J . Phys. Chem. 1990, 94, 5767-5772
5767
Thermally Activated Emission of Adamantanethione, Thiofenchone, and Thiocamphor K. J. Falkt and R. P.Steer* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0 (Received: November 28, 1989; In Final Form: February 9, 1990)
Measurements of the emission, absorption, and excitation spectra and triplet lifetimes of adamantanethione (I), thiofenchone (11), and thiocamphor (111) as a function of temperature and solvent polarity have established that their lowest triplets are of r,r*configuration. In all three molecules thermally activated phosphorescence is observed from a second, higher energy triplet of n,r* character which remains in thermal equilibrium with the lowest triplet at temperatures greater than 77 K. Triplet lifetimes of 101 and 43.5 ps are obtained for I in frozen perfluoro-1,3-dimethylcyclohexaneat 77 K and in infinitely dilute fluid perfluoro-l,3-dimethylcyclohexane at 295 K, respectively. The photophysical processes associated with the decay of the lowest triplet states are discussed,
Introduction Polyatomic molecules possessing closely spaced excited electronic states generally exhibit rather complex photochemistry and photophysics. Variations in the nature of the solvent, the temperature, and the nature and position of substituents all have important influences on the rates and mechanisms of excited-state relaxation as neighboring states either reorder in energy or couple with one another more or less effectively.' Various models involving intramolecular vibronic and spin-orbit coupling and intermolecular interactions have been invoked to assist in interpreting the experimental data in such systems2 In particular when n,r* and r,r* triplet states are in close proximity, as in xanthonef6 for example, or when states in the singlet and triplet manifolds are nearly isoergic, as in aromatic thiones,'-I0 both spin-orbit and vibronic coupling effects need to be considered in interpreting the electronic spectroscopy and the dynamics of photochemical and photophysical processes. Thermally activated delayed fluorescencell and thermally activated are sometimes observed in such systems. Although the mechanisms leading to the appearance of delayed luminescence have been known for many years,I2 the phenomena are observed relatively infrequently, and examples of thermally activated phosphorescence in polyatomic molecular systems are particularly rare. There is ample evidence that the lowest n,r* singlet state (S,) and the two lowest triplet states (TIand T2; one n,r* and one r,r*)all lie in close proximity in polyatomic t h i ~ n e s . & I ~ JThis ~'~ leads to the previous observation of thermally activated delayed fluorescence" from SIand inversion of the two triplets with changing solvent polarity* in aromatic thiones like xanthione. Strong interstate coupling effects have been suggested to explain both the photophysical and low-temperature phosphorescence behavior of these molecules. The latter e x p e r i m e n t ~ ~ * 'reveal ~J~-'~ unusually large zero-field splittings (ID*I 16-28 cm-') of the triplet sublevels of several aromatic thiones which have been attributed to effective spin-orbit coupling between T, and T, sublevels. Although the photophysics of the aromatic thiones has been investigated thoroughly,' the aliphatic thiones remain relatively unexplored. This is perhaps surprising because several alicyclic thiones have been the subject of very detailed photochemical investigations, and one, adamantanethione, has been described as a model compound"-22 for such work. In this paper we report the first observations of thermally activated emission in three alicyclic thiones, adamantanethione, thiofenchone, and thiocamphor (Figure l), and interpret these in terms of the interactions among nearby electronic states. Experimental Section All emission and emission excitation spectra were taken on a Spex Model 222 spectrofluorometer equipped with a "Datamate" N
To whom correspondence should be addressed. 'Resent address: National Research Council of Canada, Ottawa, Ontario KIA OR6.
0022-3654/90/2094-5767$02.50/0
computer. Excitation spectra were recorded by using a rhodamine B quantum counter, and emission spectra were corrected by using the manufacturer's correction curve, the accuracy of which was verified by measuring the emission spectra of standard samples (Applied Photophysics, Set 8F). Absorption spectra were measured on a Cary I18 spectrophotometer. Emission quantum yields were measured by a relative method, using quinine sulfate in 0.1 N H2S04(& = 0.52)23as a reference and taking care to correct for solvent refractive index effects. The details of the techniques employed in these measurements have been recorded elsewhere.24v2s All low-temperature measurements were performed with a variable-temperature liquid nitrogen cryostat (Oxford Instruments, Model DN704) equipped with a digital temperature controller. Temperatures in the cell were measured to accuracies better than f l K with a platinum thermocouple which was calibrated from 77 to 315 K by use of solvent slushes. Although the sapphire windows supplied with the cryostat yielded sufficient emission to render impossible the measurement of weak (4f lo4) S2 So fluorescence in the 280-500-nm range,26 phosphorescence measurements at h > 500 nm were not seriously impeded. N
-
(1) Lim, E. C. In Excited States; Lim, E. C., Ed.;Academic Press: New York, 1977; Vol. 3, p 305. (2) Lim, E. C. J : Phys. Chem. 1986, 90, 6670 and references therein. (3) Connors, R. E.; Sweeney, R. J.; Cerio, F. J . Phys. Chem. 1987, 91,819. (4) Connors, R. E.; Christian, W. R. J . Phys. Chem. 1982, 86, 1524. (5) Griesser, H. J.; Bramley, R. Chem. Phys. Lett. 1981, 83, 287. (6) Scaiano, J. C. J . Am. Chem. SOC.1980, 102, 7747. (7) Ramamurthy, V.; Steer, R. P. Acc. Chem. Res. 1988, 21, 380 and references therein. (8) Maciejewski, A.; Szymanski, M.; Steer, R. P. Chem. Phys. Lett. 1988, 143, 559. (9) Maki, A. H.; Ghosh, S.;Petrin, M. J . Chem. Phys. 1988, 120, 299. (10) Maki, A. H.; Svejda, P.; Huber, J. R. Chem. Phys. 1978, 32, 369. (1 I ) Maciejewski, A.; Szymanski, M.; Steer, R. P. J . Phys. Chem. 1986, 90, 6314. (12) Parker, C. A. Photoluminescence of Solutions; Elsevier: Amsterdam, 1968. (13) Burland, D. M. J . Chem. Phys. 1981, 75, 2635. (14) Taherian, M. R.; Maki, A. H. Chem. Phys. Lett. 1983, 96, 541. (15) Taherian, M. R.; Maki, A. H. Chem. Phys. 1982, 68, 179. (16) Ghosh, S.; Petrin, M.; Maki, A. H. J . Phys. Chem. 1986, 90, 1643. (17) Ramamurthy, V. Org. Photochem. 1985, 7, 231 and references therein. (18) de Mayo, P. Acc. Chem. Res. 1976, 9, 52. (19) de Mayo, P.; Law, K. Y. J . Am. Chem. SOC.1979, 101, 3251. (20) Scaiano, J . C.; Tremblay, J . P.; Ingold, K. U. Can. J . Chem. 1976, 54, 3407. (21) Bolton, J. R.; Chen, K. S.; Lawrence, A. H.; de Mayo, P. J . Am. Chem. Soc. 1975, 97, 1832. (22) Blackwell, D. S. L.; Liao, C. C.; Loutfy, R. 0.; de Mayo, P.; Paszyc, S.Mol. Photochem. 1972, 4, 171. (23) Meech, S.; Phillips, D. J . Photochem. 1983, 23, 193. (24) Maciejewski, A.; Steer, R. P. J . Am. Chem. Soc. 1983, 105, 6738. (25) Maciejewski, A.; Demmer, D. R.; James, D. R.; Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. J . Am. Chem. SOC.1985, 107, 2831. (26) Falk, K. J.; Steer, R. P. J . Am. Chem. SOC.1989, 111, 6518.
0 1990 American Chemical Society
5168
The Journal of Physical Chemistry, Vol. 94, No. 15, I990
Falk and Steer 1.15 a 1
0
4
7
>
I1 -
I -
I11
5 v) t
Figure 1. Structures of adamantanethione (I), thiofenchone (It), and thiocamphor (I1 I ) I
(L
Triplet lifetimes were measured by using two methods. A cavity-dumped C W argon ion laser operating at 488 nm and a time-correlated single-photon-counting detection system, which have been described in detail elsewhere,27 were used in early experiments. With this apparatus, 30 ns wide laser pulses were used for excitation at a repetition rate chosen to allow at least I O phosphorescence lifetimes between pulses. Pulse pile-up was avoided by counting at a rate less than 3% of the excitation rate. Although relatively long data accumulation times were required to achieve good counting statistics, background noise of only ca. 20 counts per channel was observed when the cavity dumper was adjusted to give the maximum extinction ratio. Deconvolution was unnecessary owing to the shortness of the excitation pulses relative to the measured phosphorescence decay times. In later experiments an excimer-pumped dye laser and transient digitizer were used to record phosphorescence decays. The excitation system consisted of a Lumonics Hyper-ex 400 excimer laser operating on XeCl at 308 nm which pumped a Lumonics Hyper-dye 300 dye laser. For most experiments coumarin 540A dye in ethanol was used as the active medium, the output of which was frequency doubled by using an angle-tuned Inrad KDP-A crystal to produce 5-11s pulses at ca. 265 nm. Emission was collected at right angles to the exciting beam through a 7.5 cm focal length lens, and was focused onto the cathode of a Hamamatsu R955 photomultiplier tube. Cutoff filters were used to eliminate scattered exciting light. The PMT signal was fed directly to a Tektronix Model 7D20 transient digitizer which was triggered by signals from a second PMT viewing a small fraction of the excitation pulse. Data stored in the transient digitizer were dumped via an IEEE interface to a IBM personal computer equipped with a GPIB-PC interface card (National Instruments) where the data were massaged into a suitable format. The data were then dumped serially by using Kermit-32 software to the university’s VAX 8600 time-shared computer system for analysis. The decays were analyzed by using a least-squares fitting routine (LINFIT) described by Bevington.28 The “goodness of fit” was judged by the magnitude of the reduced chi-square value, x2, and the behavior of the autocorrelation function (C(t,))of the weighted residuals. The best fit of trial decay parameters to the experimental data is one exhibiting a x2 value approaching 1, while C(ti)exhibits high-frequency, low-amplitude random oscillations about zero. In all cases the observed phosphorescence decays were well described by single-exponential functions. Several measurements were made on identical samples using the two excitation and detection systems. The measured phosphorescence lifetimes agreed within 3% in all such tests. The methods used for sample preparation, sample and solvent purification, and sample degassing have been described in detail All samples were rigorously degassed prior to their use.
Results end Discussion Like ketones,29thiones excited to their lowest triplet states are known to decay in part by photochemical reaction in fluid hy(27) James, D. R.;Demmer, D. R. M.; Verrall, R. E.; Steer, R. P. Rev. Sei. Instrum. 1983, 54, 1 121. ( 2 8 ) Bevington, P. B. Data Reduction and Error Analysis for rhe Physical Sciences; Wiley: New York, 1970. (29) Parker, C . A.; Joyce, T. A . Trans. Faraday Soc. 1969, 65, 2823.
a
d I 0 WAVELENGTH ( n m )
Figure 2. Absorption (-) and emission (---) spectra of adamantanethione in perfluoro-1,3-dimethylcyclohexaneat 295 K.
drogen-containing In order to avoid such complications we chose to carry out all experiments (except those in which the effects of changing the solvent were to be examined) in inert perfluoroalkane solvents.33 The physical properties of perfluoroalkanes and their use in photochemical experiments recently have been reviewed thoroughly by M a ~ i e j e w s k i . ~ ~ The UV-visible absorption and emission spectra of adamantanethione taken a t room temperature in perfluoro-l,3-dimethylcyclohexane are shown in Figure 2. These are representative of the three alicyclic thiones studied here. The visible portion of the absorption spectrum is known from solvent shift,22 magnetic circular d i c h r o i ~ m electronic ,~~ circular dichroism,36 numerous computational studies on model compound^,^^ and vibrational analyses of the electronic spectra of small thiocarbonyls7 to consist of two overlapping transitions. The weak, longest wavelength feature (at 555 nm in adamantanethione) has been assigned unequivocally to a direct ground state to triplet absorption band. This is overlapped to the blue by the ground state to lowest excited singlet absorption, which is stronger by a factor of ca. 5 than the transition to the adjacent triplet and extends through much of the visible region to about 400 nm. Blackwell et a1.22 have shown that these two overlapping absorptions shift to the blue by approximately the same amount on freezing the solution to 77 K or on dissolving the compound in more polar solvents at room temperature. Our experiments confirm this. On the basis of these and other observations, the longest wavelength absorption systems have been assigned to transitions to upper states which are both of primarily n,r* character. The lowest energy feature is therefore associated with a (3A2) X(IA,) transition whereas t_he overlapping features to higher energy are associated with an A(’A,) X(’A,) transition, assuming C, local symmetry about the )C=S group. There is no evidence in the absorption spectrum under these conditions for a second triplet state of 3(r,r*) character (3A1 symmetry). This implies that, if present, a transition to a second triplet state must carry a significantly smaller oscillator strength than that of the observed 3A2 ‘Al transition. The emission spectrum obtained on S2 So excitation of each of the three thiones in fluid solution consists of a very weak, broad S2 So fluorescence band in the visible region26 and a much stronger, broad band with a maximum near 650 nm and a shoulder on its short-wavelength side. Lowering the temperature results in a monotonic increase in the overall intensity of emission in the
-
-
-
- -
(30) Coyle, J. D. Tetrahedron 1985, 41, 5393. (31) Maciejewski, A. J . Photochem. Photobiol. 1988, A43, 303. (32) Bruhlmann, V.; Huber, J. R. Chem. Phys. Lett. 1978.54, 606. (33) Szymanski, M.; Maciejewski, A,; Steer, R. P. Chem. Phys. 1988, 124, 143. (34) Maciejewski, A. J . Photochem. Photobiol. 1990, ASI, 87. (35) Engelbrecht, J. P.; Anderson, G . D.; Linder, R.E.; Barth, G.; Bunnenberg, E.; Djerassi, C.; Seamans, L.; Moscowitz, A. Spectrochim. Acta 1975, 3 l A , 507 and references therein. (36) Schippers, P. H.; Dekkers, H. P. J. M. Chem. Phys. 1982, 69, 19. Peyerimhoff, S. D.; Buenker, R. J. Chem. Phys. 1982, (37)Burton, P. G.; 73. 8 3 .
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5769
Photophysics of Thiones
-I
-3
a c (
\ 0 I Y
-c
-5
-7
Figure 4. Natural logarithm of the ratio IJI,, vs the inverse of temperature for adamantanethione in perfluoro-l,3-dimethylcyclohexane.
j a 0
u,
, & , - ,
,
,
,
650 WAVELENGTH ( n m )
500
1 800
Figure 3. Emission spectra of adamantanethione in perfluoro- 1,3-dimethylcyclohexane: (A) at 295 and 77 K, normalized to their maxima, and (a) the difference spectrum.
550-800-nm range. However, not all segments of the spectrum behave in the same way. This is illustrated by comparing the normalized emission spectrum of adamantanethione in liquid perfluoro- 1,3-dimethylcyclohexane at room temperature with that in the frozen solution at 77 K, as shown in Figure 3A. Note that the low-temperature emission spectrum is broad and unstructured and that the pronounced shoulder on the blue edge of the room temperature spectrum has disappeared at 77 K. This shoulder is assigned to thermally activated emission. The spectrum of this thermally activated emission is revealed by taking the difference between the high- and low-temperature spectra, as shown in Figure 3B. Because the overall red emission spectrum narrows slightly on lowering the temperature, the difference spectrum may not be a completely faithful representation of the thermally activated emission. Nevertheless, this spectrum exhibits a broadly structured pattern of intensity with a maximum at 560 nm, and a second major feature at 598 nm, ca. 1100 cm-' to the red. For adamantanethione at room temperature in perfluoro-l,3-dimethylcyclohexane this thermally activated emission constitutes about 14% of the total intensity. The spectra of thiofenchone and thiocamphor are similar, except that in each case the feature near 600 nm in the thermally activated emission spectrum is less well resolved. Consider the following minimal mechanism for the decay of phosphorescence and thermally activated emission in these systems. (Excitation may be accomplished by illuminating in any of the visible or near-UV absorptions since radiationless relaxation from the singlet to the triplet manifolds occurs very rapidly and with high quantum efficiency.26)
Et
iS
T1 TI
Et
K = kfwd/(krcv expl-AE/kTl)
(1)
-
(2)
-
SO+ hvp
TI
SO
El
+
kr,T knr.T kr.E knr,E
(3)
(4) (5)
Here El is the thermally activated emitter (of as yet unspecified identity), AE is the electronic energy difference between the lowest triplet, T I , and the thermally activated emitter, and the rest of
TABLE I: Spectroscopic Energy Difference, AE(SI-TI), and Energies for Thermal Activation of Emission, AE, for Three Alicyclic Thiones compound solvent AE(SI-TI),cm-' AE, cm-I adamantanethione PFDMCH' 1.5 X IO3 510 f 90 n-pentane 490 f 150 thiocamphor PFDMCH 1.4 X IO3 380 f 80 thiofenchone PFDMCH 1.5x 103
PFDMCH = perfluoro- 1,3-dimethyIcyclohexane.
the notation is obvious. Using a steady-state treatment for the case of continuous excitation in the visible region, one obtains, following Parker et a1.12 1, -=-
Ip
krcvkr,E
exp(-AE/kTj
kfwdknr.E
Figure 4 is an example of a graph of the natural logarithm of the ratio of the intensity of the thermally activated emission, I,, to that of the remainder of the emission, I as a function of 1/T. A good fit between the data and a straigEt line is obtained, the slope of which can be used to determine AE. Table I gives these "activation energies" and compares them with the n,a*singlettriplet energy gaps which are obtained from the origins of the weak overlapping absorption spectra (vide infra). It is clear that the values of AE obtained from the thermal activation experiments do not correspond with the n,r* singlet-triplet energy gaps in these compounds. Therefore, unlike the aromatic thiones," the emission I , is not thermally activated, E-type12 delayed fluorescence from S1. It is actually more appropriate to use singlet-triplet energy spacings obtained from emission spectra (in which the emitting molecules are surrounded by orientationally relaxed solvent species) when attempting to identify the thermally activated state in the above manner." Unfortunately, in the present cases it is impossible to determine precisely the location of the electronic origin bands of the TI-Sotransitions from solvent-relaxed excited states because these compounds exhibit unstructured phosphorescence spectra (at least at T > 77 K). However, for aromatic thiones in perfluoroalkane solvents" the differences between the energies of the SI and T I states measured from the positions of the origin bands in absorption are no more than ca.300 cm-l larger than those measured from emission spectra. Because the differences between AE and AE(S,-TI) (Table I) are about 1000 cm-' in adamantanethione and thiocamphor, solvent relaxation effects cannot account for the discrepancy. This again suggests that the thermally activated emitting state is not SI. We therefore assign the thermally activated emitting state to the second triplet, T2, because this is the only electronic state other than TI or SI which could lie at the required energy. We now proceed to validate this assignment and to identify TI and T2. Almost all thiones examined to date have lowest triplets
5770 The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
(TI) which are of n,r* character in nonpolar solvents. This is signaled by solvent shift experiments in which a blue shift of both the longest wavelength absorption band and the phosphorescence spectrum with increasing solvent polarity has been observed by several groups.8-11-22 In several aromatic thiones, the '(r,r*)state is situated siffuciently close to the '(n,r*) state that the n,r* and r,r* triplets can be inverted in energy in polar solvents.8 When this occurs the '(r,r*)phosphorescence is generally unstructured in fluid solutions at room temperature, whereas the phosphorescence from '(n,r*) states retains some residual vibronic structure.22 The latter is characterized by a spacing of ca. I100 cm-I between adjacent strong features due to overlapping ring deformation and C-S stretching vibration^.^.'^ These observations have exact parallels in the aromatic ketone, ~ a n t h o n e . ~ " , ' ~In~ this molecule TI is of ' ( r , x * )character in nonpolar media, and at low temperature (4 K) exhibits a vibronically cluttered spectrum with a Franck-Condon profile characteristic of a displaced oscillator system. However, at higher temperatures (77 K) a stronger phosphorescence assigned to the T2, ' ( n , r * ) state is ~ b s e r v e d . ~ The latter emission is dominated by a single progression in the C=O stretching mode and exhibits a very strong origin band. The similarities between these observations in xanthone and the spectra of Figure 3 are striking. We therefore tentatively assign the 'Al, 3(r,r*) state to T,, and the 'A,, 3(n,r*) state to T2 for solutions of the three alicyclic thiones in perfluoroalkane solvents at room temperature. This assignment is confirmed by comparing the effect of varying solvent polarity on the phosphorescence spectra of adamantanethione with those of aromatic thiones and conjugated ene-thiones, previously reported by de Mayo et aLz2and others. Only adamantanethione (the only unconjugated thione previously examined in detail) exhibits a phosphorescence spectrum which does not shift to the blue in more polar solvents. Our experiments confirm these observations and show that thiocamphor and thiofenchone behave similarly. We also note that the origin bands of the T, So transitions in absorption and emission (the lowest energy observable feature in the absorption spectrum and the strongest feature in the thermally activated emission spectrum, respectively) differ, owing to solvent relaxation effects, by ca. 200 f 50 cm-' in adamantanethione, 300 f 100 cm-l in thiocamphor, and 500 f 250 cm-I in thiofenchone-all measured in perfluoroalkane solvents. These differences are nearly the same as those due to relaxation in the same solvents measured for the n,r* singlet and triplet states of the aromatic thionesll-about 220-270 cm-l. Therefore, contrary to previous worker^,^^.^' we conclude that the 3Al state is lower in energy than the 'A2 state in adamantanethione, thiocamphor, and thiofenchone, even in nonpolar media such as perfluoro- I ,3-dimethylcyclohexane. One implication of this conclusion is that the T2 So transition must carry a significantly higher oscillator strength than the TI ** So transition so that the latter is not seen in absorption (Figure 2) under the conditions used in our experiments. There can be little doubt that the lowest energy feature obserued in the absorption spectrum of Figure 2 does in fact belong to the 'A2 state since this feature and the adjacent ]AAz state (both of n,r* configuration) behave similarly with changing temperature and solvent. By the above arguments, TI must therefore be connected to So by a radiative transition which has an oscillator strength at least a factor of 3 or 4 smaller than that of T2 So. This is entirely feasible because the )A2 'Al transition will "borrow"42 its intensity via spin-orbit coupling from the very strong B(IA1) X(lA1) transition which is next highest in energy in these molecules,26 whereas the 'A1 'AI transition will derive its intensity mostly from weaker (but nevertheless electric dipole allowed), higher energy Rydberg C( 'B,) X ( ' A , ) and b(IBI) X('A1) transition^.^^
-
-
-
-
-
-
-
(38) Mahaney, M.; Huber, J . R. J . Mol. Specfrosc. 1981, 87, 438. (39) Chakrabarti, A.; Hirota, N. J . Phys. Chem. 1976, 80, 2966. (40) Griesser, H. J.; Bramley, R. Chem. Phys. 1982, 67. 361 and 373. (41) Bhattacharyya, K.; Kumar. C. V.; Das, P. K.; Jayasree, B.; Ramamurthy, V J . Chem. Soc., Faraday Trans. 2 1985.81, 1383. (42) Orlandi, G.; Siebrand. W. J . Chem. Phys. 1973, 58, 4513. (43) Falk, K . J.: Steer, R . P. Can. J . Chem. 1987, 66, 5 7 5 .
Falk and Steer ABSORPTION
-
EMISSION RI
RI
-- - ----
- sz
Figure 5. Jablonski diagram for adamantanethione for an insert perfluoroalkane solvent. Energies are in cm-'.
-
-
It is well-knownu that both the 'A2 'Al and 'A, 'AI transitions could derive radiative intensity by vibronic coupling ?f the uppetstate with the nearby 'A2 (SI)state. However, the A('A2) X(lA1) (SI So) transition is itself electric dipole forbidden cfand should therefore contribute little to the intensities of these singlet-triplet transitions. It is also well-known45 that second-order perturbations can provide mechanisms for the interaction between singlet and triplet states of the same orbital symmetry even though no direct spin-orbit coupling between them is possible. On the basis of the known behavior of aromatic t h i o r ~ e s ? J ~ J however, ~-'~ we assume here that the most effective interstate coupling mechanism involves direct spin-orbit coupling between the z sublevel of the '(n,r*) state (of AI overall symmetry) and the I(r,r*), (lA1) second excited singlet state. Placing the 3Al state below that of the 'A2 state requires that the singlet-triplet splitting associated with their common thiocarbonyl T,T* electron configuration (S2-TI) be ca. 19000 cm-'. This is unusually large, but is reasonable in these molecules. The r,r* excited states of these alicyclic compounds possess two unpaired electrons confined largely to the thiocarbonyl moiety, in orbitals having very substantial spatial overlap. The correlation energy must therefore be extremely large, much larger than in the corresponding states in the aromatic thiones where delocalization over the r system results in S2-T2 ('AI-'AI) gaps of between about 6000 and 12000 cm-l. The latter estimates are based in part on the observations of optically detected magnetic resonance (ODMR) spectra and large zero-field splitting parameters in the triplet states of aromatic thionesPJ0-13-16 data which are not available in the present case. Nevertheless, indirect evidence for the magnitude of the r,x* singlet-triplet splittings in the alicyclic thiones can be obtained from large-scale CI calculations on thi~formaldehyde.~'A value of E(S2,1AI) - E(T2,'A,) = 3.46 eV (27 900 cm-I) is calculated for the vertical transition in this molecule. Because C H S is a reasonable model upon which estimates of the analogous singlet-triplet splittings might be based, a value of 23000 cm-I for the corresponding vertical energy difference (calculated from the spectra and Stokes shifts) in the alicyclic thiones appears reasonable. These data for adamantanethione in inert perfluoroalkane solvents are summarized in the Jablonski diagram, Figure 5.
-
-
(44) McGlynn, S. P.; Azumi, T.;Kinoshita, M. Molecular Spectroscopy of the Triplet Srate; Prentice-Hall: Englewood Cliffs, NJ, 1969. (45) Stevens, C . G.; Brand, J. C. D. J . Chem. Phys. 1973, 58, 3324.
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5771
Photophysics of Thiones
TABLE 11: Phosphorescence Lifetimes, Quantum Yields, and Radiative, Nonradiative, and Self-Quenching Rate Constants of Adamantanethione, Thiofenchone, and Thiocamphor in Several Solvents at Room Temperature
compound adamantanethione
@,O(S,) 0.020 0.004 0.022 0.004 0.012 f 0.002 0.072 f 0.015 0.029 0.005
solvent PFDMCH" PFHb
**
C6H12
thiofenchone thiocamphor
" PFDMCH
Tpo,
PFDMCH PFDMCH
c1s
43.3 f 6.OC 38.8 6.6 17.6 f 1.5 154 f 85 46.3 f 15
*
*
= perfluoro-l,3-dimethylcyclohexane.b P F H = perfluoro-n-hexane.
10-2 kr, s-I
10-4 knr,s-I
4.7 5.7 6.8 5.3 6.3
2.3 2.5 5.6 0.6 2.1
103 &steady M-1 %Ute
10-9 k:p8mi M-lcv s-I
3.0 8.4 3.9 2.9 5.7
11.4 f 0.8c 5.5 0.8 3.6 f 1.4 3.8 f 1.0
f 1.Ic
f 2.4 1.5 f 2.0 f 3.2
Errors are f 2 0 on the Stern-Volmer intercept
(T;)
or slope
(k,,J.
-
0.75-
!-?
0.50-
-I
\
? 0
-
0.25-
0.0 I 0.0 1
I
l
I
I
I
1
3
4
5
6
7
8
9
I
1
1 0 1 1
I
12
t
I:
1031 T ( ~ - 1 )
Figure 6. Natural logarithm of the inverse of the measured phosphorescence lifetimes of adamantanethione (4 X 10" M) in perfluoro1,3-dimethylcyclohexane as a function of TI.The filled circles are data of ref 22 for alkane and EPA glasses. The square is a datum taken from a relative quantum yield measurement.
Phosphorescence lifetimes were measured both for fluid and solid solutions of the three thiones in several solvents. Without exception the decay of the phosphorescence is well described by a single-exponential function. The phosphorescence lifetimes were measured as a function of both emission and excitation wavelength. In all cases, other factors remaining the same, identical lifetimes were obtained for excitation at wavelengths spanning both the S2 Soand SI S, absorption bands and for emission wavelengths spanning both the thermally activated emission from T2 and the prompt phosphorescence from TI. The latter observation is particularly important because it indicates that the rate of T, T2 interconversion is much more rapid than the overall rate of decay of the triplet system. Thus, in these alicyclic thiones, TI and T2 remain in thermal equilibrium at all temperatures above 77 K. Two temperature-dependent effects are observed on the phosphorescence: the total phosphorescence intensity increases whereas the thermally activated component diminishes with decreasing temperature. The variation in total phosphorescence intensity with decreasing temperature is accompanied by a parallel variation in the measured phosphorescence lifetime. A plot of In k , , = In T-' vs 1 / T for a 4 X 10" M solution of adamantanethione in perfluore 1,3-dimethylcyclohexaneis shown in Figure 6 . The lifetime of the emitting state increases moderately rapidly as the temperature decreases from 25 OC to ca. -55 O C , the freezing point of the solvent, and then increases only slightly as the temperature falls still further to 77 K. The steeper portion of the curve yields an "activation energy" of 7.5 kJ mol-', approximately that associated with diffusion in the fluid solvent.22*M The increase in phosphorescence lifetime and in the overall intensity of the phosphorescence is therefore associated with a decrease in triplet quenching rate with decreasing temperature. Both self-quenching and quenching by residual hydrogen-containing impurities in the perfluoroalkane solvent (ca. 0.1 atom
-
-
(46) Birks, J. B.; Lumb, M. D.; Munro, 1. H. froc. R. SOC.1964, ,4280, 289.
I
0.25
I
I
0.50 0.75 CONCENTRATION ( x 10-5 M )
I
I.(
Figure 7. Stern-Volmer plot for the self-quenching of the phosphorescence of adamantanethione in perfluoro-l,3-dimethylcyclohexane at room temperature.
% by NMR) might be responsible.
Figure 7 is a Stern-Volmer plot for the self-quenching of triplet adamantanethione obtained from the phosphorescence lifetime measurements. A lifetime of 43 f 6 CLS for the triplet at infinite dilution at room temperature is obtained by extrapolation of the data in Figure 7 . This compares favorably with the lifetime of 53 f 6 CLS obtained by extrapolation of the low-temperature ( T C -55 "C) data shown in Figure 6 to room temperature. We therefore conclude that the increase in phosphorescence intensity with decreasing temperature in fluid solutions is associated almost entirely with self-quenching. Values of the phosphorescence lifetimes and quantum yields for all three thiones in fluid solutions at room temperature are given in Table 11. Also given there are the self-quenching rate constants, k,, obtained from the slopes of the corresponding Stern-Volmer plots and the radiative and nonradiative decay rate constants obtained from k, = 9pO/~pOand knr= (1 - 9qO)/~pO, respectively. The self-quenching experiments were carried out by measuring both the phosphorescence lifetimes (dynamic) and the phosphorescence intensities (steady state) as a function of thione concentration. Both static and dynamic measurements yielded the same slopes, within experimental error. The data confirm that self-quenching occurs at near the diffusion-limited rate, as has been observed previously for many aromatic and unhindered aliphatic thione triplets.32-41*47-50 The lifetime of triplet adamantanethione at 77 K in perfluoro- 1,3-dimethylcyclohexane is 101 ps which compares favorably with values of 110 and 120 ps in alkane and EPA glasses, respectively, obtained by Blackwell et a1.22 The gradual decrease in the quantum yield of prompt phosphorescence and the concomitant decrease in the triplet lifetime from -195 t o -55 O C is clearly not associated with a change in the effective radiative rate constant, since the ratio 9 p / ~remains p constant within experi(47) Maciejewski, A. Chem. Phys. Let?. 1989, 164, 166. (48) Kumar, C. V.; Qin, L.;Das,P. K.J . Chem. Soc., Faraday Trans. 2 1984, 80, 183. (49) Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. Can. J . Chem. 1983, 61. 894. (50) Ramesh, V.; Rammath, N.; Ramamurthy, V. J . Photochem. 1983, 23. 141.
5772
J . Phys. Chem. 1990, 94, 5112-5778
mental error over this temperature range. Rather, the small decrease in fP and 4pwith increasing temperature in solid solution must be associated with a concomitant increase in the rate of nonradiative decay. No direct evidence of the process involved is available, but the small effective activation energy of 0.46 kJ mol-’ (38 cm-’) likely rules out a mechanism which involves more effective T,-So coupling via the population of higher vibrational states in T,. We note with interest, however, that the measured
energy spacing, 38 cm-], is of the same magnitude as the zero-field splitting parameters, ID*I, for the triplet states of several aromatic thionesgJ4 (16-28 cm-I), suggesting that radiationless decay via an upper spin sublevel of the triplet might be responsible. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for its continuing financial support.
An ab Initio Study of the Structures, Vibrational Spectra, and Energetics of the Homocyclic Sulfur Molecules S,, n = 4-8 David A. Dixon* and E. Wasserman Central Research and Development Department,? E. I. du Pont de Nemours and Company, Experimental Station, Wilmington, Delaware 19880-0328 (Received: May 30, 1989; In Final Form: December 13, 1989)
High-level ab initio calculations have been employed to determine the geometries of cyclic S,, n = 4-8. Gradient optimization was done at the SCF level with a polarized double-f basis set. Where available, agreement with experiment for the structures is better than has previously been available from ab initio calculations. Vibrational spectra were calculated for comparison to experiment. Energies relative to S,,have been obtained for n = 5-7. Comparison of energetic and spectroscopiccalculations to experiment indicates that S4 is not a ring. The reaction S, + S8 = 2S7 has a substantial entropy contribution to AG.
Introduction The higher levels of ab initio molecular orbital calculations are increasingly of merit in providing information for the experimentalist seeking a more complete view of a system. As a growing new scientific area, the results from numerical simulations can supplement the interpretation of a variety of measurements.! The family of homocyclic sulfur molecules, S,, is a rich source of molecular variety with a single element. The experimental structures show a variation of -0.2 A, or -IO%, in S-S bond length, an unusually large range for nominal single bonds, and IOo in S-S-S bond angle.2 Due to the work of SteudeP and S ~ h m i d ta, ~number of sulfur rings have been well-characterized experimentally. On the other hand, previous theoretical studies of these rings with molecular mechanics! semiempiri~al,~ or ab initio6 approaches have not yet reproduced the structural and vibrational spectroscopic parameters for these systems to the accuracy of theoretical studies of first-row elements. We have previously employed’ numerical simulation techniques (molecular orbital theory) to study structural and spectroscopic features of pentathiepins in regions of the potential energy surface inaccessible to present experiments. We report here a high-level a b initio study of the geometries, energetics, and vibrational spectra of the sulfur rings S,, n = 4-8. The structures of n = 6-8 are well-established experimentally but are uncertain for n = 4 and 5 . This group includes many of the most interesting structural features present in elemental sulfur rings. The larger rings, n 2 9, investigated experimentally by Steudel and co-workers2 are not considered here. It is important to note that these calculations are not substitutes for more experimentally coupled approaches, e.g., force-field analyses. The ab initio computations of the force field are done in the harmonic approximation and do not include electron,correlation as they are at the S C F level. In the treatment presented here for vibrations, frequency scaling is used to compensate approximately for anharmonicity and correlation. When theoretical assignments of vibrational frequencies are at variance with experimental conclusions, improved agreement might be obtained
by the future incorporation of additional terms in the vibrational Hamiltonian. Alternatively, another assignment for the observed frequency might be considered. Since a complete theoretical treatment should correspond to correct experimental assignments, disagreements should focus attention on possible changes in one approach or the other. With ab initio calculations of second-row elements being more demanding than those for the first row, we hope that the treatments reported here represent a step toward a deeper understanding of these electron-rich species.
Calculations The calculations were done with the programs HONDO* on an IBM-3081 computer and GRADSCF’ on a CRAY-1A or CRAYXMP/24 supercomputer. We have previously shown that a polarized double-!: basis set is required to calculate accurately the S-S bond length in H2S2.6J0 All calculations were thus done with the valence double-l basis set of McLean and Chandler” augmented by a set of polarization functions (Cd = 0.6) on sulfur giving a (12s8pld)/[5s3pld] basis set. The number of basis functions ( 1 ) Schaeffer, H. F., Ill Science 1986, 231, 1100. (2) (a) Steudel, R. Top. Curr. Cbem. 1982,102, 149. (b) Steudel, R. In Studies in Inorganic Chemistry; Miller, A., Krebs, B., Eds.; Elsevier: Amsterdam, 1984; Vol. 3, p 5. (3) Schmidt, M. Angew. Cbem., Int. Ed. Engl. 1973, 12, 445. (4) Kao, J.; Allinger, N. L. Inorg. Cbem. 1977, 16, 35. (5) (a) Dewar, M. J. S.; McKec, M. L. J. Comput. Cbem. 1983.4, 84. (b) Dewar, M. J. S.; Reynolds, C. H. Ibid. 1986, 7, 140. (c) Baird, N. C. Ibid. 1984, 5, 35. (d) Jug, K.; Iffert, R. Ibid. 1987, 8, 1004. (6) (a) Laitinen, R. S.; Randolph, B.; Pakkanen, T. A. J. Comput. Chem. 1987,8,658. (b) Kao, J. Inorg. Cbem. 1977, 16, 2085. (c) Ibid. 3347. (d) Feng, W.L.; Novaro, 0. Ini. J. Quantum Chem. 1984, 26, 521. (7) Chenard, B. L.; Dixon. D. A.; Harlow, R. L.; Roe, D. C.; Fukunaga, T.J. Org. Cbem. 1987. 52, 24. (8) (a) Dupuis, M.; Rys, J.; King, H. F. J . Chem. Phys. 1976.65, 1 1 I . (b) King, H. F.; Dupuis, M.; Rys,J. National Resourcefor Computer Chemistry Software CataIoK University of California, Berkeley, CA, 1980 Vol. I , program QHOZ (HONDO). ( 9 ) GRADSCF is an ab initio gradient program system designed and written
by A. Komornicki at Polyatomics Research. (IO) Dixon, D. A.; Zeroka, D.; Wendoloski, J. J.; Wasserman, Z. R. J.
Phys. Chem. 1985,89, 5334.
‘Contribution no. 4955.
(11)
0022-3654/90/2094-5712$02.50/0
McLean. A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72. 5639
0 1990 American Chemical Society
