state relaxation paths. S3 -> S2 internal conversion is quant.
Intermol. processes dominate the decay of S2 in 3-methylpentane and other
common solvents, whereas intramol. processes are revealed in inert
perfluoroalkanes. The majority of S2 species decay via S1 to T1. However,
the thermally activated T1 -> S1 -> S0 decay process accounts for a
significant fraction of decay events at room temp. In addn., mols. having bH atoms close to the thiocarbonyl group can undergo net decay from S2 ->
S0, bypassing S1 and T1. [on SciFinder (R)] asdfgf
[76] Decay dynamics of the lowest triplet and lowest excited singlet
states of thioacetaldehyde and thioacetone
A. E. Bruno, D. C. Moule, R. P. Steer, Journal of Photochemistry and
Photobiology, A: Chemistry 1989, 46, 169-180
The lifetimes of the emission obsd. when thioacetaldehyde and
thioacetoneare excited in their ~a-~X and ~A-~X absorption systems were
measured as a function of pressure. The lifetimes of the ~a states under
collision-free conditions are in the 10 ms region, slightly varying with
vibrational energy content and, in the case of thioacetone, on deuteriation. A
collision-induced intersystem crossing was identified as a likely mechanism
of radiation-free decay of the ~a states, and was also implicated in the decay
of the ~A states in both mols. [on SciFinder (R)] asdfgf
[77] Dynamics of the second excited singlet state of thiophosgene in
solution
K. J. Falk, S. E. Sveinson, R. P. Steer, Chemical Physics Letters 1989, 163,
490-494
The dynamic behavior of the 2nd excited singlet state of Cl2CS was
investigated in liq. solns. In perfluoroalkane solvents strong ~B-~X
fluorescence (ff0 ?0.077) was obsd. when the mol. is excited to bound
vibrational states. ~B lifetimes of ?2.8 ns were obtained. Dissocn., either
directly (in competition with vibrational relaxation) at high vibrational
energies or indirectly by solvent activation when low vibrational levels are
initially populated, apparently is the main radiationless decay path.
Fluorescence is weaker in other solvents. A rate const. of 9.5 * 109 M-1 s-1
was measured for quenching by n-C6H14. [on SciFinder (R)] asdfgf
[78] Photophysics and intramolecular photochemistry of
adamantanethione, thiocamphor, and thiofenchone excited to their
second excited singlet states: evidence for subpicosecond photoprocesses
K. J. Falk, R. P. Steer, Journal of the American Chemical Society 1989, 111,
6518-6524
The absorption, emission, and excitation spectra of adamantanethione (I),
perdeuterioadamantanethione, thiofenchone (II), and thiocamphor (III) were
measured in a variety of solvents including inert perfluoroalkanes. Weak S2
-> S0 fluorescence was obsd. for the first time in these compds., and its
quantum yields were measured. Quantum yields of phosphorescence
resulting from S2 -> S0 excitation and the quantum yields of net thione
consumption in dil. solns. where photodimerization is unimportant were also
measured. The S2 states have lifetimes <1 ps and therefore, contrary to
previous reports, are not the immediate precursors responsible for the
intermol. photochem. reactions of these compds. Evidence is presented that
an intermediate, X, derived from S2 is involved, and that the most probable
decay path is S2 -> X ->T2 .dblarw. T1 -> S0. Rydberg states lie higher in
energy than S2 and are not involved. In II and III, intramol. photochem.
reaction to give the previously obsd. homothioenol product occurs via X. [on
SciFinder (R)] asdfgf
[79] Photophysics of the second excited singlet states of cyclic
enethiones
V. P. Rao, R. P. Steer, Journal of Photochemistry and Photobiology, A:
Chemistry 1989, 47, 277-286
The spectroscopic and photophys. parameters of the 2nd excited singlet
states of several enethiones were measured in inert perfluoroalkane solvents.
Two rigid, related enethiones, 1,1-dimethyl-2-thioxo-1,2dihydronaphthalene and thiocoumarin, behave in accordance with the energy
gap law and decay predominately via S2 -> S1 -> T1 radiationless transition,
1,1,3-trimethyl-2-thioxo-1,2-dihydronaphthalene, with a Me group at an sp2
carbon a to the thiocarbonyl moiety, exhibits a much faster non-radiative
decay rate. Thioisophorone and 4H-pyran-4-thione are structurally flexible
and decay non-radiatively from S2 at rates much faster than the rigid
bicyclic enethione, thioverbenone. The results are rationalized in terms of
the theory of radiationless transitions. [on SciFinder (R)] asdfgf