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Photophysics of the second excited singlet states of cyclic enethiones
Journal of Photochemistry
and Photobiology,
A: Chemistry, 47 (1989)
PHOTOPHYSICS OF THE SECOND STATES OF CYCLIC ENETHIONES V. PUSHKARA
RAOt
EXCITED
277 - 286
277
SINGLET
and R. P. STEER$
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N OWO (Canada) (Received
August
16,1988;
in revised form January 5, 1989)
Summary The spectroscopic and photophysical parameters of the second excited singlet states of several enethiones have been measured in inert perfluoroalkane solvents. Two rigid, related enethiones, l,ldimethyl-Z-thioxo-l,Zdihydronaphthalene and thiocoumarin, behave in accordance with the energy gap law and decay predominately via SZ -A-+ S1 w T, . 1,1,3Trimethyl-2-thioxo-1,2dihydronaphthalene, with a methyl group at an sp’ carbon cy to the thiocarbonyl moiety, exhibits a much faster non-radiative decay rate. Thioisophorone and 4H-pyran-4-thione are structurally flexible and decay non-radiatively from S2 at rates much faster than the rigid bicychc enethione, thioverbenone. The results are rationalized in terms of the theory of radiationless transitions.
1. Introduction Over the past 15 years the photophysics and photochemistry of molecules containing the thiocarbonyl functional group have received increasing attention [ 1 - 31. Many interesting properties of the electronic excited states of these compounds are not exhibited by their carbonyl analogs. Of importance, a growing number of thiocarbonyls are now known to fluoresce [4 - 71 and react chemically [2, 3, 81 from their second excited singlet (S,) states, in “violation” of Kasha’s rule [9]. However, despite the activity in this field, relatively few classes of thiocarbonyl-containing compounds have been investigated to date, and a full explanation of why some but not all of them exhibit S2 fluorescence or chemical reaction is still not available. Two groups of thiones have received the most study to date. The rigid aromatic thiones 14 - 6, lo], such as xanthione, are photostable on excitation to SZ in inert solvents, fluoresce from SZ to So with relatively high tPresent U.S.A. z Author
address:
Department
to whom
correspondence
1010-6030/89/$3.50
of
Chemistry,
Columbia
University,
New
York,
NY,
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
278
quantum yields, and decay primarily by internal conversion to S, at rates which are in accord with the energy gap law of radiationless transition theory Ill]. In contrast, the SZ states of the rigid alicyclic thiones [ 12, 131, such as adamantanethione, are almost non-fluorescent [14] and decay to the ground state primarily by a mechanism which does not involve direct ----A-S, internal conversion [ 151. Their intramolecular S2 decay paths s2 have not been unequivocally identified, but there is some evidence supporting a reversible photoisomerization mechanism involving the transfer of hydrogen atoms p to the thiocarbonyl group [ 16,171. Even in the aromatic thiones a small fraction of their S, states appear to undergo direct decay to So by a mechanism involving large amplitude motion of hydrogen atoms, again located “0” to the thiocarbonyl moiety [lo]. The enethiones, in which the thiocarbonyl chromophore is conjugated with a single olefinic group, provide another opportunity to examine the excited state behavior of these types of compounds. Some cyclic enethiones are relatively stable, and both aromatic and non-aromatic species have been synthesized. The photochemistry and photophysics of the lowest excited states of several such compounds have been investigated [18], and the photocycloaddition of the S2 state of one aromatic enethione to several electron deficient olefins has been reported [19]. To our knowledge, however, there has been no systematic investigation of the photophysics of the S2 states of these molecules.
2. Experimental details The four enethiones examined in this study have structures 1 to 4 in Fig. 1. All were synthesized from the corresponding ketones using methods described previously 118 - 201. They were purified by column chromatography on silica gel and their purity was verified by gas chromatography and by absorption and emission spectrophotometry. Perfluoroalkane solvents (PCR Chemicals) were purified by distillation and silica-gel column chromatography and were checked for hydrogen-containing impurities by NMR. Other solvents were purified by standard methods. Steady state absorption and emission spectra were recorded using Cary (Varian) 118C and Spex Fluorolog 2 spectrometers respectively, the latter controlled by a Datamate computer. Fluorescence quantum yields were measured by the relative method previously described [ 4,21] , taking care to use appropriate solvent refractive index corrections. Quinine sulfate in 0.1 N = 0.51 [22]) was used as a standard. Fluorescence quantum yields HzSO4 (4f could be measured with a precision of 24% and an estimated accuracy of f 10%. All samples were deoxygenated prior to use by flushing with oxygenfree argon. All measurements were taken at room temperature. Fluorescence lifetimes were measured by exciting with either the third or the fourth harmonic of a double-beam, passively mode-locked Nd-YAG laser system at the Canadian Center for Picosecond Laser Flash Photolysis,
279
a S
I
a 0
\ S
s
Fig. 1. Structures of the enethiones and related molecules. 1, thioisophorone; 2, thioverbenone; 3, 1 ,l -dimethyl-2-thioxo-1,2-dihydronaphthalene; 4, 1 ,1,3-trimethyl-2-thioxo1,2-dihydronaphthalene; 5, 4H-1-benzopyran-4-thione; 6, 2,2,3,3-tetramethylindanethione; 7, thiocoumarin; 8,4H-pyran-4-thione.
Concordia University, Montreal. The excitation pulse at half height was about 30 ps and the energy was varied between 0.5 and 2 mJ per pulse. Deconvolution of the observed, digitized emission profile from the excitation pulse was effected by obtaining the best fit to an assumed singleexponential sample fluorescence decay. Estimated absolute errors in the measured lifetimes are about 15 - 20 ps. Details of the excitation laser and the fast detection systems used to measure the emission decays have been given elsewhere [23]. 3. Results and discussion The UV-visible absorption and emission spectra of thioisophorone (1) in perfluorodimethylcyclohexane are shown in Fig. 2. The corresponding spectra of thioverbenone (2) are nearly identical to those of 1, whereas the spectra of the arylated molecules, 1 ,l -dimethyl- thioxo-I,2 dihydronaphthalene (3) and 1 ,1,3-trimethyl-2-thioxo-1,2dihydronaphthalene (4)) are similar to those shown in Fig. 1, but are shifted some 60 nm to the red and are narrower. These bands may be assigned to AI-‘ ‘ A, or IA’-lA’ transitions (taking either C, or C, local symmetry respectively), in which the upper state is the second excited singlet of 7~,n*character. The fact that the SZ energies of the arylated species are lower than those of 1 and 2 is therefore readily understood in terms of the extended conjugation of the x system in the former. In addition to the bands shown in Fig. 1, weak S, + S, and T, + So absorption systems, associated with n + 7r* one electron transitions, are located in the red region of the spectrum, whereas the T, + S,
280
% d 2
250
400
550 (nm)
WAVELENGTH
Fig. 2. Sz-So emission and absorption spectra of 1 ,I ,3-trimethyl-2-thioxo-1,2-dihydronaphthalene in perfluorodimethylcyclohexane at 295 K. The emission spectrum of the Sz + S,J absorption system. excited at h,, TABLE
was
1
Spectral properties
of a&unsaturated
Datum
Ea2*( Sz-So)
(cm-r)
e_(STSO)
(M-l
thiones in perfluoroalkane
solvents 4
1
2
3
30000
29000
25000
25000
19100
16700
17600
19500
AC r,y(S2--Se) (cm-‘)
6030
5710
4930
4930
APyg( s2-So)
(cm-‘)
4180
3900
4230
3750
Stokes shifta (cm-‘)
10300
9900
5850
5070
Egbs (Sr-So)
17000
17000
15000
15000
e,,_(,S-So)
cm-
-1)
(cm-l) (M-r
cm-
~1 1
AE(S2-Sl)b (cm-‘) aBetween maxima of absorption bApproximate. See text.
34
26 13000 and emission
12000
18 10000
24 10000
spectra.
phosphorescence bands are located in the red and near-IR. All the thiones examined in this work exhibit S, + SO fluorescence located in the spectral window between the first and second singlet absorptions. These observations are in complete accord with those for a large number of other aromatic and alicyclic thiones in non-polar solvents [4 - 8, 151. The important spectral parameters for all four thiones examined in this work are collected in Table 1. It is relatively straightforward to obtain accurate values of the lowest triplet state energies of these molecules from measurements of their vibrationally resolved phosphorescence spectra in non-polar solvents. However,
281
the locations of the origin bands of their S,-S, and S2-So systems are difficult to determine accurately because the former exhibit only poorly resolved vibrational structure and are overlapped by the relatively intense triplet absorption systems, whereas the latter show no structure at all. Crude estimates of the S1 and SZ energies may be obtained by assuming that the origin is the first observable, partially resolved vibrational feature w+-So in the S1 absorption spectrum to the blue of the T, + So origin, and that the onset of the S2 absorption approximately locates the SZ + So origin. The widths (Ai& 9 full width at half-maximum) and the Stokes shifts of the absorption and emission systems increase in the order 4 = 3 < 2 < 1, indicative of progressively larger distortions and displacements of their SZ surfaces relative to their ground state surfaces. Such observations are consistent with the notions that the extended conjugation in 4 and 3 makes these arylated molecules more rigid than the simple cyclic enethiones, and that the bicyclic thione, 2, is more rigid than the monocyclic one, 1. The S2-So absorption and fluorescence spectra of these thiones do not exhibit resolved vibrational structure either in n-hexane or in perfluoroalkane solvents. However, both the absorption and emission maxima are located furthest to the blue in perfluoroalkane solvents, and suffer a red shift of about 200 - 300 cm-l in n-hexane, in agreement with similar effects observed for several aromatic thiones [4, 51. Both spectroscopic and photophysical evidence suggest that perfluoroalkanes interact extremely weakly with dissolved solutes [24]. In other thione systems perfluoroalkane solvents are sufficiently chemically and physically inert that the excited thione decays by primarily intramolecular processes, with the solvent acting only as a classical heat bath [25]. Although not all the reasons why the interactions between perfluoroalkane and excited thione are so weak are well understood, it is nevertheless possible to eliminate some possible ambiguities in interpreting photochemical and photophysical data (uide infru) if the SZ decay parameters of the thiones are measured in perfluoroalkane solvents. The quantum yields of SZ + So fluorescence, #f, the excited state lifetimes, TV,, the S2 electronic energies, E(&), and the approximate S2-S, electronic energy gaps, AE(S,-S,), of enethiones 1 - 4, all taken in perfluoroalkane solvents, are summarized in Table 2. Also tabulated are the values of the radiative, 12,, and non-radiative, k,,, rate constants for S2 decay, calculated from k, = &/rs, and k,, = (1 - &)/T~,_ The relative errors in the lifetimes in the range ~a, < 50 ps are much larger than the relative errors in the quantum yields. Therefore more reliable estimates of the excited state lifetimes of 1 and 4 were obtained by scaling the lifetimes of 2 and 3 respectively by the ratios of the quantum yields #I(l)/+(2) and #(4)/$(3). This results in values of k,(l) = k,(2) and k,(3) = k,(4), as is reasonable because of the strong simila&ies in the absorption spectra of each pair of compounds. The cumulative errors in k,,(l) and k,,(4) may, however, be as much as a factor of 2. To demonstrate the interaction between the S,-enethione and a hydrocarbon solvent, the fluorescence quantum yields and the SZ lifetimes of the same compounds were also measured in n-hexane.
282 TABLE
2
Sz-Sa fluorescence alkanea
quantum
yields and Sz decay
Parameters
of enethiones
in perfluoro-
Datum
1
2
3
4
@f(s2-wb
0.0018
0.041
0.026
0.0033
7szc (PS) k, x lo-’ k,
(s-l)
x 1O-9 (s-l)
AE(S2-Sl)
(cm-‘)
&“l@f”” * Ts*pE/rs,Hex e
11
250
153
19
16
16
17
17
91
3.9
6.4
59
13000
12000
10000
10000
2.0 -
5.6 5.1
1.5 1.5
1.4 -
aIn Perfluorodimethylcyclohexane or perfluoro-n-hexane at room temperature. Thione] = loss M. Deoxygenated solutions. i *lo%. CEstimated accuracy for compounds 2 and 3 is +(15 - 20) ps. ~(1) = 7(2)@(l)/@(2) and 7( 4) = T( 3)$( 4)/@( 3) are calculated. *Ratio of quantum yields in perfluoroalkane to hexane solvents. eLifetimes could not be measured for 1 and 4 in n-hexane. They are presumed to be less than 30 ps.
We begin our discussion of these data by identifying the possible intermolecular and intramolecular processes which could contribute significantly to the decay. Potential intermolecular processes, namely bimolecular self-quenching by ground state thione, quenching by O2 and quenching by any other ground state solution component (M), may be eliminated by measuring the decay parameters under carefully selected conditions. Selfquenching occurs at near diffusion-controlled rates [ZI], but this process will contribute negligibly to the decay when the thione concentrations are less than lo-“ M because the S2 lifetimes are all less than a nanosecond. For the same reason, even crude deoxygenation of the perfluoroalkane solutions will easily eliminate 0, quenching even though the solubilities of oxygen in the perfluoroalkanes are unusually large. Based on a comparison of the decay parameters obtained in n-hexane with those obtained in perfluoroalkanes (Table Z), it may also be concluded that hydrocarbons are potential quenchers. Both TV, and & of thiones 1 - 4 are substantially larger in perfluoroalkanes than in n-hexane. The largest differences in rs and @r which result from substituting perfluoroalkane solvent for n-hexane are obtained for 2, whereas moderate differences are obtained for the rest of the enethiones examined. Although no general correlation between the magnitude of the solvent effect and Es or any other parameter was observed for these thiones, the fact that boih rs and & change by nearly the same relative amounts when the solvent is ‘changed should be taken as a measure of the good internal consistency of the measurements. These observations are consistent with our earlier studies on
283
aromatic thiones in which both the lifetimes and the fluorescence quantum yields of several rigid aromatic thiones vary dramatically with the choice of the solvent [4, 261. By spiking perfluoroalkane solutions with various addends, we have also shown that molecules such as 3-methylpentane and benzene efficiently quench thione Sz + S,, fluorescence despite the facts that (1) the energies of the lowest excited singlet states of these quenchers are higher than those of the thiones, (2) the quantum yields of net photuchemical consumption of the thiones are very small, and (3) no direct evidence of either ground or excited state complexes is found [4]. The details of the mechanism(s) by which an encounter leads to product formation or physical quenching remain unknown, although various cases of hydrogen bonding, charge transfer or other molecular interactions such as those described by Morokuma [27] can be envisaged. Whatever the nature of the interactions, our previously published results on aromatic thiones and the present results on the enethiones suggest that only a relatively slight intermolecular perturbation, perhaps with an energy of the order of a few hundred reciprocal centimeters [ 51, is required for the addend or solvent to assist strongly in the electronic relaxation of the excited thione. Having excluded contributions from intermolecular processes to the quenching of the second excited singlet states of the thioenones by measuring their decay parameters in dilute (lO-5 M) deoxygenated perfluoroalkane solutions, both the radiative and the non-radiative rate constants calculated from the quantum yields and the lifetimes obtained in perfluorodkanes may safely be taken as measures of the intramolecular decay characteristics of the second excited singlet states of these thioenones. The possible intramolecular Sz radiationless decay processes include internal conversion to S,, internal conversion to S1, intersystem crossing to the triplet manifold, and intramolecular photodecomposition. or photorearrangement. According to the energy gap law [ll], k,, should increase exponentially with decreasing electronic energy gap between the coupled states, all other factors influencing the transition remaining constant. Therefore, direct Sz w SO internal conversion is unlikely to compete with internal conversion to S1, considering that AE(S,-SO) B- AE(S,-S,). However, apparently no satisfactory correlation is obtained between the calculated non-radiative rate constants and the S,-S, electronic energy gaps of the enethiones examined in this study. Part of the difficulty in interpreting these results resides in the fact that the spectra do not permit accurate values of E(S2) and E(S,) to be obtained. Nevertheless, 1 and 2 exhibit almost identical spectra and must therefore have nearly identical S2-SI energy gaps. Similarly 3 and 4 must have similar values of AE(S,-S,). Nevertheless, within each pair of similar compounds, the k,, values differ by a factor of about 10 or more. S1 internal converThese observations might suggest that direct S2 sion promoted by high frequency C-H stretching vibrations is not the only dominant non-radiative decay process in some, if not all, the thiones examined. There have been suggestions that intersystem crossing from excited
284
singlet states to the triplet manifold may compete successfully with internal conversion in some molecules [ 28, 291. The lowest triplets of thiones 1 to 4 are located only 1000 - 2000 cm-’ below S, . However, the location and the nature of the upper triplets of these thiones are unknown, making it impossible to be completely unequivocal about the identity of the radiationless process responsible for relaxation of the enethione second excited singlets. Nevertheless, the differences observed in the non-radiative rate constants for the thione pairs 1 and 2, and 3 and 4, can reasonably be attributed to differences in the rigidity of the molecular framework in which the chromophore is found. Radiationless transitions from S,w T, (n > 1) need not be invoked, and, in the absence of evidence to the contrary, are assumed not to be important _ We consider the arylenethiones first, and compare their photophysical behavior with that of 4H-l-benzopyran4-thione (5), 2,2,3,3&etramethylindanethione (6), and thiocoumarin (7). The S2 state of thiocoumarin has previously been reported to be non-fluorescent in benzene and to decay to its lowest triplet state with a quantum yield $+ of 0.9 [ 181. Because its S2--S, oscillator strength is similar to those of 3 and 4, and it is also a rigid molecule, the SZ -+ S, radiative rates of all three molecules (3,4, and 7) are expected to be similar. The fact that 7 is non-fluorescent in benzene can thus be attributed to two effects. First, thiocoumarin is reported [18] to have a relatively small S2-S1 energy gap of about 6200 cm-l. Based upon the energy gap law, a relatively large Franck-Condon factor for its S2* S, internal conversion is therefore expected, compared to 3 and 4 which have larger values of AE(&-S 1). Since the quantum yield of S1 w~chfT, intersystem crossing is near unity [18 3, a large value of & reflects an equally large quantum yield of S2 M S, internal conversion in thiocoumarin, as expected if the energy gap law holds. Second, benzene is known to exhibit strong physical interactions with the SZ states of several related aromatic thiones [4]. Such interactions result in an increase in the observed rate of radiationless de&y, whereas the rate of SZ + So radiative decay is affected only slightly. For example, the lifetime of xanthione in its S2 state is 175 ps in perfluoroalkanes [4, 51, but is reduced to about 12 ps in benzene [30] _ Both intermolecular solvent assistance and the small inherent S,-S, energy gap therefore tend to favor rapid radiationless decay, resulting in a small (unobservable) fluorescence yield in benzene. 1 ,l -Dimethyl-2-thioxo-1,2dihydronaphthalene (3) Passesses two methyl groups at a saturated carbon ~1 to the thiocarbonyl group. The presence of such cxmethyl groups apparently has no significant effect on the rates of SZ radiationless decay of relatively rigid thiones because 2,2,3,3tetramethylindanethione (6) exhibits an SZ -+.-,+ S, decay rate which is completely consistent with the energy gap law [5]. The presence of a hydrogen atom at an sp2 carbon (;Yto the thiocarbonyl group also apparently has no anomalous effect on the radiationless decay because the S, states of 5 and other similar thiones also have values of k,, which are consistent with the energy gap law 153. Thus 3, which possesses both methyl groups at a
285
saturated carbon and a hydrogen atom at an sp2 carbon, both cyto the C=S group, should behave in a manner which is consistent with the other rigid aromatic thiones if intermolecular interactions are minimized through the use of perfluoroalkane solvents. It is impossible to tell if the measured value of knr= 6.4 X lo9 s-lfor 3 is consistent with the energy gap law established for the other rigid aromatic thiones because of the inability to estimate its S2-S1 energy gap accurately. Nevertheless, 3 does decay from S2 d S, -&+ T1 with high quantum yield (& = 0.8)) and in that sense behaves similarly to the other aromatic thiones [ 311. The only structural difference between 3 and 4 is that a hydrogen atom is replaced by a CH, group at the sp2 carbon CYto the thiocarbonyl moiety in the latter. Their absorption and emission spectra are very similar, as expected for molecules which are nearly structurally identical. Nevertheless, the value of k,, for 4 is about 10 times larger than that for 3. The additional methyl group must therefore serve to promote radiationless decay. The enhancement of the radiationless decay rate most likely occurs via a “free rotor” effect (see ref. 32 for a discussion) which is facilitated by a@-unsaturation in the enethione. The fact that there will be substantial shortening of the central “single” bond in the C=C-C=S structure [33], associated with its acquisition of additional r character in the S2 state, is significant. Note that the lifetime of 4 in hexane must be less than 19 ps, considerably shorter than the value of 730 ps estimated on the basis of triethylamine quenching of its S2 fluorescence in cyclohexane [19]. Similar comparisons may be made between 1,2 and k-pyran4-thione (8). Compound 8 has been investigated previously [ 5, 341 and exhibits a very low S2 + So fluorescence quantum yield (about 1 X 10m4), and an S2 lifetime shorter than 20 ps. Its absorption and emission spectra are broad and structureless and reveal a large Stokes shift of about 10 000 cm-l, similar to those of 1. Its S2 non-radiative decay rate is much faster than those of the rigid aromatic thiones having similar S2-S1 energy gaps, owing to its lack of structural rigidity [ 51. The two cyclic enethiones studied here (1 and 2) both have similar values of AE(S2-S1). Nevertheless, the S2 state of 1 exhibits a value of k,, which is considerably larger than that of 2. The presence of an additional bridging methylene group makes 2 considerably more rigid than 1. It is therefore likely that vibrations leading to skeletal deformations are promoting modes in the non-radiative decay of the S2 states of these molecules.
Acknowledgments The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support. The assistance of Dr. D. Sharma, Head of Laser Operations, Canadian Center for Picosecond Laser Flash Photolysis, and of Dr. K. J. Falk is gratefully acknowledged.
286
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
R. P. Steer, Rev. Chem. Intermed., 4 (1981) 1. J. D. Coyle, Tetrahedron, 41 (1985) 5393. V. Ramamurthy, Org. Photochem., 7 (1985) 231. A. Maciejewski, D. R. Demmer, D. R. James, A. Safarzadeh-Amiri, R. E. Verrall and R. P. Steer, J. Am. Chem. Sot., IO7 (1985) 2831. A. Maciejewski, A. Safarzadeh-Amiri, R. E. Verrall and R. P. Steer, Chem. Phys., 87 (1984) 295. M. Mahaney and J. R. Huber, Chem. Phys., 9 (1975) 371. M. H. Hui, P. de Mayo, R. Suau and W. R. Ware, Chem. Phys. Lett., 31 (1975) 257. P. de Mayo, Act. Chem. Res., 9 (1976) 52. M. Kasha, Discuss. Faraday SOC., 9 (1950) 14. S. R. Abrams, M. Green, R. P. Steer and M: Szymanski, Chem. Phys. Lett., I39 (1987) 182. R. Englman and J. Jortner, Mol. Phys., 18 (1970) 145. K. Y. Law, P. de Mayo and S. K. Wong, J. Am. Chem. Sot., 99 (1977) 5813. K. Y. Law and P. de Mayo, J. Am. Chem. Sot., 101 (1979) 3251. K. J. Falk, A. R. Knight, A. Maciejewski and R. P. Steer, J. Am. Chem. Sot., 106 (1984) 8293. K. J. Falk, Ph.D. Thesis, University of Saskatchewan, 1988. D. S. L. Blackwell, K. H. Lee, P. de Mayo, G. L. R. Petrasunias and G. Reverdy, Nouv. J. Chim., 3 (1979) 123. A. Couture, J. Gomez and P. de Mayo, J. Org. Chem., 46 (1981) 2010. K. Bhattacharyya, P. K. Das, V. Ramamurthy and V. P. Rao, J. Chem. SOC., Faraday Trans. 2. 82 (1986) 135. V. P. Rao and V. Ramamurthy, J. Org. Chem., 50 (1985) 5009. V. P. Rao and V. Ramamurthy, Tetrahedron, 41 (1985) 2169. A. Maciejewski and R. P. Steer, J. Photochem., 24 (1984) 303. S. Meech and D. Phillips, J. Photochem., 23 (1983) 193. N. Serpone, M. A. Jamieson, D. K. Sharma, R. Danesh, F. Bolletta and M. Z. Hoffman, Chem. Phys. Lett., 104 (1984) 87. D. C. Dong and M. A. Winnik, Can. J. Chem., 62 (1984) 2560. K. F. Freed and J. Jortner, J. Chem. Phys., 52 (1970) 6272. A. Maciejewski and R. P. Steer, J. Am. Chem. Sot., IO5 (1983) 6738. K. Morokuma, Act. Chem. Res., IO (1977) 294. A. Maciejewski and R. P. Steer, J. Photochem., 35 (1986) 59, and references cited therein. W. A. Wassam, Jr., and E. C. Lim, J. Chem. Phys., 68 (1978) 433. R. W. Anderson, Jr., R. M. Hoechstrasser and H. J. Pownall, Chem. Phys. Lett., 43 (1976) 224. M. Szymanski, M. Maciejewski and R. P. Steer, J. Phys. Chem., 92 (1988) 6939. N. J. Turro, Modern Molecular Photochemistry, Benjamin/Cummings, Menlo Park, CA, 1978, p. 170 ff. R. H. Judge and D. C. Moule, J. Chem. Phys., 80 (1984) 4646. M. Szymanski, R. P. Steer and A. Maciejewski, Chem. Phys. Lett., 135 (1987) 243.
and Photobiology,
A: Chemistry, 47 (1989)
PHOTOPHYSICS OF THE SECOND STATES OF CYCLIC ENETHIONES V. PUSHKARA
RAOt
EXCITED
277 - 286
277
SINGLET
and R. P. STEER$
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan S7N OWO (Canada) (Received
August
16,1988;
in revised form January 5, 1989)
Summary The spectroscopic and photophysical parameters of the second excited singlet states of several enethiones have been measured in inert perfluoroalkane solvents. Two rigid, related enethiones, l,ldimethyl-Z-thioxo-l,Zdihydronaphthalene and thiocoumarin, behave in accordance with the energy gap law and decay predominately via SZ -A-+ S1 w T, . 1,1,3Trimethyl-2-thioxo-1,2dihydronaphthalene, with a methyl group at an sp’ carbon cy to the thiocarbonyl moiety, exhibits a much faster non-radiative decay rate. Thioisophorone and 4H-pyran-4-thione are structurally flexible and decay non-radiatively from S2 at rates much faster than the rigid bicychc enethione, thioverbenone. The results are rationalized in terms of the theory of radiationless transitions.
1. Introduction Over the past 15 years the photophysics and photochemistry of molecules containing the thiocarbonyl functional group have received increasing attention [ 1 - 31. Many interesting properties of the electronic excited states of these compounds are not exhibited by their carbonyl analogs. Of importance, a growing number of thiocarbonyls are now known to fluoresce [4 - 71 and react chemically [2, 3, 81 from their second excited singlet (S,) states, in “violation” of Kasha’s rule [9]. However, despite the activity in this field, relatively few classes of thiocarbonyl-containing compounds have been investigated to date, and a full explanation of why some but not all of them exhibit S2 fluorescence or chemical reaction is still not available. Two groups of thiones have received the most study to date. The rigid aromatic thiones 14 - 6, lo], such as xanthione, are photostable on excitation to SZ in inert solvents, fluoresce from SZ to So with relatively high tPresent U.S.A. z Author
address:
Department
to whom
correspondence
1010-6030/89/$3.50
of
Chemistry,
Columbia
University,
New
York,
NY,
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
278
quantum yields, and decay primarily by internal conversion to S, at rates which are in accord with the energy gap law of radiationless transition theory Ill]. In contrast, the SZ states of the rigid alicyclic thiones [ 12, 131, such as adamantanethione, are almost non-fluorescent [14] and decay to the ground state primarily by a mechanism which does not involve direct ----A-S, internal conversion [ 151. Their intramolecular S2 decay paths s2 have not been unequivocally identified, but there is some evidence supporting a reversible photoisomerization mechanism involving the transfer of hydrogen atoms p to the thiocarbonyl group [ 16,171. Even in the aromatic thiones a small fraction of their S, states appear to undergo direct decay to So by a mechanism involving large amplitude motion of hydrogen atoms, again located “0” to the thiocarbonyl moiety [lo]. The enethiones, in which the thiocarbonyl chromophore is conjugated with a single olefinic group, provide another opportunity to examine the excited state behavior of these types of compounds. Some cyclic enethiones are relatively stable, and both aromatic and non-aromatic species have been synthesized. The photochemistry and photophysics of the lowest excited states of several such compounds have been investigated [18], and the photocycloaddition of the S2 state of one aromatic enethione to several electron deficient olefins has been reported [19]. To our knowledge, however, there has been no systematic investigation of the photophysics of the S2 states of these molecules.
2. Experimental details The four enethiones examined in this study have structures 1 to 4 in Fig. 1. All were synthesized from the corresponding ketones using methods described previously 118 - 201. They were purified by column chromatography on silica gel and their purity was verified by gas chromatography and by absorption and emission spectrophotometry. Perfluoroalkane solvents (PCR Chemicals) were purified by distillation and silica-gel column chromatography and were checked for hydrogen-containing impurities by NMR. Other solvents were purified by standard methods. Steady state absorption and emission spectra were recorded using Cary (Varian) 118C and Spex Fluorolog 2 spectrometers respectively, the latter controlled by a Datamate computer. Fluorescence quantum yields were measured by the relative method previously described [ 4,21] , taking care to use appropriate solvent refractive index corrections. Quinine sulfate in 0.1 N = 0.51 [22]) was used as a standard. Fluorescence quantum yields HzSO4 (4f could be measured with a precision of 24% and an estimated accuracy of f 10%. All samples were deoxygenated prior to use by flushing with oxygenfree argon. All measurements were taken at room temperature. Fluorescence lifetimes were measured by exciting with either the third or the fourth harmonic of a double-beam, passively mode-locked Nd-YAG laser system at the Canadian Center for Picosecond Laser Flash Photolysis,
279
a S
I
a 0
\ S
s
Fig. 1. Structures of the enethiones and related molecules. 1, thioisophorone; 2, thioverbenone; 3, 1 ,l -dimethyl-2-thioxo-1,2-dihydronaphthalene; 4, 1 ,1,3-trimethyl-2-thioxo1,2-dihydronaphthalene; 5, 4H-1-benzopyran-4-thione; 6, 2,2,3,3-tetramethylindanethione; 7, thiocoumarin; 8,4H-pyran-4-thione.
Concordia University, Montreal. The excitation pulse at half height was about 30 ps and the energy was varied between 0.5 and 2 mJ per pulse. Deconvolution of the observed, digitized emission profile from the excitation pulse was effected by obtaining the best fit to an assumed singleexponential sample fluorescence decay. Estimated absolute errors in the measured lifetimes are about 15 - 20 ps. Details of the excitation laser and the fast detection systems used to measure the emission decays have been given elsewhere [23]. 3. Results and discussion The UV-visible absorption and emission spectra of thioisophorone (1) in perfluorodimethylcyclohexane are shown in Fig. 2. The corresponding spectra of thioverbenone (2) are nearly identical to those of 1, whereas the spectra of the arylated molecules, 1 ,l -dimethyl- thioxo-I,2 dihydronaphthalene (3) and 1 ,1,3-trimethyl-2-thioxo-1,2dihydronaphthalene (4)) are similar to those shown in Fig. 1, but are shifted some 60 nm to the red and are narrower. These bands may be assigned to AI-‘ ‘ A, or IA’-lA’ transitions (taking either C, or C, local symmetry respectively), in which the upper state is the second excited singlet of 7~,n*character. The fact that the SZ energies of the arylated species are lower than those of 1 and 2 is therefore readily understood in terms of the extended conjugation of the x system in the former. In addition to the bands shown in Fig. 1, weak S, + S, and T, + So absorption systems, associated with n + 7r* one electron transitions, are located in the red region of the spectrum, whereas the T, + S,
280
% d 2
250
400
550 (nm)
WAVELENGTH
Fig. 2. Sz-So emission and absorption spectra of 1 ,I ,3-trimethyl-2-thioxo-1,2-dihydronaphthalene in perfluorodimethylcyclohexane at 295 K. The emission spectrum of the Sz + S,J absorption system. excited at h,, TABLE
was
1
Spectral properties
of a&unsaturated
Datum
Ea2*( Sz-So)
(cm-r)
e_(STSO)
(M-l
thiones in perfluoroalkane
solvents 4
1
2
3
30000
29000
25000
25000
19100
16700
17600
19500
AC r,y(S2--Se) (cm-‘)
6030
5710
4930
4930
APyg( s2-So)
(cm-‘)
4180
3900
4230
3750
Stokes shifta (cm-‘)
10300
9900
5850
5070
Egbs (Sr-So)
17000
17000
15000
15000
e,,_(,S-So)
cm-
-1)
(cm-l) (M-r
cm-
~1 1
AE(S2-Sl)b (cm-‘) aBetween maxima of absorption bApproximate. See text.
34
26 13000 and emission
12000
18 10000
24 10000
spectra.
phosphorescence bands are located in the red and near-IR. All the thiones examined in this work exhibit S, + SO fluorescence located in the spectral window between the first and second singlet absorptions. These observations are in complete accord with those for a large number of other aromatic and alicyclic thiones in non-polar solvents [4 - 8, 151. The important spectral parameters for all four thiones examined in this work are collected in Table 1. It is relatively straightforward to obtain accurate values of the lowest triplet state energies of these molecules from measurements of their vibrationally resolved phosphorescence spectra in non-polar solvents. However,
281
the locations of the origin bands of their S,-S, and S2-So systems are difficult to determine accurately because the former exhibit only poorly resolved vibrational structure and are overlapped by the relatively intense triplet absorption systems, whereas the latter show no structure at all. Crude estimates of the S1 and SZ energies may be obtained by assuming that the origin is the first observable, partially resolved vibrational feature w+-So in the S1 absorption spectrum to the blue of the T, + So origin, and that the onset of the S2 absorption approximately locates the SZ + So origin. The widths (Ai& 9 full width at half-maximum) and the Stokes shifts of the absorption and emission systems increase in the order 4 = 3 < 2 < 1, indicative of progressively larger distortions and displacements of their SZ surfaces relative to their ground state surfaces. Such observations are consistent with the notions that the extended conjugation in 4 and 3 makes these arylated molecules more rigid than the simple cyclic enethiones, and that the bicyclic thione, 2, is more rigid than the monocyclic one, 1. The S2-So absorption and fluorescence spectra of these thiones do not exhibit resolved vibrational structure either in n-hexane or in perfluoroalkane solvents. However, both the absorption and emission maxima are located furthest to the blue in perfluoroalkane solvents, and suffer a red shift of about 200 - 300 cm-l in n-hexane, in agreement with similar effects observed for several aromatic thiones [4, 51. Both spectroscopic and photophysical evidence suggest that perfluoroalkanes interact extremely weakly with dissolved solutes [24]. In other thione systems perfluoroalkane solvents are sufficiently chemically and physically inert that the excited thione decays by primarily intramolecular processes, with the solvent acting only as a classical heat bath [25]. Although not all the reasons why the interactions between perfluoroalkane and excited thione are so weak are well understood, it is nevertheless possible to eliminate some possible ambiguities in interpreting photochemical and photophysical data (uide infru) if the SZ decay parameters of the thiones are measured in perfluoroalkane solvents. The quantum yields of SZ + So fluorescence, #f, the excited state lifetimes, TV,, the S2 electronic energies, E(&), and the approximate S2-S, electronic energy gaps, AE(S,-S,), of enethiones 1 - 4, all taken in perfluoroalkane solvents, are summarized in Table 2. Also tabulated are the values of the radiative, 12,, and non-radiative, k,,, rate constants for S2 decay, calculated from k, = &/rs, and k,, = (1 - &)/T~,_ The relative errors in the lifetimes in the range ~a, < 50 ps are much larger than the relative errors in the quantum yields. Therefore more reliable estimates of the excited state lifetimes of 1 and 4 were obtained by scaling the lifetimes of 2 and 3 respectively by the ratios of the quantum yields #I(l)/+(2) and #(4)/$(3). This results in values of k,(l) = k,(2) and k,(3) = k,(4), as is reasonable because of the strong simila&ies in the absorption spectra of each pair of compounds. The cumulative errors in k,,(l) and k,,(4) may, however, be as much as a factor of 2. To demonstrate the interaction between the S,-enethione and a hydrocarbon solvent, the fluorescence quantum yields and the SZ lifetimes of the same compounds were also measured in n-hexane.
282 TABLE
2
Sz-Sa fluorescence alkanea
quantum
yields and Sz decay
Parameters
of enethiones
in perfluoro-
Datum
1
2
3
4
@f(s2-wb
0.0018
0.041
0.026
0.0033
7szc (PS) k, x lo-’ k,
(s-l)
x 1O-9 (s-l)
AE(S2-Sl)
(cm-‘)
&“l@f”” * Ts*pE/rs,Hex e
11
250
153
19
16
16
17
17
91
3.9
6.4
59
13000
12000
10000
10000
2.0 -
5.6 5.1
1.5 1.5
1.4 -
aIn Perfluorodimethylcyclohexane or perfluoro-n-hexane at room temperature. Thione] = loss M. Deoxygenated solutions. i *lo%. CEstimated accuracy for compounds 2 and 3 is +(15 - 20) ps. ~(1) = 7(2)@(l)/@(2) and 7( 4) = T( 3)$( 4)/@( 3) are calculated. *Ratio of quantum yields in perfluoroalkane to hexane solvents. eLifetimes could not be measured for 1 and 4 in n-hexane. They are presumed to be less than 30 ps.
We begin our discussion of these data by identifying the possible intermolecular and intramolecular processes which could contribute significantly to the decay. Potential intermolecular processes, namely bimolecular self-quenching by ground state thione, quenching by O2 and quenching by any other ground state solution component (M), may be eliminated by measuring the decay parameters under carefully selected conditions. Selfquenching occurs at near diffusion-controlled rates [ZI], but this process will contribute negligibly to the decay when the thione concentrations are less than lo-“ M because the S2 lifetimes are all less than a nanosecond. For the same reason, even crude deoxygenation of the perfluoroalkane solutions will easily eliminate 0, quenching even though the solubilities of oxygen in the perfluoroalkanes are unusually large. Based on a comparison of the decay parameters obtained in n-hexane with those obtained in perfluoroalkanes (Table Z), it may also be concluded that hydrocarbons are potential quenchers. Both TV, and & of thiones 1 - 4 are substantially larger in perfluoroalkanes than in n-hexane. The largest differences in rs and @r which result from substituting perfluoroalkane solvent for n-hexane are obtained for 2, whereas moderate differences are obtained for the rest of the enethiones examined. Although no general correlation between the magnitude of the solvent effect and Es or any other parameter was observed for these thiones, the fact that boih rs and & change by nearly the same relative amounts when the solvent is ‘changed should be taken as a measure of the good internal consistency of the measurements. These observations are consistent with our earlier studies on
283
aromatic thiones in which both the lifetimes and the fluorescence quantum yields of several rigid aromatic thiones vary dramatically with the choice of the solvent [4, 261. By spiking perfluoroalkane solutions with various addends, we have also shown that molecules such as 3-methylpentane and benzene efficiently quench thione Sz + S,, fluorescence despite the facts that (1) the energies of the lowest excited singlet states of these quenchers are higher than those of the thiones, (2) the quantum yields of net photuchemical consumption of the thiones are very small, and (3) no direct evidence of either ground or excited state complexes is found [4]. The details of the mechanism(s) by which an encounter leads to product formation or physical quenching remain unknown, although various cases of hydrogen bonding, charge transfer or other molecular interactions such as those described by Morokuma [27] can be envisaged. Whatever the nature of the interactions, our previously published results on aromatic thiones and the present results on the enethiones suggest that only a relatively slight intermolecular perturbation, perhaps with an energy of the order of a few hundred reciprocal centimeters [ 51, is required for the addend or solvent to assist strongly in the electronic relaxation of the excited thione. Having excluded contributions from intermolecular processes to the quenching of the second excited singlet states of the thioenones by measuring their decay parameters in dilute (lO-5 M) deoxygenated perfluoroalkane solutions, both the radiative and the non-radiative rate constants calculated from the quantum yields and the lifetimes obtained in perfluorodkanes may safely be taken as measures of the intramolecular decay characteristics of the second excited singlet states of these thioenones. The possible intramolecular Sz radiationless decay processes include internal conversion to S,, internal conversion to S1, intersystem crossing to the triplet manifold, and intramolecular photodecomposition. or photorearrangement. According to the energy gap law [ll], k,, should increase exponentially with decreasing electronic energy gap between the coupled states, all other factors influencing the transition remaining constant. Therefore, direct Sz w SO internal conversion is unlikely to compete with internal conversion to S1, considering that AE(S,-SO) B- AE(S,-S,). However, apparently no satisfactory correlation is obtained between the calculated non-radiative rate constants and the S,-S, electronic energy gaps of the enethiones examined in this study. Part of the difficulty in interpreting these results resides in the fact that the spectra do not permit accurate values of E(S2) and E(S,) to be obtained. Nevertheless, 1 and 2 exhibit almost identical spectra and must therefore have nearly identical S2-SI energy gaps. Similarly 3 and 4 must have similar values of AE(S,-S,). Nevertheless, within each pair of similar compounds, the k,, values differ by a factor of about 10 or more. S1 internal converThese observations might suggest that direct S2 sion promoted by high frequency C-H stretching vibrations is not the only dominant non-radiative decay process in some, if not all, the thiones examined. There have been suggestions that intersystem crossing from excited
284
singlet states to the triplet manifold may compete successfully with internal conversion in some molecules [ 28, 291. The lowest triplets of thiones 1 to 4 are located only 1000 - 2000 cm-’ below S, . However, the location and the nature of the upper triplets of these thiones are unknown, making it impossible to be completely unequivocal about the identity of the radiationless process responsible for relaxation of the enethione second excited singlets. Nevertheless, the differences observed in the non-radiative rate constants for the thione pairs 1 and 2, and 3 and 4, can reasonably be attributed to differences in the rigidity of the molecular framework in which the chromophore is found. Radiationless transitions from S,w T, (n > 1) need not be invoked, and, in the absence of evidence to the contrary, are assumed not to be important _ We consider the arylenethiones first, and compare their photophysical behavior with that of 4H-l-benzopyran4-thione (5), 2,2,3,3&etramethylindanethione (6), and thiocoumarin (7). The S2 state of thiocoumarin has previously been reported to be non-fluorescent in benzene and to decay to its lowest triplet state with a quantum yield $+ of 0.9 [ 181. Because its S2--S, oscillator strength is similar to those of 3 and 4, and it is also a rigid molecule, the SZ -+ S, radiative rates of all three molecules (3,4, and 7) are expected to be similar. The fact that 7 is non-fluorescent in benzene can thus be attributed to two effects. First, thiocoumarin is reported [18] to have a relatively small S2-S1 energy gap of about 6200 cm-l. Based upon the energy gap law, a relatively large Franck-Condon factor for its S2* S, internal conversion is therefore expected, compared to 3 and 4 which have larger values of AE(&-S 1). Since the quantum yield of S1 w~chfT, intersystem crossing is near unity [18 3, a large value of & reflects an equally large quantum yield of S2 M S, internal conversion in thiocoumarin, as expected if the energy gap law holds. Second, benzene is known to exhibit strong physical interactions with the SZ states of several related aromatic thiones [4]. Such interactions result in an increase in the observed rate of radiationless de&y, whereas the rate of SZ + So radiative decay is affected only slightly. For example, the lifetime of xanthione in its S2 state is 175 ps in perfluoroalkanes [4, 51, but is reduced to about 12 ps in benzene [30] _ Both intermolecular solvent assistance and the small inherent S,-S, energy gap therefore tend to favor rapid radiationless decay, resulting in a small (unobservable) fluorescence yield in benzene. 1 ,l -Dimethyl-2-thioxo-1,2dihydronaphthalene (3) Passesses two methyl groups at a saturated carbon ~1 to the thiocarbonyl group. The presence of such cxmethyl groups apparently has no significant effect on the rates of SZ radiationless decay of relatively rigid thiones because 2,2,3,3tetramethylindanethione (6) exhibits an SZ -+.-,+ S, decay rate which is completely consistent with the energy gap law [5]. The presence of a hydrogen atom at an sp2 carbon (;Yto the thiocarbonyl group also apparently has no anomalous effect on the radiationless decay because the S, states of 5 and other similar thiones also have values of k,, which are consistent with the energy gap law 153. Thus 3, which possesses both methyl groups at a
285
saturated carbon and a hydrogen atom at an sp2 carbon, both cyto the C=S group, should behave in a manner which is consistent with the other rigid aromatic thiones if intermolecular interactions are minimized through the use of perfluoroalkane solvents. It is impossible to tell if the measured value of knr= 6.4 X lo9 s-lfor 3 is consistent with the energy gap law established for the other rigid aromatic thiones because of the inability to estimate its S2-S1 energy gap accurately. Nevertheless, 3 does decay from S2 d S, -&+ T1 with high quantum yield (& = 0.8)) and in that sense behaves similarly to the other aromatic thiones [ 311. The only structural difference between 3 and 4 is that a hydrogen atom is replaced by a CH, group at the sp2 carbon CYto the thiocarbonyl moiety in the latter. Their absorption and emission spectra are very similar, as expected for molecules which are nearly structurally identical. Nevertheless, the value of k,, for 4 is about 10 times larger than that for 3. The additional methyl group must therefore serve to promote radiationless decay. The enhancement of the radiationless decay rate most likely occurs via a “free rotor” effect (see ref. 32 for a discussion) which is facilitated by a@-unsaturation in the enethione. The fact that there will be substantial shortening of the central “single” bond in the C=C-C=S structure [33], associated with its acquisition of additional r character in the S2 state, is significant. Note that the lifetime of 4 in hexane must be less than 19 ps, considerably shorter than the value of 730 ps estimated on the basis of triethylamine quenching of its S2 fluorescence in cyclohexane [19]. Similar comparisons may be made between 1,2 and k-pyran4-thione (8). Compound 8 has been investigated previously [ 5, 341 and exhibits a very low S2 + So fluorescence quantum yield (about 1 X 10m4), and an S2 lifetime shorter than 20 ps. Its absorption and emission spectra are broad and structureless and reveal a large Stokes shift of about 10 000 cm-l, similar to those of 1. Its S2 non-radiative decay rate is much faster than those of the rigid aromatic thiones having similar S2-S1 energy gaps, owing to its lack of structural rigidity [ 51. The two cyclic enethiones studied here (1 and 2) both have similar values of AE(S2-S1). Nevertheless, the S2 state of 1 exhibits a value of k,, which is considerably larger than that of 2. The presence of an additional bridging methylene group makes 2 considerably more rigid than 1. It is therefore likely that vibrations leading to skeletal deformations are promoting modes in the non-radiative decay of the S2 states of these molecules.
Acknowledgments The authors wish to thank the Natural Sciences and Engineering Research Council of Canada for continuing financial support. The assistance of Dr. D. Sharma, Head of Laser Operations, Canadian Center for Picosecond Laser Flash Photolysis, and of Dr. K. J. Falk is gratefully acknowledged.
286
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