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The phosphorescence of 4h-pyran-4-thione: large quantum yields from room-temperature fluid solutions
Volume 135, number 3
CHEMICAL PHYSICS LETTERS
THE PHOSPHORESCENCE OF 4H-PYRAN-CTHIONE: LARGE QUANTUM YIELDS FROM ROOM-TEMPERATURE
3 April 1987
FLUID SOLUTIONS
M. SZYMANSKI ‘, R.P. STEER ’ Department of Chemistry, University ofSaskatchewan, Saskatoon, Saskatchewan. Canada S7N OWO
and A. MACIEJEWSKI Faculty of Chemistry, A. Mickiewicz University, 60- 780 Poznan. Poland Received 11 November 1986
The spectra, phosphorescence quantum yields and triplet lifetimes of 4H-pyran-4-thione (PT) in fluid solution at room temperature have been measured. In inert perlluoroalkane solvents at 293 K, the phosphorescence quantum yield of PT is 0.33 on excitation to S2 and 0.47 on direct excitation to T,. The reasons for these extremely large radiative yields are discussed.
1. -Introduction Large quantum yields of T , + S,, phosphorescence, &, from organic compounds are usually only observed in glassy or crystalline solids at low temperature [ 1,2]. In fluid solution at room temperature, intermolecular quenching processes dominate, so that only a very few classes of compounds exhibit readily measurable phosphorescence quantum yields. Intermolecular solvent-induced relaxation can be minimized through the use of inert perfluoroalkane solvents which interact exceptionally weakly with electronically excited solutes [ 3-51. However, even in these cases the fraction of excited triplets which decay radiatively is relatively small; phosphorescence quantum yields of 0.034 for benzil, 0.097 for benzophenone, and 0.10 for acetophenone are representative of the ketones [ 51. Relatively intense phosphorescence has been reported for several thioketones in fluid solutions [ 6-101, but quantitative data are scarce. The most thoroughly studied thione is xanthione for which Huber and Mahaney
[ 81 have reported &=O.Ol in 3-methylpentane at 298 K. A value of &=0.024 has also been reported for 2,2,3,3_tetramethylindanethione in 3-methylpentane at room temperature [ lo]. We have previously shown that the S2 excited state lifetimes and the corresponding S2-& fluorescence quantum yields of aromatic thiones can be more than a factor of ten larger in perlluoroalkanes than in other photochemically inert solvents [ 11,121. Similar effects have also been found when the luminescence of ketones has been studied as a function of the nature of the solvent [ 3-51. It therefore seemed logical to examine the phosphorescence of the thiones in perfluoroalkane solvents. We chose 4H-pyran-4-thione (PT) as a model for this study because it has a large So-T1 energy gap [ 131 and therefore might be expected to exhibit a relatively slow T,-S, intramolecular radiationless decay rate. Studies of the absorption, emission [ 131 and triplet ODMR [ 141 spectra of PT, and its S2 decay dynamics [ 15 1, reactivity and dipole moment [ 161 have previously been reported.
’ Present address: Institute of Physics, A. Mickiewicz University, 60-780 Poznan, Poland. 2 To whom correspondence should be addressed.
0 009-2614/87/$ 03.50 0. Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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hexane (q+ 0.9 1 [ 20,2 1 ] was used as an emission standard. Quantum yields were measured with a precision of f 5% and an estimated accuracy of f 10%. All intensities reported refer to the photon flux.
2. Experimental PT was synthesized by the reaction of 4H-pyranone with P2S5 [ 17,181. The crude thione was purified by crystallization from MeOH and toluene under Ar or He and was stored at low temperature. Per( PF- 1,3-DMCH ) fluoro- 1,3-dimethylcyclohexane and 3-methylpentane (3-MP) were purified by fractional distillation followed by column chromatography. After purification these solvents showed no emission under the conditions used to study PT. All the steady-state and dynamic measurements were performed on solutions which had been deoxygenated by repeated freeze-pump-thaw cycles. Steady-state absorption and emission spectra were measured using Cary 118C (Varian) and Spex Fluorolog 2 spectrometers, the latter controlled by a “Datamate” computer. Emission lifetimes were measured using a Spectra Physics synchronously pumped cavity-dumped tunable dye laser system similar to the one previously described [ lo,19 1. Quantum yields of emission were measured by a relative method which has been described previously [ 111. 9, IO-diphenylanthracene in degassed cyclo-
-i E 0
3. Results and discussion The absorption and long wavelength emission spectra of PT in 3-MP at room temperature are shown in fig. 1. The symmetry-allowed So-& and symmetry-forbidden So-+ absorptions are found in the near UV and mid-visible regions, respectively. The longest wavelength absorption feature, located at x 582 nm, is assigned to the O-O band of the So-+T1 absorption and is clearly resolved from the So-& system. The emission consists mainly of T,-& phosphorescence, but a very weak feature at the blue edge of the spectrum is due to thermally activated delayed fluorescence from S, [ 13 1. These spectra are typical of the larger organic thiones in fluid solutions and are very similar to those obtained for 4H- 1-benzopyran4-thione and xanthione under similar conditions [ 6,221.
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LETTERS
t.
293 K
-i
zz m
;
14.
x1000
0-
--sl
z i
P
6 SO 4
WAVELENGTH Fig. 1. Absorption
244
and emission
(nm)
spectra of PT in 3-MP at 293 K.
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Volume 135, number 3
293 K
PT in 3-MP
PT in PF-I,3-DMCH
01
”
”
3
”
”
5
”
7 [PT]
9
”
II
”
13
15
1
(xIO-~M)
Fig. 2. Stem-Volmer plots for the self-quenching of PT phosphorescence in 3-MP and PF-1,3-DMCH.
The O-O bands of the So-+T1 and S,-S, absorption systems are located at 17100 and 17680 cm-‘, respectively, and have apparent molar extinction coefficients of 10.0 and 15.4 M- ’ cm- ‘, respectively, all in 3-MP. The maximum of the O-Oband of the T1+SO phosphorescence spectrum is located = 200 cm- ’ to the red of the O-O band in absorption at 16920 cm- ‘. From the absorption spectrum the Table 1 Photophysical properties of 4H-pyran-Cthione
S-T1 energy gap is therefore 580 cm-‘. The So-S2 absorption system is much more intense (en&IX = 16800 M-r cm-’ at 30490 cm-‘) and is structureless. Note that the !&So fluorescence of PT (h= 1.3x 10W4) has been omitted from fig. 1 for clarity. Sz+ S,, fluorescence is spectrally well resolved from the longer wavelength phosphorescence and delayed fluorescence and is readily distinguished by the much longer lifetime and the ease of oxygen quenching of the latter. Triplet lifetimes were measured as a function of PT concentration in 3-MP and PF-1,3-DMCH. The results are presented in fig. 2. PT was excited in its weak So-S1 absorption system, using the intense fundamental output of the R6G dye laser, and the lowest triplet was populated via efficient, rapid, radiationless decay from S,*T,. The decay of phosphorescence from Tr was well described by single exponential functions in all cases. The slopes and intercepts of the resulting Stern-Volmer plots (fig. 2) give, respectively, the bimolecular triplet selfquenching rate constants, kelB and the triplet lifetimes of PT at infinite dilution, r”(T1), in the two solvents. These data are tabulated in table 1. Self-quenching is known to occur at or near diffusion-controlled rates in thione systems. That triplet PT is no exception can be deduced from the values of kselfwhich are < 10” m-’ s-r at 293 K in both solvents. Note that k,,, is greater in the solvent of lower viscosity, 3-MP, as expected. Because kelf is so large, it was necessary to obtain r”(T1) by extrapo-
and acetophenone in solution at room temperature
PT in PF- 1,3-DMCH
4%W &TO r”UI) (ms) q(S,,T,)
kp(s-‘1 hsc (s-l) ke,r(M-Is-‘) P!$pt’, (cm-‘)
3 April 1987
CHEMICAL PHYSICS LETTERS
0.33kO.04 0.47 f. 0.07 (43+4)x10-3 0.70*0.07 1.1x104 1.2x 104 (1.36f0.06)~10’~ 16940
&,H,COCHx in PFMCH a)
R"'
0.10 0.39 1.0” 25.6 230.4 1.2x lo6 25300
4.7 0.11 0.7 430 52.1 l.lX104
in 3-MP 0.034 (6.5?0.5)xlO-3 0.91 kO.09 7.1x103 14.7x lo4 (2.6 t 0.2) x 10” 16920
a) Acetophenone in perfluorometylcyclohexane. Data from ref. [ 51. ‘) Ratio of the values of the parameters for PT in PF- 1,3-DMCHto acetophenone in PFMCH. ‘) Data from refs. [ 23-251. Values are for hydrocarbon solvents.
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lation. Data were taken at thione concentration from 2 x 10e6 to 2 x 1O- 5 M in order to avoid conditions in which self-quenching was the major deactivation channel. Good linear Stern-Volmer plots of the lifetime data were obtained, so it is unlikely that the extrapolation would introduce any significant systematic error. Values of rO(T,) of 43 + 4 and 6.5 k 0.5 J.LSare found for PT in PF-1,3-DMCH and 3-MP, respectively. Note that the lifetime is 6.6 times longer in the pertluoroalkane. In order to measure phosphorescence quantum yields, PT solutions (2 x 10e6 to 2x 10m5 M) were illuminated in the strong So-& absorption band and their corrected TI-+SO emission spectra were obtained. The phosphorescence quantum yields at each concentration, @r(S,), were then determined using the relationship: _fZ(v) drr I:
n2
@P(Sz)=~S -JZs( r) drr Z, ( n2)’
’
where & is the fluorescence quantum yield of the standard, the integrals are the areas under the corrected emission spectra, n is the refractive index of the solvent, Z, is the absorbed intensity, and the superscript S refers to the standard in each case. The data were plotted in Stem-Volmer form and extrapolated to infinite dilution to get &(S,). The results are tabulated in table 1. For PT in PF- 1,3-DMCH at 293 K a remarkably high value of &(S,) = 0.33 + 0.04 is obtained. The corresponding value in 3-MP is a factor of 10 smaller. It was not possible by similar means to measure &( T, ), the phosphorescence quantum yield in the limit of infinite dilution obtained on direct excitation to T,. The So-T, molar extinction coefficient is small, the lifetime of T, is long, and self-quenching occurs at a near-diffusion-limited rate. It was thus impossible to measure phosphorescence intensities over a large enough PT concentration range to permit accurate extrapolation to zero solute concentration. Nevertheless, accurate values of the relative quantum efficiencies of T1 phosphorescence could be obtained by selective laser irradiation in the resolved So-S, and S,-+T, absorption bands. The ratio
tl(S,,T,)=MS,)
Za(T,YMT,) Z,(S)
was measured to be 0.70+0.07 246
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CHEMICAL PHYSICS LETTERS
for PT in PF-1,3-
DMCH and 0.91+0.09
in 3-MP. Here Zr(Si) and
ZP(TI ) refer to the intensities of phosphorescence resulting from So-S, excitation with absorbed intensity Z,(S) and So+T, excitation with absorbed intensity I,( T1 ) respectively. We can calculate &(T,) from these data based on the following reasoning. Polyatomic thiones, including PT, exhibit no measurable prompt fluorescence from Si when Si is populated either directly by So-S, absorption or indirectly via the relaxation of higher excited states [ 131. Their S1 decay times must therefore lie in the picosecond range; this has been verified experimentally for xanthione by Molenkamp et al. [ 261. S, will therefore not be significantly self-quenched at concentrations up to the limit of PT’s solubility in perfluoroalkanes at room temperature ( x 1O- ’ M) , even if the quenching rate is diffusion limited. At PT concentrations greater than x5x10-’ M (cf. fig. 2) measurable phosphorescence self-quenching occurs, but this must be due exclusively to T, + So+2So, owing to the fact that T, is relatively long-lived. We have previously shown that perfluoroalkane solvents do not interact sufficiently strongly with electronically excited thiones to cause significant perturbation of the rates of their intramolecular electronic transitions [12,15]. Thus, the value of q(S,T,) should be independent of concentration (to the limit of PT’s solubility in pertluoroalkanes) and should remain constant even in the limit of infinite dilution. &(S2) is thus related to&T,) by &G2)
=@(S,,S,)
v(S,
,TiM%T,)
3
(2)
where @(S2,S1) is the quantum yield of S,*S, internal conversion. We have previously shown [ 15 ] that S2~S, internal conversion is the major radiationless decay path of photostable aromatic thiones in perfluoroalkane solvents. Because @&S2-So) is only 1.3 x 10e4 for PT and because the quantum yield of net photochemical consumption is c 10m3, we assume that $( S,,S, ) = 1.OO. Thus, &( T1 ) = &SI,Tl)l~(SI, T,), and using the measured values of &(S2) and q(S,, T,) we obtain &(T,)=0.47 f0.07 for PT in PF-1,3-DMCH at 293 K. To our knowledge this is the largest value of a phosphorescence quantum yield ever reported for a polyatomic organic molecule in fluid solution at room temperature.
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We have previously shown [ 13 ] that thiones with small S,-Tr energy gaps exhibit thermally delayed S1 fluorescence as a result of the establishment of S, ST, equilibrium. PT, with AE( S,-T1) = 580 cm-‘, is no exception, although its weak delayed fluorescence is almost completely buried under the much more intense phosphorescence (cf. fig. 1). Since S1 can decay both radiatively and non-radiatively, the existence of the S L*T, equilibrium provides additional paths for T1 decay. Thus, the value of q( S, ,T1) reflects not only the fact that the quantum yield of S,*T, intersystem crossing is < 1, but also, the fact that molecules in T1 can return to S1 via back intersystem crossing. Comparisons of the photophysical properties of PT with those of other thiones and of acetophenone and benzophenone [ 5 1, all in perfluoroalkane solvents at room temperature, reveal why PT exhibits such a large value of &( T1 ). At concentrations at which selfquenching can be neglected, (3) where kp is the T, + So radiative rate constant, kIsc is the rate constant for the Tr *So radiationless transition in the solvent used, and kRx is the rate constant for total photochemical consumption of the thione. We have previously shown that many thiones are photochemically stable in perfluoroalkane solvents [ 11,121. In the present case, the quantum yield of net photochemical consumption of PT in PF-1,3DMCH is < 10m3. Thus, in pertluoroalkanes, kRXakp+kIsc and &T,)/r”(T1) yields a value of kp=l.lx104 s-r, whereas [ 1 -&T,)]/r”(T,) yields kIsc = 1.2x 1O4s- ‘. The corresponding values of these and other relevant quantities for PT in 3-MP and for acetophenone in perfluoromethylcyclohexane [ $23-251 are given in table 1. Both kp and kIsc are much larger for PT than for acetophenone as a consequence of the more effective spin-orbit coupling in the S-containing molecule [ 2,6,14]. Note that the spin-orbit coupling matrix element might be expected to be particularly large in triplet PT because it is a relatively small molecule requiring unpaired electron density to be more highly localized on the “heavy” S atom. The very large zerofield splitting parameters ( (D 1= 24 and 28 cm-’ for two trap sites in an n-pentane matrix) are, in part,
LETTERS
3 April 1987
indicative of this effect [ 141. PT’s radiative rate constant is thus 430 times larger than that of acetophenone [ 5 1, but is only about a factor of 2 greater than those of related, larger thiones such as xanthione and 2,6-dimethyl-4H-pyran-4-thione [ 271. PT differs from these other thiones by virtue of its relatively large Ti-So energy separation. According to the energy gap law of radiationless transition theory [ 281, this should lead to a relatively small Franck-Condon factor for the TImSO transition and, hence, a relatively small value of k,,,. A relatively small kIsc (compared with other thiones), together with a relatively large kp is thus responsible for PT’s large &!(T, ) in perfluoroalkane solvents at room temperature. The rate constant for radiationless T1 decay is a factor of 12 larger in 3-MP than in PF-1,3-DMCH and is largely responsible for the small observed values of & and r”( T1 ) in the former solvent. The fact that kIsc is much higher in the hydrocarbon solvent for both PT and several ketones [ 3-51 is additional evidence that pertluoroalkane solvents cause minimal perturbation of the intramolecular electronic relaxation rates in these excited molecules. The mechanism whereby 3-MP and other alkanes increase the overall radiationless decay rate of triplet PT has not yet been established. The interaction does not lead to net photochemistry because the quantum yield of PT consumption remains small in 3-MP. No emission which could be attributed to an exciplex has been observed. Nevertheless, based on analogies with other systems [ 29,301, it is not unreasonable to propose that dark exciplexes are formed. Similar effects have been observed by us in the electronic relaxation of SZ thiones [ 11,12,15 1. The rates of intermolecular triplet PT-alkane quenching are some five orders of magnitude smaller than those of thione S2-alkane quenching under similar conditions. Nevertheless, because triplet PT is intrinsically long-lived, intermolecular interaction is still responsible for 3 85% of its total decay in alkane solvents. The phosphorescence of PT in n-pentane matrices at low temperature has been examined in detail by Taherian and Maki [ 141. They obtained a triplet lifetime, r( T,, 77 K) = 85 + 5 J.LS but did not measure phosphorescence quantum yields. An estimate of the phosphorescence quantum yield of PT in an n-pen247
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CHEMICAL PHYSICS LETTERS
tane matrix at 77 K may be made by assuming that the radiative rate constant is solvent and temperature independent and evaluating the quantity &( T,, 293 K) r(T,, 77 K)/r’(T,, 293 K), which equals 0.93 f 0.18. On this basis we conclude that PT decays almost exclusively by radiative means at temperatures 677 K. Xanthione, on the other hand, has a phosphorescence quantum yield of 0.11 in glasses at 77 K [ 61 and its triplet thus decays by largely nonradiative means. Care must therefore be taken to avoid unwarranted comparisons between the structural and low temperature photophysical properties of these two molecules.
Acknowledgement
The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for their continuing financial support. One of us (MS) wishes to thank the College of Graduate Studies and Research of the University of Saskatchewan for the award of a University Postdoctoral Fellowship.
References [ 1 ] C.A. Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968). [2] S.P. McGlynn, T. Azumi and M. Kinoshita, Molecular spectroscopy of the triplet state (Prentice-Hall, Englewood Cliffs, 1969). [ 31 C.A. Parker and T.A. Joyce, Chem. Commun. (1968) 749. [ 41 C.A. Parker and T.A. Joyce, Chem. Commun. ( 1968) 142 1. [ 51C.A. Parker and T.A. Joyce, Trans. Faraday Sot. 65 (1969) 2823. [6] D.A. Capitanio, H.J. Pownall and J.R. Huber, J. Photothem. 3 (1974) 225.
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[ 71 M. Mahaney and J.R. Huber, J. Photochem. 5 (1976) 333. [ 81 J.R. Huber and M. Mahaney, Chem. Phys. Letters 30 (1975) 410. [9] D. Seratimov, U. Bruhlmann and J.R. Huber, Ber. Bunsenges. Physik. Chem. 79 (1975) 202. [lo] A. Safarzadeh-Amiri, R.E. Verrall and R.P. Steer, Can. J. Chem. 61 (1983) 894. [ 111 A. Maciejewski and R.P. Steer, Chem. Phys. Letters 100 (1983) 540. [ 121 A. Maciejewski, D.R. Demmer, D.R. James, A. SafarzadehAmiri, R.E. Verrall and R.P. Steer, J. Am. Chem. Sot. 107 (1985) 2831. [ 131 A. Maciejewski, M. Szymanski and R.P. Steer, J. Phys. Chem., submitted for publication. [ 141 M.R. Taherian and A.H. Maki, Chem. Phys. Letters 96 (1983) 541. [ 151 A. Maciejewski, A. Safanadeh-Amiri, R.E. Vet-ml and R.P. Steer, Chem. Phys. 87 (1984) 295. [ 161 R. Mayer, W. Broy and R. Zahradnik, Advan. Heterocyclic Chem. 8 (1967) 219. [ 171 J.W. Scheerrner, R.H.J. Ooms and R.J.F. Nivard, Synthesis (1973) 149. [ 181 B.S. Pedersen, S. Scheibye, N.H. Nilsson and S.D. Lawesson, Bull. Sot. Chim. Belg. 87 (1978) 223. [ 191 D.R. James, D.R.M. Demmer, R.E. Verrall and R.P. Steer, Rev. Sci. Instr. 54 (1983) 1121. [ 201 S. Hamal and F. Hirayama, J. Phys. Chem. 87 (1983) 83. [ 211 S. Meech and D. Phillips, J. Photochem 23 (1983) 193. [22] A. Maciejewski, M. Szymanski and R.P. Steer, to be published. [ 231 R.F. Borkman and D.R. Keams, J. Chem. Phys. 44 (1966) 945. [ 241 A.A. Lamola and G.S. Hammond, J. Chem. Phys. 43 (1965) 2129. [25] K. Sandros,Acta. Chem. Stand. 23 (1969) 2815. [26] L.W. Mole&, D.P. Weitekamp and D.A. Wiersma, Chem. Phys. Letters 99 (1983) 382. [27] M. Szymanski and R.P. Steer, to be published. [ 281 R. Engleman and J. Jortner, Mol. Phys. 18 (1970) 145. [ 291 H. Shizuka, H. Hagiwara and M. Fukushima, J. Am. Chem. Sot. 107 (1985) 7816. [ 301 S.L. Mattes and S. Farid, Accounts Chem. Res. 15 (1982) 80.
CHEMICAL PHYSICS LETTERS
THE PHOSPHORESCENCE OF 4H-PYRAN-CTHIONE: LARGE QUANTUM YIELDS FROM ROOM-TEMPERATURE
3 April 1987
FLUID SOLUTIONS
M. SZYMANSKI ‘, R.P. STEER ’ Department of Chemistry, University ofSaskatchewan, Saskatoon, Saskatchewan. Canada S7N OWO
and A. MACIEJEWSKI Faculty of Chemistry, A. Mickiewicz University, 60- 780 Poznan. Poland Received 11 November 1986
The spectra, phosphorescence quantum yields and triplet lifetimes of 4H-pyran-4-thione (PT) in fluid solution at room temperature have been measured. In inert perlluoroalkane solvents at 293 K, the phosphorescence quantum yield of PT is 0.33 on excitation to S2 and 0.47 on direct excitation to T,. The reasons for these extremely large radiative yields are discussed.
1. -Introduction Large quantum yields of T , + S,, phosphorescence, &, from organic compounds are usually only observed in glassy or crystalline solids at low temperature [ 1,2]. In fluid solution at room temperature, intermolecular quenching processes dominate, so that only a very few classes of compounds exhibit readily measurable phosphorescence quantum yields. Intermolecular solvent-induced relaxation can be minimized through the use of inert perfluoroalkane solvents which interact exceptionally weakly with electronically excited solutes [ 3-51. However, even in these cases the fraction of excited triplets which decay radiatively is relatively small; phosphorescence quantum yields of 0.034 for benzil, 0.097 for benzophenone, and 0.10 for acetophenone are representative of the ketones [ 51. Relatively intense phosphorescence has been reported for several thioketones in fluid solutions [ 6-101, but quantitative data are scarce. The most thoroughly studied thione is xanthione for which Huber and Mahaney
[ 81 have reported &=O.Ol in 3-methylpentane at 298 K. A value of &=0.024 has also been reported for 2,2,3,3_tetramethylindanethione in 3-methylpentane at room temperature [ lo]. We have previously shown that the S2 excited state lifetimes and the corresponding S2-& fluorescence quantum yields of aromatic thiones can be more than a factor of ten larger in perlluoroalkanes than in other photochemically inert solvents [ 11,121. Similar effects have also been found when the luminescence of ketones has been studied as a function of the nature of the solvent [ 3-51. It therefore seemed logical to examine the phosphorescence of the thiones in perfluoroalkane solvents. We chose 4H-pyran-4-thione (PT) as a model for this study because it has a large So-T1 energy gap [ 131 and therefore might be expected to exhibit a relatively slow T,-S, intramolecular radiationless decay rate. Studies of the absorption, emission [ 131 and triplet ODMR [ 141 spectra of PT, and its S2 decay dynamics [ 15 1, reactivity and dipole moment [ 161 have previously been reported.
’ Present address: Institute of Physics, A. Mickiewicz University, 60-780 Poznan, Poland. 2 To whom correspondence should be addressed.
0 009-2614/87/$ 03.50 0. Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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hexane (q+ 0.9 1 [ 20,2 1 ] was used as an emission standard. Quantum yields were measured with a precision of f 5% and an estimated accuracy of f 10%. All intensities reported refer to the photon flux.
2. Experimental PT was synthesized by the reaction of 4H-pyranone with P2S5 [ 17,181. The crude thione was purified by crystallization from MeOH and toluene under Ar or He and was stored at low temperature. Per( PF- 1,3-DMCH ) fluoro- 1,3-dimethylcyclohexane and 3-methylpentane (3-MP) were purified by fractional distillation followed by column chromatography. After purification these solvents showed no emission under the conditions used to study PT. All the steady-state and dynamic measurements were performed on solutions which had been deoxygenated by repeated freeze-pump-thaw cycles. Steady-state absorption and emission spectra were measured using Cary 118C (Varian) and Spex Fluorolog 2 spectrometers, the latter controlled by a “Datamate” computer. Emission lifetimes were measured using a Spectra Physics synchronously pumped cavity-dumped tunable dye laser system similar to the one previously described [ lo,19 1. Quantum yields of emission were measured by a relative method which has been described previously [ 111. 9, IO-diphenylanthracene in degassed cyclo-
-i E 0
3. Results and discussion The absorption and long wavelength emission spectra of PT in 3-MP at room temperature are shown in fig. 1. The symmetry-allowed So-& and symmetry-forbidden So-+ absorptions are found in the near UV and mid-visible regions, respectively. The longest wavelength absorption feature, located at x 582 nm, is assigned to the O-O band of the So-+T1 absorption and is clearly resolved from the So-& system. The emission consists mainly of T,-& phosphorescence, but a very weak feature at the blue edge of the spectrum is due to thermally activated delayed fluorescence from S, [ 13 1. These spectra are typical of the larger organic thiones in fluid solutions and are very similar to those obtained for 4H- 1-benzopyran4-thione and xanthione under similar conditions [ 6,221.
A
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3 April 1987
LETTERS
t.
293 K
-i
zz m
;
14.
x1000
0-
--sl
z i
P
6 SO 4
WAVELENGTH Fig. 1. Absorption
244
and emission
(nm)
spectra of PT in 3-MP at 293 K.
1
Volume 135, number 3
293 K
PT in 3-MP
PT in PF-I,3-DMCH
01
”
”
3
”
”
5
”
7 [PT]
9
”
II
”
13
15
1
(xIO-~M)
Fig. 2. Stem-Volmer plots for the self-quenching of PT phosphorescence in 3-MP and PF-1,3-DMCH.
The O-O bands of the So-+T1 and S,-S, absorption systems are located at 17100 and 17680 cm-‘, respectively, and have apparent molar extinction coefficients of 10.0 and 15.4 M- ’ cm- ‘, respectively, all in 3-MP. The maximum of the O-Oband of the T1+SO phosphorescence spectrum is located = 200 cm- ’ to the red of the O-O band in absorption at 16920 cm- ‘. From the absorption spectrum the Table 1 Photophysical properties of 4H-pyran-Cthione
S-T1 energy gap is therefore 580 cm-‘. The So-S2 absorption system is much more intense (en&IX = 16800 M-r cm-’ at 30490 cm-‘) and is structureless. Note that the !&So fluorescence of PT (h= 1.3x 10W4) has been omitted from fig. 1 for clarity. Sz+ S,, fluorescence is spectrally well resolved from the longer wavelength phosphorescence and delayed fluorescence and is readily distinguished by the much longer lifetime and the ease of oxygen quenching of the latter. Triplet lifetimes were measured as a function of PT concentration in 3-MP and PF-1,3-DMCH. The results are presented in fig. 2. PT was excited in its weak So-S1 absorption system, using the intense fundamental output of the R6G dye laser, and the lowest triplet was populated via efficient, rapid, radiationless decay from S,*T,. The decay of phosphorescence from Tr was well described by single exponential functions in all cases. The slopes and intercepts of the resulting Stern-Volmer plots (fig. 2) give, respectively, the bimolecular triplet selfquenching rate constants, kelB and the triplet lifetimes of PT at infinite dilution, r”(T1), in the two solvents. These data are tabulated in table 1. Self-quenching is known to occur at or near diffusion-controlled rates in thione systems. That triplet PT is no exception can be deduced from the values of kselfwhich are < 10” m-’ s-r at 293 K in both solvents. Note that k,,, is greater in the solvent of lower viscosity, 3-MP, as expected. Because kelf is so large, it was necessary to obtain r”(T1) by extrapo-
and acetophenone in solution at room temperature
PT in PF- 1,3-DMCH
4%W &TO r”UI) (ms) q(S,,T,)
kp(s-‘1 hsc (s-l) ke,r(M-Is-‘) P!$pt’, (cm-‘)
3 April 1987
CHEMICAL PHYSICS LETTERS
0.33kO.04 0.47 f. 0.07 (43+4)x10-3 0.70*0.07 1.1x104 1.2x 104 (1.36f0.06)~10’~ 16940
&,H,COCHx in PFMCH a)
R"'
0.10 0.39 1.0” 25.6 230.4 1.2x lo6 25300
4.7 0.11 0.7 430 52.1 l.lX104
in 3-MP 0.034 (6.5?0.5)xlO-3 0.91 kO.09 7.1x103 14.7x lo4 (2.6 t 0.2) x 10” 16920
a) Acetophenone in perfluorometylcyclohexane. Data from ref. [ 51. ‘) Ratio of the values of the parameters for PT in PF- 1,3-DMCHto acetophenone in PFMCH. ‘) Data from refs. [ 23-251. Values are for hydrocarbon solvents.
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lation. Data were taken at thione concentration from 2 x 10e6 to 2 x 1O- 5 M in order to avoid conditions in which self-quenching was the major deactivation channel. Good linear Stern-Volmer plots of the lifetime data were obtained, so it is unlikely that the extrapolation would introduce any significant systematic error. Values of rO(T,) of 43 + 4 and 6.5 k 0.5 J.LSare found for PT in PF-1,3-DMCH and 3-MP, respectively. Note that the lifetime is 6.6 times longer in the pertluoroalkane. In order to measure phosphorescence quantum yields, PT solutions (2 x 10e6 to 2x 10m5 M) were illuminated in the strong So-& absorption band and their corrected TI-+SO emission spectra were obtained. The phosphorescence quantum yields at each concentration, @r(S,), were then determined using the relationship: _fZ(v) drr I:
n2
@P(Sz)=~S -JZs( r) drr Z, ( n2)’
’
where & is the fluorescence quantum yield of the standard, the integrals are the areas under the corrected emission spectra, n is the refractive index of the solvent, Z, is the absorbed intensity, and the superscript S refers to the standard in each case. The data were plotted in Stem-Volmer form and extrapolated to infinite dilution to get &(S,). The results are tabulated in table 1. For PT in PF- 1,3-DMCH at 293 K a remarkably high value of &(S,) = 0.33 + 0.04 is obtained. The corresponding value in 3-MP is a factor of 10 smaller. It was not possible by similar means to measure &( T, ), the phosphorescence quantum yield in the limit of infinite dilution obtained on direct excitation to T,. The So-T, molar extinction coefficient is small, the lifetime of T, is long, and self-quenching occurs at a near-diffusion-limited rate. It was thus impossible to measure phosphorescence intensities over a large enough PT concentration range to permit accurate extrapolation to zero solute concentration. Nevertheless, accurate values of the relative quantum efficiencies of T1 phosphorescence could be obtained by selective laser irradiation in the resolved So-S, and S,-+T, absorption bands. The ratio
tl(S,,T,)=MS,)
Za(T,YMT,) Z,(S)
was measured to be 0.70+0.07 246
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for PT in PF-1,3-
DMCH and 0.91+0.09
in 3-MP. Here Zr(Si) and
ZP(TI ) refer to the intensities of phosphorescence resulting from So-S, excitation with absorbed intensity Z,(S) and So+T, excitation with absorbed intensity I,( T1 ) respectively. We can calculate &(T,) from these data based on the following reasoning. Polyatomic thiones, including PT, exhibit no measurable prompt fluorescence from Si when Si is populated either directly by So-S, absorption or indirectly via the relaxation of higher excited states [ 131. Their S1 decay times must therefore lie in the picosecond range; this has been verified experimentally for xanthione by Molenkamp et al. [ 261. S, will therefore not be significantly self-quenched at concentrations up to the limit of PT’s solubility in perfluoroalkanes at room temperature ( x 1O- ’ M) , even if the quenching rate is diffusion limited. At PT concentrations greater than x5x10-’ M (cf. fig. 2) measurable phosphorescence self-quenching occurs, but this must be due exclusively to T, + So+2So, owing to the fact that T, is relatively long-lived. We have previously shown that perfluoroalkane solvents do not interact sufficiently strongly with electronically excited thiones to cause significant perturbation of the rates of their intramolecular electronic transitions [12,15]. Thus, the value of q(S,T,) should be independent of concentration (to the limit of PT’s solubility in pertluoroalkanes) and should remain constant even in the limit of infinite dilution. &(S2) is thus related to&T,) by &G2)
=@(S,,S,)
v(S,
,TiM%T,)
3
(2)
where @(S2,S1) is the quantum yield of S,*S, internal conversion. We have previously shown [ 15 ] that S2~S, internal conversion is the major radiationless decay path of photostable aromatic thiones in perfluoroalkane solvents. Because @&S2-So) is only 1.3 x 10e4 for PT and because the quantum yield of net photochemical consumption is c 10m3, we assume that $( S,,S, ) = 1.OO. Thus, &( T1 ) = &SI,Tl)l~(SI, T,), and using the measured values of &(S2) and q(S,, T,) we obtain &(T,)=0.47 f0.07 for PT in PF-1,3-DMCH at 293 K. To our knowledge this is the largest value of a phosphorescence quantum yield ever reported for a polyatomic organic molecule in fluid solution at room temperature.
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We have previously shown [ 13 ] that thiones with small S,-Tr energy gaps exhibit thermally delayed S1 fluorescence as a result of the establishment of S, ST, equilibrium. PT, with AE( S,-T1) = 580 cm-‘, is no exception, although its weak delayed fluorescence is almost completely buried under the much more intense phosphorescence (cf. fig. 1). Since S1 can decay both radiatively and non-radiatively, the existence of the S L*T, equilibrium provides additional paths for T1 decay. Thus, the value of q( S, ,T1) reflects not only the fact that the quantum yield of S,*T, intersystem crossing is < 1, but also, the fact that molecules in T1 can return to S1 via back intersystem crossing. Comparisons of the photophysical properties of PT with those of other thiones and of acetophenone and benzophenone [ 5 1, all in perfluoroalkane solvents at room temperature, reveal why PT exhibits such a large value of &( T1 ). At concentrations at which selfquenching can be neglected, (3) where kp is the T, + So radiative rate constant, kIsc is the rate constant for the Tr *So radiationless transition in the solvent used, and kRx is the rate constant for total photochemical consumption of the thione. We have previously shown that many thiones are photochemically stable in perfluoroalkane solvents [ 11,121. In the present case, the quantum yield of net photochemical consumption of PT in PF-1,3DMCH is < 10m3. Thus, in pertluoroalkanes, kRXakp+kIsc and &T,)/r”(T1) yields a value of kp=l.lx104 s-r, whereas [ 1 -&T,)]/r”(T,) yields kIsc = 1.2x 1O4s- ‘. The corresponding values of these and other relevant quantities for PT in 3-MP and for acetophenone in perfluoromethylcyclohexane [ $23-251 are given in table 1. Both kp and kIsc are much larger for PT than for acetophenone as a consequence of the more effective spin-orbit coupling in the S-containing molecule [ 2,6,14]. Note that the spin-orbit coupling matrix element might be expected to be particularly large in triplet PT because it is a relatively small molecule requiring unpaired electron density to be more highly localized on the “heavy” S atom. The very large zerofield splitting parameters ( (D 1= 24 and 28 cm-’ for two trap sites in an n-pentane matrix) are, in part,
LETTERS
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indicative of this effect [ 141. PT’s radiative rate constant is thus 430 times larger than that of acetophenone [ 5 1, but is only about a factor of 2 greater than those of related, larger thiones such as xanthione and 2,6-dimethyl-4H-pyran-4-thione [ 271. PT differs from these other thiones by virtue of its relatively large Ti-So energy separation. According to the energy gap law of radiationless transition theory [ 281, this should lead to a relatively small Franck-Condon factor for the TImSO transition and, hence, a relatively small value of k,,,. A relatively small kIsc (compared with other thiones), together with a relatively large kp is thus responsible for PT’s large &!(T, ) in perfluoroalkane solvents at room temperature. The rate constant for radiationless T1 decay is a factor of 12 larger in 3-MP than in PF-1,3-DMCH and is largely responsible for the small observed values of & and r”( T1 ) in the former solvent. The fact that kIsc is much higher in the hydrocarbon solvent for both PT and several ketones [ 3-51 is additional evidence that pertluoroalkane solvents cause minimal perturbation of the intramolecular electronic relaxation rates in these excited molecules. The mechanism whereby 3-MP and other alkanes increase the overall radiationless decay rate of triplet PT has not yet been established. The interaction does not lead to net photochemistry because the quantum yield of PT consumption remains small in 3-MP. No emission which could be attributed to an exciplex has been observed. Nevertheless, based on analogies with other systems [ 29,301, it is not unreasonable to propose that dark exciplexes are formed. Similar effects have been observed by us in the electronic relaxation of SZ thiones [ 11,12,15 1. The rates of intermolecular triplet PT-alkane quenching are some five orders of magnitude smaller than those of thione S2-alkane quenching under similar conditions. Nevertheless, because triplet PT is intrinsically long-lived, intermolecular interaction is still responsible for 3 85% of its total decay in alkane solvents. The phosphorescence of PT in n-pentane matrices at low temperature has been examined in detail by Taherian and Maki [ 141. They obtained a triplet lifetime, r( T,, 77 K) = 85 + 5 J.LS but did not measure phosphorescence quantum yields. An estimate of the phosphorescence quantum yield of PT in an n-pen247
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tane matrix at 77 K may be made by assuming that the radiative rate constant is solvent and temperature independent and evaluating the quantity &( T,, 293 K) r(T,, 77 K)/r’(T,, 293 K), which equals 0.93 f 0.18. On this basis we conclude that PT decays almost exclusively by radiative means at temperatures 677 K. Xanthione, on the other hand, has a phosphorescence quantum yield of 0.11 in glasses at 77 K [ 61 and its triplet thus decays by largely nonradiative means. Care must therefore be taken to avoid unwarranted comparisons between the structural and low temperature photophysical properties of these two molecules.
Acknowledgement
The authors are grateful to the Natural Sciences and Engineering Research Council of Canada for their continuing financial support. One of us (MS) wishes to thank the College of Graduate Studies and Research of the University of Saskatchewan for the award of a University Postdoctoral Fellowship.
References [ 1 ] C.A. Parker, Photoluminescence of solutions (Elsevier, Amsterdam, 1968). [2] S.P. McGlynn, T. Azumi and M. Kinoshita, Molecular spectroscopy of the triplet state (Prentice-Hall, Englewood Cliffs, 1969). [ 31 C.A. Parker and T.A. Joyce, Chem. Commun. (1968) 749. [ 41 C.A. Parker and T.A. Joyce, Chem. Commun. ( 1968) 142 1. [ 51C.A. Parker and T.A. Joyce, Trans. Faraday Sot. 65 (1969) 2823. [6] D.A. Capitanio, H.J. Pownall and J.R. Huber, J. Photothem. 3 (1974) 225.
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[ 71 M. Mahaney and J.R. Huber, J. Photochem. 5 (1976) 333. [ 81 J.R. Huber and M. Mahaney, Chem. Phys. Letters 30 (1975) 410. [9] D. Seratimov, U. Bruhlmann and J.R. Huber, Ber. Bunsenges. Physik. Chem. 79 (1975) 202. [lo] A. Safarzadeh-Amiri, R.E. Verrall and R.P. Steer, Can. J. Chem. 61 (1983) 894. [ 111 A. Maciejewski and R.P. Steer, Chem. Phys. Letters 100 (1983) 540. [ 121 A. Maciejewski, D.R. Demmer, D.R. James, A. SafarzadehAmiri, R.E. Verrall and R.P. Steer, J. Am. Chem. Sot. 107 (1985) 2831. [ 131 A. Maciejewski, M. Szymanski and R.P. Steer, J. Phys. Chem., submitted for publication. [ 141 M.R. Taherian and A.H. Maki, Chem. Phys. Letters 96 (1983) 541. [ 151 A. Maciejewski, A. Safanadeh-Amiri, R.E. Vet-ml and R.P. Steer, Chem. Phys. 87 (1984) 295. [ 161 R. Mayer, W. Broy and R. Zahradnik, Advan. Heterocyclic Chem. 8 (1967) 219. [ 171 J.W. Scheerrner, R.H.J. Ooms and R.J.F. Nivard, Synthesis (1973) 149. [ 181 B.S. Pedersen, S. Scheibye, N.H. Nilsson and S.D. Lawesson, Bull. Sot. Chim. Belg. 87 (1978) 223. [ 191 D.R. James, D.R.M. Demmer, R.E. Verrall and R.P. Steer, Rev. Sci. Instr. 54 (1983) 1121. [ 201 S. Hamal and F. Hirayama, J. Phys. Chem. 87 (1983) 83. [ 211 S. Meech and D. Phillips, J. Photochem 23 (1983) 193. [22] A. Maciejewski, M. Szymanski and R.P. Steer, to be published. [ 231 R.F. Borkman and D.R. Keams, J. Chem. Phys. 44 (1966) 945. [ 241 A.A. Lamola and G.S. Hammond, J. Chem. Phys. 43 (1965) 2129. [25] K. Sandros,Acta. Chem. Stand. 23 (1969) 2815. [26] L.W. Mole&, D.P. Weitekamp and D.A. Wiersma, Chem. Phys. Letters 99 (1983) 382. [27] M. Szymanski and R.P. Steer, to be published. [ 281 R. Engleman and J. Jortner, Mol. Phys. 18 (1970) 145. [ 291 H. Shizuka, H. Hagiwara and M. Fukushima, J. Am. Chem. Sot. 107 (1985) 7816. [ 301 S.L. Mattes and S. Farid, Accounts Chem. Res. 15 (1982) 80.