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Photophysics of thione triplets in solution: Factors controlling the rates of radiationless decay
Chemical Physics 124 (1988) 143-154 North-Holland, Amsterdam
PHOTOPHYSICS OF THIONE TRIPLETS IN SOLUTION: FACTORS CONTROLLING THE RATES OF RADIATIONLESS
DECAY
M. SZYMAfiSIU ‘, A. MACIEJEWSKI ’ and R.P. STEER 3 Department of Chemistry University ofSaskatchewan. Sashztoon, Saskatchewan, Canada S7N 0 WO Received 26 February 1988
Several spectroscopic, photochemical, and/or photophysical properties of the lowest triplet states of eleven molecules containing the thiocarbonyl group have been measured in 3-methylpentane and perlluoro-1,3-dimethylcyclohexane solvents at room temperature. Values of the phosphorescence lifetimes, phosphorescence quantum yields and quantum yields of net photochemical consumption of thione at infinite dilution have been determined. A quantitative analysis of the intramolecular relaxation processes of the triplet thiones in the absence of complicating intermolecular self-quenching and solvent-excited solute interactions is possible, using data obtained with inert pertluoroalkane solutions. Photochemical processes contribute negligibly to the rates of triplet decay. With one exception, the radiative rate constants fall within a narrow range between about (3-l 1) x lo3 s-‘. These are about a factor of 1O5larger than those of the triplet hydrocarbons, and are indicative of strong mixing of the triplet and singlet manifolds. The non-radiative rate constants of the rigid thiones are consonant with the energy gap law and are governed by the Franck-Condon factors for the T, w.5, non-radiative transitions. A normal deuterium isotope effect is observed in xanthione.
1. Introduction
Interest in the properties of thiones in their lowest triplet ( T1 ) states has grown considerably in the last decade [ l-3 1. Of particular note are the exceptionally large zero-field splitting parameters of xanthione (XT), benzopyranthione (BPT), and 4H-pyran-4thione (PT) triplets [ 4,5 1, the huge quantum yields of phosphorescence of PT in fluid solutions at room temperature [ 61, the large oscillator strengths of thione S,-+T, transitions [ 7,8] (which permit direct, selective excitation to T, ), their relatively short, concentration-dependent lifetimes [ 9- 111 (which facilitate studies of their intermolecular interactions), and the occurrence of delayed fluorescence from S, by thermal activation from the triplet manifold [ 121. The spectroscopic properties of thione triplets have been examined in some detail. There is now little ’ On leave from Institute of Physics, A. Mickiewicz University, Poznab, Poland. 2 Faculty of Chemistry, A. Mickiewicz University, Poznatl, Poland. 3 To whom correspondence should be addressed.
doubt that most thiones possess S, and T, states which are of n, 7c*electronic configuration in both low temperature matrices and non-polar solvents at room temperature [ 8,131. The x, n* triplet companion of the ’ (?r, A*) Sz state is of only slightly higher energy than T1 in some thiones and the small T2-T1 gap is thought to be largely responsible for the existence of large values of the T1 zero-field splitting parameter, )P I , in thiones such as XT and PT [ 4,5 1. There is also some evidence of T,-T2 reordering as the polarity of the solvent is increased [ 14 1. Nevertheless, for the present study which is primarily concerned with the photophysical properties of thiones in inert, nonpolar solvents, the T, state is clearly of n, n* configuration and 3A2character, assuming Czv local symmetry about the thiocarbonyl group. in these molecules. The decay dynamics of thione triplets have previously been studied by measuring their phosphorescence lifetimes and quantum yields [ 7,9,15,16], by monitoring their triplet-triplet absorption intensities in laser flash photolysis [ 10, 11,17 1, by observing the yields of singlet molecular oxygen in T,-oxygen quenching experiments [ 18-201, and by determining the nature, yields, and rates of formation of the
0301-0104/88/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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M. Szymariski et al. /Photophysics of thione triplets in solution
products of intermolecular and intramolecular photochemical reactions [ 2,3,2 11. Thermally activated delayed fluorescence from S I has also been observed in several thiones [ 12 1, and this indirect route of T, relaxation has provided valuable kinetic information concerning the decay processes involved. In fluid solutions bimolecular self-quenching of the triplet states of sterically unhindered [ 10,201 thiones occurs at near diffusion controlled rates [ 19,22,23]. The rates of decay of thione triplets are therefore dominated by self-quenching processes when the concentration of thiones are greater than about 10e4 M. If the firstorder decay parameters characteristic of thione triplets in fluid solution are desired, then it is necessary to make measurements at very low thione concentrations and to extrapolate the results to infinite dilution [9]. Almost all of the previous work on thione triplets has been carried out using a variety of solvents, most of which would commonly be considered to be inert. We have shown, however, that the second excited singlet states of a number of thiones exhibit electronic relaxation rates which are highly solvent dependent [ 24,25 1. Of the solvents examined to date, only the perfluoroalkanes appear to act as classical heat baths [ 261, thereby permitting the intramolecular decay parameters of the S, thiones to be observed. Similar effects on the rates of decay of ketone triplets in fluid solution were observed by Parker and Joyce [ 271 some twenty years ago. Replacing even a photochemically unreactive solvent such as benzene or a chlorofluorocarbon by a perfluoroalkane was observed to increase the T, lifetime and phosphorescence quantum yield of benzophenone by at least an order of magnitude. It is understood from many photochemical and photophysical studies that thione triplets are much less reactive than their Sz counterparts. Nevertheless, the lifetimes of the T1 states are three to five orders of magnitude longer than those of Sz, and it therfore appeared to be important to examine the decay dynamics of triplet thiones in a variety of solvents, including the pertluoroalkanes. If, as with the Sz states, solvent perturbation of the triplet decay could be minimized in the perfluoroalkanes, then an examination of the parameters governing intramolecular T, decay might be undertaken. This paper presents the results of such a study.
2. Experimental The thiones used in the present work (fig. 1) were synthesized by sulfuration of the corresponding ketones using established techniques [ 28-30 1. Xanthione (XT) was selectively deuterated as previously described [ 3 11. Three compounds of good isotopic and chemical purity were recovered, XT-d, containing > 99% D, XT-d, containing > 93OhD at positions 1, 3, 4, 5, 6 and 8 and XT-d, containing ~93% D at positions 1, 4, 5 and 8, contaminated with 6Ohand 12% D at the “unlabelled” 2, 7 and 3, 6 positions respectively. Deuterium substitution patterns and contents were determined by high-resolution NMR and GC MS as previously described [ 3 11. The thiones were purified by recrystallization and silica gel column chromatography and were analyzed for impurities by gas chromatography. Hydrocarbon solvents were purified by standard methods, and perfluoroalkane solvents (PCR Research Chemicals) were purified by distillation followed by column chromatography. Solvent impurities contributed negligible emission when the neat liquid was excited at wavelengths from the near UV through the visible. All experiments were performed on samples which had been rigorously deoxygenated, because O2 quenches and/or reacts with thione triplets at near diffusion controlled rates and the solubility of oxygen in pertluoroalkane solvents is unusually high. This was accomplished either by 7 to 9 freeze-pump-thaw cycles to a residual pressure of < 10e4 Torr at 77 K or by air displacement using prolonged bubbling with He which had itself been desoxygenated by passage through a hot copper column. Absorption measurements were made with either a Cary 118C or a Specord M-40 (Carl Zeiss, Jena) spectrophotometer. Steady state emission spectra and quantum yields were measured using either a Spex Fluorolog 222 or a modified Perkin-Elmer MPF-3 spectrofluorometer, as previously described [ 32,33 1. Phosphorescence lifetimes were measured using a Spectra Physics argon ion/dye laser combination which was either cavity-dumped to provide nanosecond excitation pulses in the visible or mode-locked, synchronously pumped, cavity-dumped and frequency-doubled to provide picosecond excitation pulses in the UV. Time-correlated single-photon counting was used, as previously described [ 341, to
145
A4. Szymariski et al. /Photophysics of thione triplets in solution
TMIT
PT
TXT
BPT
XT
DM BTPT
DMTEP
Fig. 1. Structures and abbreviations for the thiones studied in the present work. TMIT, 2,2,3,3-tetramethylindanethione; PT, 4H-pyran4-thione; BPT, 4H-1-benzopyran-4-thione; XT, xanthione; TXT, tbioxantbione; DMBTPT, 2,6dimetbyl_4H-l-benzotbiopyran-4-thione; DMTBP, p,p’-dimethoxythiobenzophenone.
accumulate phosphorescence decay profiles. Cavitydumping and photon-counting rates were selected to ensure that pulse pile-up did not distort the decays. Deconvolution of the observed microsecond decays from the subnanosecond instrument response function was unnecessary. The rate of photochemical consumption of thione was monitored routinely by UV absorption spectrophotometry and the net percentage decomposition was kept negligibly small for quantum yield and lifetime all emission measurements. The quantum yields of net photochemical consumption of thione were measured by following the decrease in absorbance of the thione in its strong (t> lo4 M-’ cm-‘) So+& absorption band, and sometimes by determining the concentration of thione using gas chromatography. The absorbed intensity of the exciting radiation was measured by Reinecke salt actinometry, taking account of the thermal elimination of thiocyanate [ 35 1.
absorption and is sufficiently well resolved from the singlet system to permit clean, direct excitation to the triplet state. The long wavelength emission at room temperature consists mainly of T, + So phosphorescence, but is accompanied by a small amount ( < ~O%I of the total) of thermally activated delayed SI+SO fluorescence [ 121. The latter appears as a weak, poorly resolved, temperature-sensitive feature to the blue of the highest energy strong band in the phosphorescence spectrum. The phosphorescence spectra of most rigid aromatic thiones exhibit a number of strong, broad features separated by = 1100-1200 cm-‘, likely assignable either to single quantum changes of the
BPT
in
PF-1.3-DMCH
I t 5
v-so
- IO ‘5
I ii
E 0 c
II ::
c
D
3 3. Results
-5
:: z
The absorption and emission spectra of the thiones examined in the present work have either been previously reported [6,9,12,14,32] or are similar to them. The spectra of BPT in perfluoro- 1,3-dimethylcyclohexane (PF- 1,3-DMCH ) are representative and are given in fig. 2. One should note that, as with previous examples, the relatively strong S,+T, absorption lies to the red of the corresponding !%-+S,
'\ '\ '. 400
500 WAVELENGTH
600
700
if z -
800
(nm)--
Fig. 2. Absorption (--) and emission (---) spectra of BPT in pertluoro-1,3dimethylcyclohexane at 295 K. Absorption spectrum in the 450-650 nm ranges taken in 3-MP. Note large absorbance scale change at 450 nm.
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M. Szymariski et al. /Photophysics of thione triplets in solution
ground state C=S stretching frequency or, more likely, to a congestion of ring vibrational modes [ 361 having frequencies of = 1030- 1340 cm- ‘. The progressions exhibit strong origin bands, and decline in intensity rapidly with increasing wavelength. Nevertheless, the phosphorescence spectra of some thiones (e.g. thioxanthione, TXT) extend substantially to the red of 800 nm where photomultiplier tubes exhibit rapidly declining sensitivities. The accurate determination of the phosphorescence quantum yields of such compounds is therefore more difficult because a modest correction factor accounting for the fraction of the emission at il> 800 nm must be applied. Both the absorption and emission spectra of thiones are better resolved in pertluoroalkane solvents than in alkanes such as 3-methylpentane (3-MP). A similar effect has been noted previously for the absorption and Sz-& emission spectra of these same thiones [24,25]. The inertness of the pertluoroalkanes is clearly revealed by the distinct narrowing of the origin band in the phosphorescence spectrum in these solvents compared with 3-MP at the same temperature. Very small shifts in the frequencies of the band maxima ( x 30 cm- ’ ) on substituting perlluoroalkanes for 3-MP are also observed in these spectra but are insignificant in the context of the present work. The spectroscopic properties of the triplet states of eight thiones in both 3-MP and perfluoro- 1,3-dimethylcyclohexane are summarized in table 1. In this table flaabs and IT,, are the frequencies of the peaks of the origin bands in absorption and emission, e is the decadic molar extinction coefficient of the origin band in the absorption spectrum (if resolved), and Avz&’ and Ar~kc are the widths of the origin bands in absorption (at 3/4 height due to resolution difficulties) and in emission (at l/2 height). The key photophysical data for this study were obtained by measuring the lifetimes of the phosphorescing triplet state, r-r,, and the quantum yields of phosphorescence, I&, as a function of thione concentration and excitation wavelength in both 3-MP and PF- 1,3-DMCH. The phosphorescence decays were all well described by single exponential functions (xf approaching unity and random distributions of residuals). The resulting lifetimes were independent of exciting wavelength (excitation to S2, S, and Ti ) for a given thione concentration in a given solvent at a given temperature. (Note, however, that subnanose-
cond !&-So fluorescence [ 25,371 could also be observed when exciting to S2, but this did not contaminate the phosphorescence significantly.) Typical examples of phosphorescence decay curves have been previously published [ 9,16 1. The lifetime and quantum yield data were plotted in Stern-Volmer form, yielding good linear relationships for l/r-r, versus thione concentration. An example is given in fig. 3. Values of r$, were determined from the intercepts of these plots at infinite dilution. The quantum yields of phosphorescence, q&, were measured at the same concentrations as those used to measure rT, . The @gvalues were then obtained using the scaling relationship @g=&,r$,/rT,. The data are presented in tables 2 and 3. The errors in z?, quoted in these tables are f 3 standard deviations on the intercepts of the Stem-Volmer plots. It is immediately apparent that the values of 7$ are more precise than those of @Xsince the latter contain the cumulative errors in 7l, and rT5,, , so that the uncertainties in the radiative and non-radiative rate constants calculated from these quantities are due mostly to uncertainties in @. The slopes of the Stem-Volmer plots, together with the values of 7$, , were used in calculating the bimolecular self-quenching rate constants, kq. The values of kw fall in narrow ranges between 4.5 x 1O9 and 1.8~10’~ M-’ s-’ in PF-1,3-DMCH and between 1.7~10’~and2.6x10’~M-‘s-‘in3-MP. The results of the measurement of the quantum yields of net photochemical consumption, &,, on excitation of XT, BPT and PT in their n-+x* absorptions in both 3-MP and PD- 1,3-DMCH are given in table 4. The thione concentrations for these experiments were chosen so that self-quenching contributed G 10% of the total T1 decay rate. A detailed discussion of these results will not be undertaken here. It is sufficient for our present purpose to note that the values of @,,are generally small in 3-MP and are extremely small ( < 6 x 10F4) in PF-1,3-DMCH.
4. Discussion We begin by describing the conditions under which it should be possible to measure the rates of intramolecular decay of rigid thione triplets in fluid solution at room temperature. Both chemical and physical criteria must be met.
M. Szymariski et al. /Photophysics of thione triplets in solution
147
Table 1 T,-spectral properties of several thiones at room temperature Thione
Solvent
r&(0,0) (cm-‘)
kn(O> 0) (cm-‘)
e(O,O) (M-l cm-‘)
Akz(O, 0) (cm-‘)
A9:A2(0, 0) (cm-‘)
PT
PF- 1,3-DMCH 3-MP
17060 17100
17010 16980
10.0 9.9
90 140
560 - 700
BPT
PF- 1,3-DMCH 3-MP
16030 16030
15930 15900
10.4 10.1
110 145
380 530
TMIT
PF- 1,3-DMCH 3-MP
16180 16180
15860 15810
3.6 4.0
b) b)
590 710
Br-TMIT
PF-1,3-DMCH 3-MP
15870 15870
15560 15490
3.1 3.5
b) b)
631 680
XT-d,,
PF- 1,3-DMCH 3-MP
a) 15130
14970 14940
a)
a) b)
390 510
DMBTPT
PF- 1,3-DMCH 3-MP
b, b)
14880 14860
b) b)
b) b)
b) b)
DMTBP
PF-1,3-DMCH 3-MP
b, b)
14030 13930
b) b)
b) b)
b) b)
TXT
PF-1,3-DMCH 3-MP
b, b)
13560 13610
b) b)
b) b)
b) b)
8.5
a) Not measured due to the low solubitily of XT in PF-1,3-DMCH. b, Not calculated due to strong overlap with the (0,O) band of S,+S,.
From a chemical perspective we wish to observe thione triplet decay under conditions in which the excited molecule undergoes neither intramolecular nor intermolecular photochemical reaction at significant rates. The data of table 4 clearly demonstrate 0.16 _ 0.‘6-
BPT 293
0.06,
, 0
I 2
in PF-1,3-OMCH K
,
I 4
,
I 6
,
[BPT]
, 6
,
I , IO
(x IO+
I , 12
I 14
I , 16
that net photochemical consumption of these thiones excited in the visible in their n-+x* absorption systems is negligible in perfluoroalkane solvents, since 9p< 6 x 10m4. Net photochemical consumption of thione is still small in 3-MP (@p< 5 x 10W2), but the fraction of T1 decay events which proceed by a photochemical mechanism could conceivably be much greater than fir, (and therefore significant in 3-MP) if the initial photochemical event were reversible. For example Law and de Mayo [38] have shown that x 90% of free radical pairs formed by the abstraction of H from protic solvents by excited thiones reform reactants by disproportionation in the solvent cage. Hydrogen abstraction from solvent molecules which are not fully fluorinated is possible, but &, is so small in the perfluoroalkanes employed as to render even this process insignificant. Self-quenching either may lead to distinct photochemical products ( d as high as 1 #I) or may proceed via an almost entirely physical mechanism [ 16,391
Ml
Fig. 3. Stem-Volmer plot of 1/7T, versus BPT concentration in PF-1,3-DMCH at 293 K.
+I’When adamantanethione is excited to S2,the quantum yield of dimer is one when extrapolated to infinite thione concentration.
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M. Szymariski et al. / Photophysics of thione triplets in solution
Table 2 Parameters describing the photophysical decay of the triplet states of several thiones in PF-1,3-DMCH solution at 295 K kJ
@;(T,)a’
Thione PT BPT TMIT Br-TMIT XT-d, XT-d, XT-d, XT-d, DMBTPT DMTBP TXT
43*4 16+2 38?3 21.7kO.3 8.2f0.4 18.5f2.0 21*2 26+3 8.5kO.6 3.04+ 0.06 1.01 f0.02
0.47f0.11 b) 0.13f0.03 0.12f0.02 0.13+0.03 0.064f0.013 0.11f0.02 0.12+0.02 0.14f0.03 0.08 f 0.02 (1.5+0.3)x10-3 (9+2)x lo-’
‘) Errors are t3u. @O,(T,)=&(S,) b, Fromref. [6].
x 10-3)
k.,(
x 10-4)
AWT,-So)
(s-l)
(s-l)
(cm-‘)
11 8.1 3.2 6.0 7.8 5.9 5.7 5.4 9.4 0.49 8.9
1.2 5.4 2.3 4.0 11 4.8 4.2 3.3 11 33 98
17010 15930 15860 15530 14970
q+,(T,)/@,(S,) where both @(S,) and the ratio @iJT,)/@&$)
14880 14030 13560
were experimentally measured.
Table 3 Parameters describing the photophysical decay of PT, BPT, XT-d,, and TXT in 3-MP solution at 295 K Thione PT BPT XT-d, TXT
4; (T,) =) (5.6fl.l)xlO-* (5.4+1.1)x10-* (5.4fl.l)Xlo-* (8*2)x 10-r
0 a) rTI
k, (x lo-‘)
k.,(~lO-~)“’
(CLS)
(s-l)
(s-l)
6.5f0.5 7.3kO.4 7.1kO.4 0.99kO.01
8.6 7.4 7.6 8.1
1.4 1.3 1.3 10.0
‘) Erors are +3. @~(T,)=@~(S2) @,(T,)/@,(Sz) where both @(S,) and the ratio &(T,)/&,(S,) b, Sum for all first-order and pseudo-first-order radiationless decay processes.
0.086 0.42 0.85 0.98
were experimentally measured.
Table 4 Quantum yields of net photochemical consumption of PT, BPT and XT illuminated in their n-x* absorptions in PF-1,3-DMCH and 3MP Thione
Solvent PF- 1,3-DMCH
PT BPT XT-d,
3-MP
conc.>( 1O-6 (M)
4D
cont. x 1O-6 (M)
9D
0.5 1 2
6x 1O-4 5x 1o-4 5x 1o-4
1 1 1
5x10-2 3x 10-2 8xlOW
involving the net process T1 + So+2So. Regardless of the mechanism by which self-quenching occurs, the process is known to proceed at rates which are nearly diffusion-limited for almost all thione triplets which are not sterically hindered [ 10,19,20,22,23]. The values of k,, obtained in the present work are not ex-
ceptional and demonstrate (cf. fig. 3 ) that very low concentrations of thione are required if the rate of T1 decay is not to be completely dominated by selfquenching in fluid solutions. The following simple calculation is illustrative. A typical unquenched T1 lifetime is z 10 us in pertluoroalkane solvents,
M. Szymariski et al. /Photophysics of thione triplets in solution
AE (T, - So)
x lO-3
(cm-‘)
-
Fig. 4. Semilogarithmic plot of k,, versus AE for the aromatic thiones in perfluoroalkane solution at room temperature. The filled circle is for XT-d,.
whereas kq is z 10” M- ’ s- *. The concentration of thione required to reduce the observed lifetime by 10% due to self-quenching is therefore x 1 x 10m6 M. If one is interested in determining predominantly intramolecular decay parameters of the thiones in fluid solution at room temperature, it is therefore imperative either to do the measurements at thione concentrations < low6 M (which is extremely difficult) or to make measurements at somewhat higher concentrations and extrapolate the data to infinite dilution. Because excellent linear Stern-Volmer plots are obtained (cf. fig. 3) for 1/r=, versus thione concentration, and because only short extrapolations are required, we are confident that the data for 7?, and @ reported in tables 2 and 3 are precise and correctly reflect the values of these quantities in the absence of intermolecular interactions with ground state thione. Because self-quenching can occur by an almost exclusively physical mechanism (leading to no net consumption of thione), it is also imperative to carry out the measurements of @p at thione concentrations which are sufficiently small to avoid masking a possible efficient intramolecular photochemical process by self-quenching. Thus the data in table 4 are reported for concentrations of thione in the 1OH6M region where self-quenching will contribute in only a minor way ( x < 10%) to the overall rate of decay. The fact that extremely small values of &, are obtained for the three thiones studied is proof that intramolecular photochemical decomposition
149
contributes negligibly to the overall rate of their decay in perfluoroalkane solvents. We assume this to be the case for all the thione triplets examined in the present work since they all have rather low triplet energies (ET < x 200 kJ mol- ’ [ 1,2 ] ) and PT, which has the highest ET= 203 kJ mol- ’ [ 61, exhibits &,=6x 10-4. The physical requirements for observing “intramolecular” radiationless transitions in dense media have been discussed in detail by Freed and Jortner [ 261. The medium should neither modify the spinorbit coupling matrix elements nor enhance the coupling between states of the same multiplicity. To avoid electrostatic interactions which may mix the electronic states of the guest (solute) and the host (solvent), the lowest electronic state of the host should be considerably higher than that of the excited guest. The medium should not modify the order or position of the electronic levels of the guest. Finally, the vibrational modes of the medium should not affect the vibrational energies of the guest so that promoting modes for radiationless transitions in the guest correspond only to intramolecular vibrations. The medium does provide a heat bath in which vibrational relaxation within a given electronic state may take place efficiently, and which, at finite temperatures, serves to enhance the population of vibrational states characterized by quantum numbers vi > 0. Perlluoroalkane solvents appear to fulfill these requirements in all respects when considering the radiationless decay of thione triplet guests at room temperature. The thione absorption and emission spectra exhibit significant band narrowing in perfluoroalkanes compared with 3-MP (itself a relatively inert solvent) as may be seen by examining the values of Avid,” and Arti in table 1. Both absorption and emission origin bands exhibit almost the same frequencies in 3-MP and in perfluoroalkanes (cf. table 1). Indeed, Hara and Ware [ 401 and Dong and Winnik [ 4 1 ] have shown that peifluoroalkane solvents cause only minor displacements in the frequencies and intensities of the environmentally sensitive Ham bands of pyrene. The lifetimes, 7$,, and the phosphorescence quantum yields, @z, are larger and the radiationless decay constants, k,,, are smaller in perlluoroalkanes than in any other solvent employed to date, and are the same within experimental error, for a number of different pertluoroalkanes (PF- 1,3-
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M. Szymariski et al. /Photophysics of thione triplets in solution
DMCH, pertluoro-n-hexane, perfluorodecalin, perfluoromethylcyclohexane) having a variety of structures and physical properties. It is possible that +E, r:, , and 1/k,,, could be larger still in a hypothetical ensemble of isolated thione triplets exhibiting a room temperature Boltzmann distribution of internal energies. Nevertheless, the facts that the thione triplets having the lowest ET exhibit nearly the same values of these parameters in 3-MP and PF- 1,3-DMCH, whereas the thiones having larger triplet energies (and larger effects of replacing 3-MP by PF- 1,3-DMCH, table 3) exhibit the same values of @, r”, and l/h, in a variety of perfluoroalkane solvents suggest that the perfluoroalkanes are inert in the sense of causing minimal perturbation of the interstate electronic coupling energies in the guest molecules. The energies of the lowest excited electronic states of the perfluoroalkanes [ 42 ] are certainly far higher than those of the thione triplets. Finally, we note that replacing H by F has the effect of eliminating all the high-frequency C-H stretching vibrations in the solvent, so that perturbations of the high-frequency C-H promoting modes in the excited guest molecules by the host should be minimized. The perfluoroalkanes do, however, act as efficient vibrational thermalizers, even on a time scale of a few picoseconds. Perfluoroalkanes have previously been used successfully to provide inert media for the study of the second excited singlet states of these same thiones [ 24,25 1. The lifetimes of these states range from less than live to several hundreds of picoseconds, but even here vibrational relaxation in the perlluoroalkane solvents is sufficiently rapid that the shapes of the S2-+ So emission spectra are completely independent of the initial vibrational energy content of the SZ state. There can be no doubt that the longerlived triplets are fully vibrationally and rotationally relaxed. One should also note that the triplet energies are sufficiently large ( > 13000 cm-’ ) that these molecules would be classified as statistical limit cases even in the absence of a condensed host medium. The phosphorescence spectra of several of the thiones examined in the present work have been reported previously for a variety of host media at temperatures between 1.0 and 295 K [4,5,7,36]. In nonpolar, inert solvents the T, states are clearly of n, rc* electron configuration and ‘AZ electronic character. Both semi-empirical calculations [ 7,431 and zero-
field splitting parameter measurements [ 4,5,43] demonstrate that the second triplet state, of 3A, character, lies with ~2500 cm-’ of the ‘A2 state in PT, BPT and XT. Although we have good evidence that the ‘A2 and ‘A1 states are inverted in polar solvents such as acetonitrile [ 141, there is no doubt that T1 is of 3A2 electronic character in pertluoroalkanes at room temperature or in low temperature Shpolskii matrices [ 36 1. The phosphorescence spectra of the rigid aromatic thiones in non-polar solvents at room temperature invariably consist of a number of broad bands which diminish monotonically in intensity to longer wavelengths [ 7,8,14,15 1, and which exhibit total widths of z 3700-4500 cm-‘. (The latter is similar to the phosphorescence spectra of aromatic hydrocarbons [ 44 1.) This structure becomes better resolved in less strongly interacting solvents, becomes still sharper at 77 K, and consists of narrow lines at liquid He temperatures [ 4,5,36]. The most useful phosphorescence spectrum has been reported by Mahaney and Huber [ 361 who measured well-resolved corrected spectra of XT in a Shpolskii matrix at 15 K. The unresolved structure at higher temperatures is shown to consist of contributions from several ring vibrational modes, with a minor contribution from the C=S stretching frequency. The origin is the strongest band in the spectrum, carrying z 25% of the total oscillator strength, and bands in various progressions fall off rapidly in intensity with decreasing energy. Non-rigid thiones like DMTBP shown little vibronic structure, likely indicating that a considerable fraction of the oscillator strength is carried by lowfrequency torsional modes in these molecules. Corrected spectra at 77 K [ 45 ] and uncorrected spectra at x < 1 K [ 5 ] suggest that TXT may suffer a distortion in its T1 state which is somewhat greater than other rigid thiones, as evidenced by its more slowly diminishing phosphorescence intensity profile towards longer wavelengths. With the possible exception of TXT, however, the balance of the rigid thiones studied here exhibit remarkably similar emission profiles and widths. One concludes that the equilibrium structures of the S, and T1 states at energies near their zero-point levels are quite similar in XT and the other rigid thiones. Having established that unperturbed intramolecular decay can be observed for dilute solutions of trip-
M. Szymariskiet al. /Photophysicsof thionetripletsin solution
lets in perfluoroalkanes, we now begin an analysis of the observed decay rates. The radiative, k,, and nonradiative, k,,,, rate constants for T1 decay in PF-1,3DMCH are calculated from k, = qS”,/rg, and k,,, = ( 1 -@) /r$, and are given in table 2. In 3-MP, similar expressions are used, but the non-radiative constants obtained (table 3) will be the sum of a number of first-order and pseudo-first-order terms, Ek,,,, whose detailed elaboration is unimportant hereN2. Note that the values of k, are identical, within experimental error, for the two solvents and, with the exception of the non-rigid DMTBP, fall within a narrow range between 3.2 x 10’ and 11 x IO3 s- ‘. The most remarkable aspect of these data is that the values of k, are so large - a factor of x lo5 larger than those of typical aromatic hydrocarbon triplets whose radiative lifetimes often fall within the l-30 s range [ 461. The large values of k, are, of course, completely consistent with the observations of S,+T, absorptions having oscillator strengths comparable to those of the companion n+rr.*, S,-+S, transitions in the same compounds. The qualitative conclusion is that the T, states possess large admixtures of singlet character as a result of strong spin-orbit coupling. We now consider the non-radiative rate constants, k,,. Previous experiments in which thione triplet lifetimes and phosphorescence quantum yields were measured in fluid solutions at room temperature produced data which exhibited only a qualitative energy gap dependence [ 7,16,2 1,401. The reasons for this are now apparent. Previous experim,ents were done either at thione concentrations too large to afford accurate estimates of &,0 and rp,, or in solvents commonly considered to be inert but which nevertheless stongly perturb the excited thione (particularly those with large ET) and substantially increase their apparent non-radiative rate constants (cf. table 3). The present data, which provide accurate values of @gand r?, in non-perturbing pertluoroalkane solvents, provide a good quantitative energy gap law correlation (vide infra ) . The spectroscopic data suggest that, in the rigid thiones, the T, surface is not strongly displaced relative to the So surface, whereas the photochemical data suggest that the T, states are bound. Therefore the #*For a discussion of a detailed similar analysis applied to the Sz state see ref. [ 251.
151
weak coupling limit described by Englman and Jortner [ 471 and others, known to apply to the radiationless decay of aromatic hydrocarbon triplets, should provide an adequate model for our purposes. In this model k, = (21r)“2c2 YG r A(fiOM A.E)1/2exp ( - -> fifz0h.j ’
(1)
where, in our case, bE is the T,-So electronic energy gap (table 2), fro, is the energy of the highest frequency vibrational modes in the molecule, and C is the electronic matrix element coupling the initial and final states. The parameter y is given by y=ln (2AE/&fz0&&) - 1 ,
(2)
where AM and dare the reduced displacement and the degeneracy of the mode of maximum frequency, and contains the pertinent information about the relative displacements of the two potential energy surfaces. We assume, for the moment, that only the high-frequency C-H stretching modes need be considered as important accepting modes in the So state, as is the case for aromatic hydrocarbon triplets having sufficiently large ET [ 48 1. Assuming C is constant for the thiones under consideration, and ignoring the weak dependence of the preexponential term on AE, a linear correlation between log k,,, and AE is to be expected. A plot of the data is given in fig. 4. A reasonably good linear correlation is obtained by taking the data for all the undeuterated compounds. The intercept is 2.8 x lOI SK’, and from the slope a value of ~~4.22 is obtained. From the latter and using A,!?=15000 cm-’ where k,,=1.6x105 s-‘, we calculate C= 1 x lo3 cm-‘. The observed linear relationship between log k,, and AE suggests that the energy gap law is operative, i.e. the variation of k,, with AE is due to the exponential decrease in the FranckCondon factor for the T, wSo transition with increasing AE. However, the value of the electronic matrix element, C= 1 x 1O3 cm- ’ is apparently much too large for a radiationless transition which is nominally spin forbidden. We thus consider a more detailed analysis of the problem following the work of Siebrand and coworkers [ 48-5 11. The data are plotted, fig. 5, in the empirical form
M. Szymariski et al. /Photophysics of thione triplets in solution
152
Fig. 5. Semilogarithmic plot of k,, versus (ET-E,,) /q. A value of 4000 cm- ’ is used for EO. The filled circle is for XT-d,, for which a value of ED= 5500 cm-’ is used.
ET -Eo
log L& = log k!, - -
rl
f,
(3)
where q is taken as nH/nT, the fraction of the total number (&) of atoms in the molecule which are hydrogen andfis a proportionality constant (the slope) which is related to the inverse of the wavenumber of the C-H(D) stretching vibration cfx 10m4 cm in the aromatic hydrocarbons). E. is the energy below which C-C stretching or other modes are more effective than C-H stretching modes as sinks for the vibrational energy generated in the final state as a result of the radiationless transition. E. is taken as 4000 cm-’ because the full widths of the phosphorescence spectra of the rigid thiones are nearly the same as those of the aromatic hydrocarbons on which Siebrand did the original analysis. In the present work those molecules possessing free -CH3 or -OCH3 groups present a problem because one does not know, a priori, the extent to which the high-frequency C-H stretches in these groups can act as accepting or promoting modes. Including either all or none of the H atoms in these groups in the calculation of q results in points which fall well off the line drawn for those four thiones, PT, BPT, XT-d, and TXT, which have only ring H atoms. If the -CH3 and -OCH3 groups are ignored and replaced (for calculation purposes) by H or -CX, (X #H), then the resulting values of r] cause the log k,,versus (E-E,) / q data for these molecules to correlate much better with the other data. This suggests that the C-H
stretching vibrations in these nearly freely rotating groups are very weakly coupled to those of the ring carbon framework in these molecules. Nevertheless, because of the uncertainty in our understanding of how these modes participate in the radiationless decay, we do not consider TMIT, Br-TMIT, DMTBPT and DMTBP in any further analysis. The slope of the line drawn for the perhydromolecules is 2.2 x lop4per cm-‘, very nearly the same as that for the triplet hydrocarbons [ 48-5 11. Thus the Franck-Condon factors vary with ET in the same way for both classes of compound. This indicates that the intramolecularly unperturbed thione and hydrocarbon triplets behave similarly insofar as Franck-Condon prohibition of their T,*So radiationless transition rates is concerned. We conclude that the high-frequency C-H stretching modes are the most effective accepting vibrations in So at the energies spanned by the compounds examined. Only one deuterated compound, XT, has been examined. It is nevertheless possible to use the radiationless decay data for this one compound to test if the above Franck-Condon analysis for the perhydro compounds is reasonable. Assuming the intercepts at (ET- E,)/q=O are the same for both the perhydro and perdeutero compounds, one obtains a value of the ratio of the slopes of 1.27 f 0.07. This should be compared with a value of 1.36 expected on the basis of the square root of the ratio of the reduced masses, (IUCH/~cD)“2, and a value of 1.35 observed for the aromatic hydrocarbon triplets [ r48-5 11. The effect of perdeuteration is thus as expected if Franck-Condon factors for the C-H(D) stretching modes primarily govern the rates of T1 -So raditionless decay. The intercepts of these plots can be employed to provide some semi-quantitative information about the electronic matrix elements, C,, of the TL*SO transitions in these thiones. For triplet hydrocarbons =3.5x lo5 s-1, whereas for triplet thiones CL,HC s-l. Since kzr,T/kzr,HC= k:r,T = (9.1&2)x10” cc/c”,, = 2.6~ 106, using C,,x 1 x lO-*lcm-’ we calculate C -,-x 16 cm- ‘. Given the considerable uncertainty in the determination of kt,,,, the fact that C, is of the same order as the zero-field splitting parameter, 1D* 1, observed by Maki and co-workers [ 5,431 for XT triplets (24 cm-‘) must be considered fortuitous. Nevertheless, the calculation of C, is certainly correct to well within an order of magni-
M. Szymariski et al. /Photophysics of thione triplets in solution
tude, and clearly illustrates the very large interstate coupling energies in these molecules. The variation of AY,,, with deuterium substitution pattern in XT indicates that all C-H oscillators contribute significantly, but perhaps not equally, to the overall Franck-Condon factor for the molecule. The data show that k,,, decreases monotonically with increasing deuterium substitution, but that substitution of the first four D atoms (at positions 1,5,6 and 8 ) has a bigger effect than substitution of the last four. We expect that substitutions at C ( 1) and C (8 ) have the greatest effect since the largest change in ring carbon electron density occurs at these atoms, and previous work [ 7,52,53] has demonstrated that the two effects are well correlated. The overall effect of perdeuteration is apparently normal, however, at least for XT triplets. The triplet thione thus behaves differently from the second excited singlet state in which we recently showed [ 541 that large amplitude motion of H atoms at the C ( 1) and C (8 ) positions promotes S2* So decay.
Acknowledgement
The authors wish to acknowledge with gratitude the continuing financial support of the Natural Sciences and Engineering Research Council of Canada. Financial support under Polish Research Project CPRB 8.14 is also gratefully acknowledged.
References [ 1] R.P. Steer, Rev. Chem. Intermed. 4 ( 1981) 1. [ 21 V. Ramamurthy, Org. Photochem. 7 ( 1985) 23 1. [3] J.D. Coyle, Tetrahedron 41 (1985) 5393. [4] D.M. Burland, J. Chem. Phys. 75 (1981) 2635.’ [ 5] M.R. Taherian and A.H. Maki, Chem. Phys. 68 ( 1982) 179; Chem. Phys. Letters 96 (1983) 541; M.J. Petrin, S. Ghosh and A.H. Maki, Chem. Phys. 120 (1988) 299. [ 61 M. Szymanski, R.P. Steer and A. Maciejewski, Chem Phys. Letters 135 (1987) 243. [7] D.A. Capitanio, H.J. Pownall and J.R. Huber, J. Photothem. 3 (1974) 225. [ 81 C.A. Emeis and L.J. Oosterhoff, J. Chem. Phys. 54 ( 1971) 4809. [9] A. Maciejewski, M. Szymanski and R.P. Steer, J. Phys. Chem., to be published.
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[lo] K. Bhattacharyya, V. Ramamurthy and P.K. Das, J. Phys. Chem. 91 (1987) 5626. [ 111 K. Bhattacharyya, P.K. Das, V. Ramamurthy and V.P. Rao, J. Chem. Sot. Faraday Trans. II 82 ( 1986) 135. [ 121 A. Maciejewski, M. Szymanski and R.P. Steer, J. Phys. Chem. 90 (1986) 6314. [ 131 J.P. Englebrecht, G.D. Anderson, R.E. Linder, G. Barth, E. Bunnenberg, C. Djerassi, L. Seamans and A. Moscowitz, Spectrochim. Acta 31 A (1975) 507. [ 141 A. Maciejewski, M. Szymadski and R.P. Steer, Chem. Phys. Letters 143 (1988) 559. [ 151 D.S.L. Blackwell, CC. Liao, R.O. Loutfy, P. de Mayo and S. Paszyc, Mol. Photochem. 4 (1972) 171. [ 161 A. Safarzadeh-Amiri, R.E. Verrall and R.P. Steer, Can. J. Chem. 61 (1983) 894. [ 171 D.R. Kemp and P. de Mayo, J. Chem. Sot. Chem. Commun. (1972) 233. [ 181 C.V. Kimar, L. Qin and P.K. Das, J. Chem. Sot. Faraday Trans. II 80 ( 1984) 783. [ 191 A. Safarzadeh-Amiri, D.A. Condirston, R.E. Verrall and R.P. Steer, Chem. Phys. Letters 77 (198 1) 99. [20] V. Ramesh, N. Ramnath and V. Ramamurthy, J. Photothem. 18 (1982) 293; 23 (1983) 141. [21] U. Bruhlmann and J.R. Huber, J. Photochem. 10 ( 1979) 205. [22] R. Rajee and V. Ramamurthy, J. Photochem. 11 (1979) 135. [23] K. Bhattacharyya, C.V. Kumar, P.K. Das, B. Jayasree, V. Ramamurthy, J. Chem. Sot. Faraday Trans. II 81 (1985) 1983. [24] A. Maciejewski and R.P. Steer, J. Am. Chem. Sot. 105 (1983) 6738. [25] A. Maciejewski, D.R. Demmer, D.R. James, A. SafarzadehAmiri, R.E. Verrall and R.P. Steer, J. Am. Chem. Sot. 107 (1985) 2831. [ 261 K.F. Freed and J. Jortner, J. Chem. Phys. 52 ( 1970) 6272. [27] C.A. Parker and T.A. Joyce, Trans. Faraday Sot. 65 ( 1969) 2823. [28] F. Arndt and L. Lorenz, Berichte 63 (1930) 3121. [29] B.S. Pedersen, S. Scheibye, N.H. Nilsson and S.O. Lawesson, Bull. Sot. Chim. Belg. 87 (1978) 223. [30] J.W. Scheermer, P.H.J. Ooms and R.J.F. Nivard, Synthesis (1973) 149. [ 311 S.R. Abrams, J. Label. Comp. Radiopharm. 24 ( 1986) 941. [32] M. Szymanski, A. Maciejewski and R.P. Steer, J. Phys. Chem. 92 (1988) 2485. [33] A. Maciejewski and R.P. Steer, J. Photochem. 35 (1986) 59. [ 341 D.R. James, D.R.M. Demmer, R.E. Verrall and R.P. Steer, Rev. Sci. Instr. 54 (1983) 1121. [35] J.N. Demas, in: Creation and detection of the excited state,. Vol. 4, ed. W.R. Ware (Dekker, New York, 1976) p. 1. [ 361 M. Mahaney and J.R. Huber, J. Mol. Spetry. 87 ( 1981) 438. [ 371 A. Maciejewski, A. Safarzedeh-Amiri, R.E. Verrall and R.P. Steer, Chem. Phys. 87 (1984) 295. [38]K.Y.LawandP.deMayo,J.Am.Chem.Soc. 101 (1979) 3251.
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[ 391 A.H. Lawrence, CC. Liao, P. de Mayo and V. Ramamur-
thy, J. Am. Chem. Sot. 98 (1976) 3572. [ 401 K. Hara and W.R. Ware, Chem. Phys. 5 1 ( 1980) 61. [41] D.C. Dong and M.A. Winnik, Can. J. Chem. 62 ( 1984) 2560. [ 42 ] C.W. Lawson, F. Hirayama and S. Lipsky, J. Chem. Phys. 51 (1969) 1590. [43] A.H. Maki, P. Svejda and J.R. Huber, Chem. Phys. 32 (1978) 369. [ 44 ] C.A. Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968). [45] M. Mahaney and J.R. Huber, Chem. Phys. 9 (1975) 371. [46] S.L. Murov, Handbook of photochemistry (Dekker, New York, 1973) pp. 39 ff.
[47] R. Englman and J. Jortner, Mol. Phys. 18 (1970) 145. [48] W. Siebrand, J. Chem. Phys. 47 (1967) 2411. [49] W. Siebrand, J. Chem. Phys. 44 (1966) 4055. [ 501 W. Siebrand, J. Chem. Phys. 46 ( 1967) 440. [ 51) W. Siebrand and D.F. William, J. Chem. Phys. 49 (1968) 1860. [ 521 B.R. Henry and W. Siebrand, J. Chem. Phys. 54 (1971) 1072. [53] B.R. Henry and J.L. Charlton, J. Am. Chem. Sot. 95 (1973) 2782. [54] S.R. Abrams, M. Green, R.P. Steer and M. Szymanski, Chem. Phys. Letters I39 ( 1987) 182.
PHOTOPHYSICS OF THIONE TRIPLETS IN SOLUTION: FACTORS CONTROLLING THE RATES OF RADIATIONLESS
DECAY
M. SZYMAfiSIU ‘, A. MACIEJEWSKI ’ and R.P. STEER 3 Department of Chemistry University ofSaskatchewan. Sashztoon, Saskatchewan, Canada S7N 0 WO Received 26 February 1988
Several spectroscopic, photochemical, and/or photophysical properties of the lowest triplet states of eleven molecules containing the thiocarbonyl group have been measured in 3-methylpentane and perlluoro-1,3-dimethylcyclohexane solvents at room temperature. Values of the phosphorescence lifetimes, phosphorescence quantum yields and quantum yields of net photochemical consumption of thione at infinite dilution have been determined. A quantitative analysis of the intramolecular relaxation processes of the triplet thiones in the absence of complicating intermolecular self-quenching and solvent-excited solute interactions is possible, using data obtained with inert pertluoroalkane solutions. Photochemical processes contribute negligibly to the rates of triplet decay. With one exception, the radiative rate constants fall within a narrow range between about (3-l 1) x lo3 s-‘. These are about a factor of 1O5larger than those of the triplet hydrocarbons, and are indicative of strong mixing of the triplet and singlet manifolds. The non-radiative rate constants of the rigid thiones are consonant with the energy gap law and are governed by the Franck-Condon factors for the T, w.5, non-radiative transitions. A normal deuterium isotope effect is observed in xanthione.
1. Introduction
Interest in the properties of thiones in their lowest triplet ( T1 ) states has grown considerably in the last decade [ l-3 1. Of particular note are the exceptionally large zero-field splitting parameters of xanthione (XT), benzopyranthione (BPT), and 4H-pyran-4thione (PT) triplets [ 4,5 1, the huge quantum yields of phosphorescence of PT in fluid solutions at room temperature [ 61, the large oscillator strengths of thione S,-+T, transitions [ 7,8] (which permit direct, selective excitation to T, ), their relatively short, concentration-dependent lifetimes [ 9- 111 (which facilitate studies of their intermolecular interactions), and the occurrence of delayed fluorescence from S, by thermal activation from the triplet manifold [ 121. The spectroscopic properties of thione triplets have been examined in some detail. There is now little ’ On leave from Institute of Physics, A. Mickiewicz University, Poznab, Poland. 2 Faculty of Chemistry, A. Mickiewicz University, Poznatl, Poland. 3 To whom correspondence should be addressed.
doubt that most thiones possess S, and T, states which are of n, 7c*electronic configuration in both low temperature matrices and non-polar solvents at room temperature [ 8,131. The x, n* triplet companion of the ’ (?r, A*) Sz state is of only slightly higher energy than T1 in some thiones and the small T2-T1 gap is thought to be largely responsible for the existence of large values of the T1 zero-field splitting parameter, )P I , in thiones such as XT and PT [ 4,5 1. There is also some evidence of T,-T2 reordering as the polarity of the solvent is increased [ 14 1. Nevertheless, for the present study which is primarily concerned with the photophysical properties of thiones in inert, nonpolar solvents, the T, state is clearly of n, n* configuration and 3A2character, assuming Czv local symmetry about the thiocarbonyl group. in these molecules. The decay dynamics of thione triplets have previously been studied by measuring their phosphorescence lifetimes and quantum yields [ 7,9,15,16], by monitoring their triplet-triplet absorption intensities in laser flash photolysis [ 10, 11,17 1, by observing the yields of singlet molecular oxygen in T,-oxygen quenching experiments [ 18-201, and by determining the nature, yields, and rates of formation of the
0301-0104/88/$03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division )
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M. Szymariski et al. /Photophysics of thione triplets in solution
products of intermolecular and intramolecular photochemical reactions [ 2,3,2 11. Thermally activated delayed fluorescence from S I has also been observed in several thiones [ 12 1, and this indirect route of T, relaxation has provided valuable kinetic information concerning the decay processes involved. In fluid solutions bimolecular self-quenching of the triplet states of sterically unhindered [ 10,201 thiones occurs at near diffusion controlled rates [ 19,22,23]. The rates of decay of thione triplets are therefore dominated by self-quenching processes when the concentration of thiones are greater than about 10e4 M. If the firstorder decay parameters characteristic of thione triplets in fluid solution are desired, then it is necessary to make measurements at very low thione concentrations and to extrapolate the results to infinite dilution [9]. Almost all of the previous work on thione triplets has been carried out using a variety of solvents, most of which would commonly be considered to be inert. We have shown, however, that the second excited singlet states of a number of thiones exhibit electronic relaxation rates which are highly solvent dependent [ 24,25 1. Of the solvents examined to date, only the perfluoroalkanes appear to act as classical heat baths [ 261, thereby permitting the intramolecular decay parameters of the S, thiones to be observed. Similar effects on the rates of decay of ketone triplets in fluid solution were observed by Parker and Joyce [ 271 some twenty years ago. Replacing even a photochemically unreactive solvent such as benzene or a chlorofluorocarbon by a perfluoroalkane was observed to increase the T, lifetime and phosphorescence quantum yield of benzophenone by at least an order of magnitude. It is understood from many photochemical and photophysical studies that thione triplets are much less reactive than their Sz counterparts. Nevertheless, the lifetimes of the T1 states are three to five orders of magnitude longer than those of Sz, and it therfore appeared to be important to examine the decay dynamics of triplet thiones in a variety of solvents, including the pertluoroalkanes. If, as with the Sz states, solvent perturbation of the triplet decay could be minimized in the perfluoroalkanes, then an examination of the parameters governing intramolecular T, decay might be undertaken. This paper presents the results of such a study.
2. Experimental The thiones used in the present work (fig. 1) were synthesized by sulfuration of the corresponding ketones using established techniques [ 28-30 1. Xanthione (XT) was selectively deuterated as previously described [ 3 11. Three compounds of good isotopic and chemical purity were recovered, XT-d, containing > 99% D, XT-d, containing > 93OhD at positions 1, 3, 4, 5, 6 and 8 and XT-d, containing ~93% D at positions 1, 4, 5 and 8, contaminated with 6Ohand 12% D at the “unlabelled” 2, 7 and 3, 6 positions respectively. Deuterium substitution patterns and contents were determined by high-resolution NMR and GC MS as previously described [ 3 11. The thiones were purified by recrystallization and silica gel column chromatography and were analyzed for impurities by gas chromatography. Hydrocarbon solvents were purified by standard methods, and perfluoroalkane solvents (PCR Research Chemicals) were purified by distillation followed by column chromatography. Solvent impurities contributed negligible emission when the neat liquid was excited at wavelengths from the near UV through the visible. All experiments were performed on samples which had been rigorously deoxygenated, because O2 quenches and/or reacts with thione triplets at near diffusion controlled rates and the solubility of oxygen in pertluoroalkane solvents is unusually high. This was accomplished either by 7 to 9 freeze-pump-thaw cycles to a residual pressure of < 10e4 Torr at 77 K or by air displacement using prolonged bubbling with He which had itself been desoxygenated by passage through a hot copper column. Absorption measurements were made with either a Cary 118C or a Specord M-40 (Carl Zeiss, Jena) spectrophotometer. Steady state emission spectra and quantum yields were measured using either a Spex Fluorolog 222 or a modified Perkin-Elmer MPF-3 spectrofluorometer, as previously described [ 32,33 1. Phosphorescence lifetimes were measured using a Spectra Physics argon ion/dye laser combination which was either cavity-dumped to provide nanosecond excitation pulses in the visible or mode-locked, synchronously pumped, cavity-dumped and frequency-doubled to provide picosecond excitation pulses in the UV. Time-correlated single-photon counting was used, as previously described [ 341, to
145
A4. Szymariski et al. /Photophysics of thione triplets in solution
TMIT
PT
TXT
BPT
XT
DM BTPT
DMTEP
Fig. 1. Structures and abbreviations for the thiones studied in the present work. TMIT, 2,2,3,3-tetramethylindanethione; PT, 4H-pyran4-thione; BPT, 4H-1-benzopyran-4-thione; XT, xanthione; TXT, tbioxantbione; DMBTPT, 2,6dimetbyl_4H-l-benzotbiopyran-4-thione; DMTBP, p,p’-dimethoxythiobenzophenone.
accumulate phosphorescence decay profiles. Cavitydumping and photon-counting rates were selected to ensure that pulse pile-up did not distort the decays. Deconvolution of the observed microsecond decays from the subnanosecond instrument response function was unnecessary. The rate of photochemical consumption of thione was monitored routinely by UV absorption spectrophotometry and the net percentage decomposition was kept negligibly small for quantum yield and lifetime all emission measurements. The quantum yields of net photochemical consumption of thione were measured by following the decrease in absorbance of the thione in its strong (t> lo4 M-’ cm-‘) So+& absorption band, and sometimes by determining the concentration of thione using gas chromatography. The absorbed intensity of the exciting radiation was measured by Reinecke salt actinometry, taking account of the thermal elimination of thiocyanate [ 35 1.
absorption and is sufficiently well resolved from the singlet system to permit clean, direct excitation to the triplet state. The long wavelength emission at room temperature consists mainly of T, + So phosphorescence, but is accompanied by a small amount ( < ~O%I of the total) of thermally activated delayed SI+SO fluorescence [ 121. The latter appears as a weak, poorly resolved, temperature-sensitive feature to the blue of the highest energy strong band in the phosphorescence spectrum. The phosphorescence spectra of most rigid aromatic thiones exhibit a number of strong, broad features separated by = 1100-1200 cm-‘, likely assignable either to single quantum changes of the
BPT
in
PF-1.3-DMCH
I t 5
v-so
- IO ‘5
I ii
E 0 c
II ::
c
D
3 3. Results
-5
:: z
The absorption and emission spectra of the thiones examined in the present work have either been previously reported [6,9,12,14,32] or are similar to them. The spectra of BPT in perfluoro- 1,3-dimethylcyclohexane (PF- 1,3-DMCH ) are representative and are given in fig. 2. One should note that, as with previous examples, the relatively strong S,+T, absorption lies to the red of the corresponding !%-+S,
'\ '\ '. 400
500 WAVELENGTH
600
700
if z -
800
(nm)--
Fig. 2. Absorption (--) and emission (---) spectra of BPT in pertluoro-1,3dimethylcyclohexane at 295 K. Absorption spectrum in the 450-650 nm ranges taken in 3-MP. Note large absorbance scale change at 450 nm.
146
M. Szymariski et al. /Photophysics of thione triplets in solution
ground state C=S stretching frequency or, more likely, to a congestion of ring vibrational modes [ 361 having frequencies of = 1030- 1340 cm- ‘. The progressions exhibit strong origin bands, and decline in intensity rapidly with increasing wavelength. Nevertheless, the phosphorescence spectra of some thiones (e.g. thioxanthione, TXT) extend substantially to the red of 800 nm where photomultiplier tubes exhibit rapidly declining sensitivities. The accurate determination of the phosphorescence quantum yields of such compounds is therefore more difficult because a modest correction factor accounting for the fraction of the emission at il> 800 nm must be applied. Both the absorption and emission spectra of thiones are better resolved in pertluoroalkane solvents than in alkanes such as 3-methylpentane (3-MP). A similar effect has been noted previously for the absorption and Sz-& emission spectra of these same thiones [24,25]. The inertness of the pertluoroalkanes is clearly revealed by the distinct narrowing of the origin band in the phosphorescence spectrum in these solvents compared with 3-MP at the same temperature. Very small shifts in the frequencies of the band maxima ( x 30 cm- ’ ) on substituting perlluoroalkanes for 3-MP are also observed in these spectra but are insignificant in the context of the present work. The spectroscopic properties of the triplet states of eight thiones in both 3-MP and perfluoro- 1,3-dimethylcyclohexane are summarized in table 1. In this table flaabs and IT,, are the frequencies of the peaks of the origin bands in absorption and emission, e is the decadic molar extinction coefficient of the origin band in the absorption spectrum (if resolved), and Avz&’ and Ar~kc are the widths of the origin bands in absorption (at 3/4 height due to resolution difficulties) and in emission (at l/2 height). The key photophysical data for this study were obtained by measuring the lifetimes of the phosphorescing triplet state, r-r,, and the quantum yields of phosphorescence, I&, as a function of thione concentration and excitation wavelength in both 3-MP and PF- 1,3-DMCH. The phosphorescence decays were all well described by single exponential functions (xf approaching unity and random distributions of residuals). The resulting lifetimes were independent of exciting wavelength (excitation to S2, S, and Ti ) for a given thione concentration in a given solvent at a given temperature. (Note, however, that subnanose-
cond !&-So fluorescence [ 25,371 could also be observed when exciting to S2, but this did not contaminate the phosphorescence significantly.) Typical examples of phosphorescence decay curves have been previously published [ 9,16 1. The lifetime and quantum yield data were plotted in Stern-Volmer form, yielding good linear relationships for l/r-r, versus thione concentration. An example is given in fig. 3. Values of r$, were determined from the intercepts of these plots at infinite dilution. The quantum yields of phosphorescence, q&, were measured at the same concentrations as those used to measure rT, . The @gvalues were then obtained using the scaling relationship @g=&,r$,/rT,. The data are presented in tables 2 and 3. The errors in z?, quoted in these tables are f 3 standard deviations on the intercepts of the Stem-Volmer plots. It is immediately apparent that the values of 7$ are more precise than those of @Xsince the latter contain the cumulative errors in 7l, and rT5,, , so that the uncertainties in the radiative and non-radiative rate constants calculated from these quantities are due mostly to uncertainties in @. The slopes of the Stem-Volmer plots, together with the values of 7$, , were used in calculating the bimolecular self-quenching rate constants, kq. The values of kw fall in narrow ranges between 4.5 x 1O9 and 1.8~10’~ M-’ s-’ in PF-1,3-DMCH and between 1.7~10’~and2.6x10’~M-‘s-‘in3-MP. The results of the measurement of the quantum yields of net photochemical consumption, &,, on excitation of XT, BPT and PT in their n-+x* absorptions in both 3-MP and PD- 1,3-DMCH are given in table 4. The thione concentrations for these experiments were chosen so that self-quenching contributed G 10% of the total T1 decay rate. A detailed discussion of these results will not be undertaken here. It is sufficient for our present purpose to note that the values of @,,are generally small in 3-MP and are extremely small ( < 6 x 10F4) in PF-1,3-DMCH.
4. Discussion We begin by describing the conditions under which it should be possible to measure the rates of intramolecular decay of rigid thione triplets in fluid solution at room temperature. Both chemical and physical criteria must be met.
M. Szymariski et al. /Photophysics of thione triplets in solution
147
Table 1 T,-spectral properties of several thiones at room temperature Thione
Solvent
r&(0,0) (cm-‘)
kn(O> 0) (cm-‘)
e(O,O) (M-l cm-‘)
Akz(O, 0) (cm-‘)
A9:A2(0, 0) (cm-‘)
PT
PF- 1,3-DMCH 3-MP
17060 17100
17010 16980
10.0 9.9
90 140
560 - 700
BPT
PF- 1,3-DMCH 3-MP
16030 16030
15930 15900
10.4 10.1
110 145
380 530
TMIT
PF- 1,3-DMCH 3-MP
16180 16180
15860 15810
3.6 4.0
b) b)
590 710
Br-TMIT
PF-1,3-DMCH 3-MP
15870 15870
15560 15490
3.1 3.5
b) b)
631 680
XT-d,,
PF- 1,3-DMCH 3-MP
a) 15130
14970 14940
a)
a) b)
390 510
DMBTPT
PF- 1,3-DMCH 3-MP
b, b)
14880 14860
b) b)
b) b)
b) b)
DMTBP
PF-1,3-DMCH 3-MP
b, b)
14030 13930
b) b)
b) b)
b) b)
TXT
PF-1,3-DMCH 3-MP
b, b)
13560 13610
b) b)
b) b)
b) b)
8.5
a) Not measured due to the low solubitily of XT in PF-1,3-DMCH. b, Not calculated due to strong overlap with the (0,O) band of S,+S,.
From a chemical perspective we wish to observe thione triplet decay under conditions in which the excited molecule undergoes neither intramolecular nor intermolecular photochemical reaction at significant rates. The data of table 4 clearly demonstrate 0.16 _ 0.‘6-
BPT 293
0.06,
, 0
I 2
in PF-1,3-OMCH K
,
I 4
,
I 6
,
[BPT]
, 6
,
I , IO
(x IO+
I , 12
I 14
I , 16
that net photochemical consumption of these thiones excited in the visible in their n-+x* absorption systems is negligible in perfluoroalkane solvents, since 9p< 6 x 10m4. Net photochemical consumption of thione is still small in 3-MP (@p< 5 x 10W2), but the fraction of T1 decay events which proceed by a photochemical mechanism could conceivably be much greater than fir, (and therefore significant in 3-MP) if the initial photochemical event were reversible. For example Law and de Mayo [38] have shown that x 90% of free radical pairs formed by the abstraction of H from protic solvents by excited thiones reform reactants by disproportionation in the solvent cage. Hydrogen abstraction from solvent molecules which are not fully fluorinated is possible, but &, is so small in the perfluoroalkanes employed as to render even this process insignificant. Self-quenching either may lead to distinct photochemical products ( d as high as 1 #I) or may proceed via an almost entirely physical mechanism [ 16,391
Ml
Fig. 3. Stem-Volmer plot of 1/7T, versus BPT concentration in PF-1,3-DMCH at 293 K.
+I’When adamantanethione is excited to S2,the quantum yield of dimer is one when extrapolated to infinite thione concentration.
148
M. Szymariski et al. / Photophysics of thione triplets in solution
Table 2 Parameters describing the photophysical decay of the triplet states of several thiones in PF-1,3-DMCH solution at 295 K kJ
@;(T,)a’
Thione PT BPT TMIT Br-TMIT XT-d, XT-d, XT-d, XT-d, DMBTPT DMTBP TXT
43*4 16+2 38?3 21.7kO.3 8.2f0.4 18.5f2.0 21*2 26+3 8.5kO.6 3.04+ 0.06 1.01 f0.02
0.47f0.11 b) 0.13f0.03 0.12f0.02 0.13+0.03 0.064f0.013 0.11f0.02 0.12+0.02 0.14f0.03 0.08 f 0.02 (1.5+0.3)x10-3 (9+2)x lo-’
‘) Errors are t3u. @O,(T,)=&(S,) b, Fromref. [6].
x 10-3)
k.,(
x 10-4)
AWT,-So)
(s-l)
(s-l)
(cm-‘)
11 8.1 3.2 6.0 7.8 5.9 5.7 5.4 9.4 0.49 8.9
1.2 5.4 2.3 4.0 11 4.8 4.2 3.3 11 33 98
17010 15930 15860 15530 14970
q+,(T,)/@,(S,) where both @(S,) and the ratio @iJT,)/@&$)
14880 14030 13560
were experimentally measured.
Table 3 Parameters describing the photophysical decay of PT, BPT, XT-d,, and TXT in 3-MP solution at 295 K Thione PT BPT XT-d, TXT
4; (T,) =) (5.6fl.l)xlO-* (5.4+1.1)x10-* (5.4fl.l)Xlo-* (8*2)x 10-r
0 a) rTI
k, (x lo-‘)
k.,(~lO-~)“’
(CLS)
(s-l)
(s-l)
6.5f0.5 7.3kO.4 7.1kO.4 0.99kO.01
8.6 7.4 7.6 8.1
1.4 1.3 1.3 10.0
‘) Erors are +3. @~(T,)=@~(S2) @,(T,)/@,(Sz) where both @(S,) and the ratio &(T,)/&,(S,) b, Sum for all first-order and pseudo-first-order radiationless decay processes.
0.086 0.42 0.85 0.98
were experimentally measured.
Table 4 Quantum yields of net photochemical consumption of PT, BPT and XT illuminated in their n-x* absorptions in PF-1,3-DMCH and 3MP Thione
Solvent PF- 1,3-DMCH
PT BPT XT-d,
3-MP
conc.>( 1O-6 (M)
4D
cont. x 1O-6 (M)
9D
0.5 1 2
6x 1O-4 5x 1o-4 5x 1o-4
1 1 1
5x10-2 3x 10-2 8xlOW
involving the net process T1 + So+2So. Regardless of the mechanism by which self-quenching occurs, the process is known to proceed at rates which are nearly diffusion-limited for almost all thione triplets which are not sterically hindered [ 10,19,20,22,23]. The values of k,, obtained in the present work are not ex-
ceptional and demonstrate (cf. fig. 3 ) that very low concentrations of thione are required if the rate of T1 decay is not to be completely dominated by selfquenching in fluid solutions. The following simple calculation is illustrative. A typical unquenched T1 lifetime is z 10 us in pertluoroalkane solvents,
M. Szymariski et al. /Photophysics of thione triplets in solution
AE (T, - So)
x lO-3
(cm-‘)
-
Fig. 4. Semilogarithmic plot of k,, versus AE for the aromatic thiones in perfluoroalkane solution at room temperature. The filled circle is for XT-d,.
whereas kq is z 10” M- ’ s- *. The concentration of thione required to reduce the observed lifetime by 10% due to self-quenching is therefore x 1 x 10m6 M. If one is interested in determining predominantly intramolecular decay parameters of the thiones in fluid solution at room temperature, it is therefore imperative either to do the measurements at thione concentrations < low6 M (which is extremely difficult) or to make measurements at somewhat higher concentrations and extrapolate the data to infinite dilution. Because excellent linear Stern-Volmer plots are obtained (cf. fig. 3) for 1/r=, versus thione concentration, and because only short extrapolations are required, we are confident that the data for 7?, and @ reported in tables 2 and 3 are precise and correctly reflect the values of these quantities in the absence of intermolecular interactions with ground state thione. Because self-quenching can occur by an almost exclusively physical mechanism (leading to no net consumption of thione), it is also imperative to carry out the measurements of @p at thione concentrations which are sufficiently small to avoid masking a possible efficient intramolecular photochemical process by self-quenching. Thus the data in table 4 are reported for concentrations of thione in the 1OH6M region where self-quenching will contribute in only a minor way ( x < 10%) to the overall rate of decay. The fact that extremely small values of &, are obtained for the three thiones studied is proof that intramolecular photochemical decomposition
149
contributes negligibly to the overall rate of their decay in perfluoroalkane solvents. We assume this to be the case for all the thione triplets examined in the present work since they all have rather low triplet energies (ET < x 200 kJ mol- ’ [ 1,2 ] ) and PT, which has the highest ET= 203 kJ mol- ’ [ 61, exhibits &,=6x 10-4. The physical requirements for observing “intramolecular” radiationless transitions in dense media have been discussed in detail by Freed and Jortner [ 261. The medium should neither modify the spinorbit coupling matrix elements nor enhance the coupling between states of the same multiplicity. To avoid electrostatic interactions which may mix the electronic states of the guest (solute) and the host (solvent), the lowest electronic state of the host should be considerably higher than that of the excited guest. The medium should not modify the order or position of the electronic levels of the guest. Finally, the vibrational modes of the medium should not affect the vibrational energies of the guest so that promoting modes for radiationless transitions in the guest correspond only to intramolecular vibrations. The medium does provide a heat bath in which vibrational relaxation within a given electronic state may take place efficiently, and which, at finite temperatures, serves to enhance the population of vibrational states characterized by quantum numbers vi > 0. Perlluoroalkane solvents appear to fulfill these requirements in all respects when considering the radiationless decay of thione triplet guests at room temperature. The thione absorption and emission spectra exhibit significant band narrowing in perfluoroalkanes compared with 3-MP (itself a relatively inert solvent) as may be seen by examining the values of Avid,” and Arti in table 1. Both absorption and emission origin bands exhibit almost the same frequencies in 3-MP and in perfluoroalkanes (cf. table 1). Indeed, Hara and Ware [ 401 and Dong and Winnik [ 4 1 ] have shown that peifluoroalkane solvents cause only minor displacements in the frequencies and intensities of the environmentally sensitive Ham bands of pyrene. The lifetimes, 7$,, and the phosphorescence quantum yields, @z, are larger and the radiationless decay constants, k,,, are smaller in perlluoroalkanes than in any other solvent employed to date, and are the same within experimental error, for a number of different pertluoroalkanes (PF- 1,3-
150
M. Szymariski et al. /Photophysics of thione triplets in solution
DMCH, pertluoro-n-hexane, perfluorodecalin, perfluoromethylcyclohexane) having a variety of structures and physical properties. It is possible that +E, r:, , and 1/k,,, could be larger still in a hypothetical ensemble of isolated thione triplets exhibiting a room temperature Boltzmann distribution of internal energies. Nevertheless, the facts that the thione triplets having the lowest ET exhibit nearly the same values of these parameters in 3-MP and PF- 1,3-DMCH, whereas the thiones having larger triplet energies (and larger effects of replacing 3-MP by PF- 1,3-DMCH, table 3) exhibit the same values of @, r”, and l/h, in a variety of perfluoroalkane solvents suggest that the perfluoroalkanes are inert in the sense of causing minimal perturbation of the interstate electronic coupling energies in the guest molecules. The energies of the lowest excited electronic states of the perfluoroalkanes [ 42 ] are certainly far higher than those of the thione triplets. Finally, we note that replacing H by F has the effect of eliminating all the high-frequency C-H stretching vibrations in the solvent, so that perturbations of the high-frequency C-H promoting modes in the excited guest molecules by the host should be minimized. The perfluoroalkanes do, however, act as efficient vibrational thermalizers, even on a time scale of a few picoseconds. Perfluoroalkanes have previously been used successfully to provide inert media for the study of the second excited singlet states of these same thiones [ 24,25 1. The lifetimes of these states range from less than live to several hundreds of picoseconds, but even here vibrational relaxation in the perlluoroalkane solvents is sufficiently rapid that the shapes of the S2-+ So emission spectra are completely independent of the initial vibrational energy content of the SZ state. There can be no doubt that the longerlived triplets are fully vibrationally and rotationally relaxed. One should also note that the triplet energies are sufficiently large ( > 13000 cm-’ ) that these molecules would be classified as statistical limit cases even in the absence of a condensed host medium. The phosphorescence spectra of several of the thiones examined in the present work have been reported previously for a variety of host media at temperatures between 1.0 and 295 K [4,5,7,36]. In nonpolar, inert solvents the T, states are clearly of n, rc* electron configuration and ‘AZ electronic character. Both semi-empirical calculations [ 7,431 and zero-
field splitting parameter measurements [ 4,5,43] demonstrate that the second triplet state, of 3A, character, lies with ~2500 cm-’ of the ‘A2 state in PT, BPT and XT. Although we have good evidence that the ‘A2 and ‘A1 states are inverted in polar solvents such as acetonitrile [ 141, there is no doubt that T1 is of 3A2 electronic character in pertluoroalkanes at room temperature or in low temperature Shpolskii matrices [ 36 1. The phosphorescence spectra of the rigid aromatic thiones in non-polar solvents at room temperature invariably consist of a number of broad bands which diminish monotonically in intensity to longer wavelengths [ 7,8,14,15 1, and which exhibit total widths of z 3700-4500 cm-‘. (The latter is similar to the phosphorescence spectra of aromatic hydrocarbons [ 44 1.) This structure becomes better resolved in less strongly interacting solvents, becomes still sharper at 77 K, and consists of narrow lines at liquid He temperatures [ 4,5,36]. The most useful phosphorescence spectrum has been reported by Mahaney and Huber [ 361 who measured well-resolved corrected spectra of XT in a Shpolskii matrix at 15 K. The unresolved structure at higher temperatures is shown to consist of contributions from several ring vibrational modes, with a minor contribution from the C=S stretching frequency. The origin is the strongest band in the spectrum, carrying z 25% of the total oscillator strength, and bands in various progressions fall off rapidly in intensity with decreasing energy. Non-rigid thiones like DMTBP shown little vibronic structure, likely indicating that a considerable fraction of the oscillator strength is carried by lowfrequency torsional modes in these molecules. Corrected spectra at 77 K [ 45 ] and uncorrected spectra at x < 1 K [ 5 ] suggest that TXT may suffer a distortion in its T1 state which is somewhat greater than other rigid thiones, as evidenced by its more slowly diminishing phosphorescence intensity profile towards longer wavelengths. With the possible exception of TXT, however, the balance of the rigid thiones studied here exhibit remarkably similar emission profiles and widths. One concludes that the equilibrium structures of the S, and T1 states at energies near their zero-point levels are quite similar in XT and the other rigid thiones. Having established that unperturbed intramolecular decay can be observed for dilute solutions of trip-
M. Szymariskiet al. /Photophysicsof thionetripletsin solution
lets in perfluoroalkanes, we now begin an analysis of the observed decay rates. The radiative, k,, and nonradiative, k,,,, rate constants for T1 decay in PF-1,3DMCH are calculated from k, = qS”,/rg, and k,,, = ( 1 -@) /r$, and are given in table 2. In 3-MP, similar expressions are used, but the non-radiative constants obtained (table 3) will be the sum of a number of first-order and pseudo-first-order terms, Ek,,,, whose detailed elaboration is unimportant hereN2. Note that the values of k, are identical, within experimental error, for the two solvents and, with the exception of the non-rigid DMTBP, fall within a narrow range between 3.2 x 10’ and 11 x IO3 s- ‘. The most remarkable aspect of these data is that the values of k, are so large - a factor of x lo5 larger than those of typical aromatic hydrocarbon triplets whose radiative lifetimes often fall within the l-30 s range [ 461. The large values of k, are, of course, completely consistent with the observations of S,+T, absorptions having oscillator strengths comparable to those of the companion n+rr.*, S,-+S, transitions in the same compounds. The qualitative conclusion is that the T, states possess large admixtures of singlet character as a result of strong spin-orbit coupling. We now consider the non-radiative rate constants, k,,. Previous experiments in which thione triplet lifetimes and phosphorescence quantum yields were measured in fluid solutions at room temperature produced data which exhibited only a qualitative energy gap dependence [ 7,16,2 1,401. The reasons for this are now apparent. Previous experim,ents were done either at thione concentrations too large to afford accurate estimates of &,0 and rp,, or in solvents commonly considered to be inert but which nevertheless stongly perturb the excited thione (particularly those with large ET) and substantially increase their apparent non-radiative rate constants (cf. table 3). The present data, which provide accurate values of @gand r?, in non-perturbing pertluoroalkane solvents, provide a good quantitative energy gap law correlation (vide infra ) . The spectroscopic data suggest that, in the rigid thiones, the T, surface is not strongly displaced relative to the So surface, whereas the photochemical data suggest that the T, states are bound. Therefore the #*For a discussion of a detailed similar analysis applied to the Sz state see ref. [ 251.
151
weak coupling limit described by Englman and Jortner [ 471 and others, known to apply to the radiationless decay of aromatic hydrocarbon triplets, should provide an adequate model for our purposes. In this model k, = (21r)“2c2 YG r A(fiOM A.E)1/2exp ( - -> fifz0h.j ’
(1)
where, in our case, bE is the T,-So electronic energy gap (table 2), fro, is the energy of the highest frequency vibrational modes in the molecule, and C is the electronic matrix element coupling the initial and final states. The parameter y is given by y=ln (2AE/&fz0&&) - 1 ,
(2)
where AM and dare the reduced displacement and the degeneracy of the mode of maximum frequency, and contains the pertinent information about the relative displacements of the two potential energy surfaces. We assume, for the moment, that only the high-frequency C-H stretching modes need be considered as important accepting modes in the So state, as is the case for aromatic hydrocarbon triplets having sufficiently large ET [ 48 1. Assuming C is constant for the thiones under consideration, and ignoring the weak dependence of the preexponential term on AE, a linear correlation between log k,,, and AE is to be expected. A plot of the data is given in fig. 4. A reasonably good linear correlation is obtained by taking the data for all the undeuterated compounds. The intercept is 2.8 x lOI SK’, and from the slope a value of ~~4.22 is obtained. From the latter and using A,!?=15000 cm-’ where k,,=1.6x105 s-‘, we calculate C= 1 x lo3 cm-‘. The observed linear relationship between log k,, and AE suggests that the energy gap law is operative, i.e. the variation of k,, with AE is due to the exponential decrease in the FranckCondon factor for the T, wSo transition with increasing AE. However, the value of the electronic matrix element, C= 1 x 1O3 cm- ’ is apparently much too large for a radiationless transition which is nominally spin forbidden. We thus consider a more detailed analysis of the problem following the work of Siebrand and coworkers [ 48-5 11. The data are plotted, fig. 5, in the empirical form
M. Szymariski et al. /Photophysics of thione triplets in solution
152
Fig. 5. Semilogarithmic plot of k,, versus (ET-E,,) /q. A value of 4000 cm- ’ is used for EO. The filled circle is for XT-d,, for which a value of ED= 5500 cm-’ is used.
ET -Eo
log L& = log k!, - -
rl
f,
(3)
where q is taken as nH/nT, the fraction of the total number (&) of atoms in the molecule which are hydrogen andfis a proportionality constant (the slope) which is related to the inverse of the wavenumber of the C-H(D) stretching vibration cfx 10m4 cm in the aromatic hydrocarbons). E. is the energy below which C-C stretching or other modes are more effective than C-H stretching modes as sinks for the vibrational energy generated in the final state as a result of the radiationless transition. E. is taken as 4000 cm-’ because the full widths of the phosphorescence spectra of the rigid thiones are nearly the same as those of the aromatic hydrocarbons on which Siebrand did the original analysis. In the present work those molecules possessing free -CH3 or -OCH3 groups present a problem because one does not know, a priori, the extent to which the high-frequency C-H stretches in these groups can act as accepting or promoting modes. Including either all or none of the H atoms in these groups in the calculation of q results in points which fall well off the line drawn for those four thiones, PT, BPT, XT-d, and TXT, which have only ring H atoms. If the -CH3 and -OCH3 groups are ignored and replaced (for calculation purposes) by H or -CX, (X #H), then the resulting values of r] cause the log k,,versus (E-E,) / q data for these molecules to correlate much better with the other data. This suggests that the C-H
stretching vibrations in these nearly freely rotating groups are very weakly coupled to those of the ring carbon framework in these molecules. Nevertheless, because of the uncertainty in our understanding of how these modes participate in the radiationless decay, we do not consider TMIT, Br-TMIT, DMTBPT and DMTBP in any further analysis. The slope of the line drawn for the perhydromolecules is 2.2 x lop4per cm-‘, very nearly the same as that for the triplet hydrocarbons [ 48-5 11. Thus the Franck-Condon factors vary with ET in the same way for both classes of compound. This indicates that the intramolecularly unperturbed thione and hydrocarbon triplets behave similarly insofar as Franck-Condon prohibition of their T,*So radiationless transition rates is concerned. We conclude that the high-frequency C-H stretching modes are the most effective accepting vibrations in So at the energies spanned by the compounds examined. Only one deuterated compound, XT, has been examined. It is nevertheless possible to use the radiationless decay data for this one compound to test if the above Franck-Condon analysis for the perhydro compounds is reasonable. Assuming the intercepts at (ET- E,)/q=O are the same for both the perhydro and perdeutero compounds, one obtains a value of the ratio of the slopes of 1.27 f 0.07. This should be compared with a value of 1.36 expected on the basis of the square root of the ratio of the reduced masses, (IUCH/~cD)“2, and a value of 1.35 observed for the aromatic hydrocarbon triplets [ r48-5 11. The effect of perdeuteration is thus as expected if Franck-Condon factors for the C-H(D) stretching modes primarily govern the rates of T1 -So raditionless decay. The intercepts of these plots can be employed to provide some semi-quantitative information about the electronic matrix elements, C,, of the TL*SO transitions in these thiones. For triplet hydrocarbons =3.5x lo5 s-1, whereas for triplet thiones CL,HC s-l. Since kzr,T/kzr,HC= k:r,T = (9.1&2)x10” cc/c”,, = 2.6~ 106, using C,,x 1 x lO-*lcm-’ we calculate C -,-x 16 cm- ‘. Given the considerable uncertainty in the determination of kt,,,, the fact that C, is of the same order as the zero-field splitting parameter, 1D* 1, observed by Maki and co-workers [ 5,431 for XT triplets (24 cm-‘) must be considered fortuitous. Nevertheless, the calculation of C, is certainly correct to well within an order of magni-
M. Szymariski et al. /Photophysics of thione triplets in solution
tude, and clearly illustrates the very large interstate coupling energies in these molecules. The variation of AY,,, with deuterium substitution pattern in XT indicates that all C-H oscillators contribute significantly, but perhaps not equally, to the overall Franck-Condon factor for the molecule. The data show that k,,, decreases monotonically with increasing deuterium substitution, but that substitution of the first four D atoms (at positions 1,5,6 and 8 ) has a bigger effect than substitution of the last four. We expect that substitutions at C ( 1) and C (8 ) have the greatest effect since the largest change in ring carbon electron density occurs at these atoms, and previous work [ 7,52,53] has demonstrated that the two effects are well correlated. The overall effect of perdeuteration is apparently normal, however, at least for XT triplets. The triplet thione thus behaves differently from the second excited singlet state in which we recently showed [ 541 that large amplitude motion of H atoms at the C ( 1) and C (8 ) positions promotes S2* So decay.
Acknowledgement
The authors wish to acknowledge with gratitude the continuing financial support of the Natural Sciences and Engineering Research Council of Canada. Financial support under Polish Research Project CPRB 8.14 is also gratefully acknowledged.
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