J. Phys. Chem. 1988, 92, 2485-2489
2485
Radiationless Decay of Aromatic Thlones in Solution Selectively Excited to Their S,, S,, S,, and T, States M. Szymanski,+A. Maciejewski,l Faculty of Chemistry and Institute of Physics, A. Mickiewicz University, Grunwaldzka 6, 60- 780 Poznan, Poland
and R. P. Steer* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO (Received: October 12, 1987)
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Seven aromatic thiones having a variety of electronic energy gaps and structures have been subjected to spectroscopic and photophysical examination. Accurate measurements of the relative TI So phosphorescence quantum yields obtained following direct excitation into the Sj, Sz, SI,and TI states have been made in perfluoroalkane and hydrocarbon solvents. Together with S2 lifetime and fluorescence quantum yield measurements, these data have been used to obtain quantitative information S2 is quantitative. Intermolecular processes dominate the decay of S2 in about the excited-state relaxation paths. S3 3-methylpentane and other common solvents, whereas intramolecular processes are revealed in inert perfluoroalkanes. The SI So decay process accounts for majority of S2species decay via SI to TI. However, the thermally activated TI a significant fraction of decay events at room temperature. In addition, molecules having “p” H atoms close to the thiocarbonyl So, bypassing SI and TI. group can undergo net decay from S2
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Introduction
The vast majority of photostable organic compounds exhibit rapid nonradiative decay to their lowest excited singlet states (SI) following excitation to higher singlet states (S,, n > 1) in solution.lJ Usually only after S, is populated does further decay to So or to the triplet manifold occur. Weak fluorescence from S, can sometimes be ~ b s e r v e d , ~but ” only in a few molecules is nonradiative decay of the highly excited state sufficiently slow to permit emission to account for anything but a tiny fraction of decay events. A~ulene’-~and its derivatives’OJ1and structurally rigid aromatic t h i o n e ~ l ~constitute -’~ the most notable cases in which So) can be large quantum yields of fluorescence (from S2 observed. Sz So fluorescence quantum yields and Sz lifetimes of photostable, rigid thiones are extremely sensitive to the nature of the ~ o l v e n t . ’ ~ JThe ~ analysis of this solvent dependence, together with bimolecular Sz thione-addend quenching studies,” has led us to propose that the decay of thiones in their Sz states is dominated by intermolecular interactions in most common solvents, including alkanes and cycloalkanes. Perfluoroalkanes, however, appear to interact sufficiently weakly with these excited thiones to permit the latter to behave as though they were embedded in a classical inert heat bath.’* Using perfluoroalkane solutions, we have demonstrated that an excellent linear correlation exists between the logarithm of the nonradiative decay constants (knr= (1 - 4?)/7f”2) of the Sz thiones and their S2-S1electronic energy gaps.’* This observation forms the basis for our proposal that Sz SI internal conversion accounts for the majority of the intramolecular Sz decay events in such solutions, in accordance with the energy gap lawI9 of radiationless transition theory. Two lines of evidence now suggest, however, that thiones in their Sz states may not decay exclusively via SI. First, Das et al. have noted in flash photolysis experiments that the quantum yield of TI formation is smaller on excitation to Sz than on excitation to SI for a series of four aromatic thionesZ0and a second series of cyclic enethiones.z’ Second, we have recently reportedz2that the quantum yields of phosphorescence from 4H-pyran-4-thione are lower on excitation to Sz than on direct excitation to TI. Since the radiative and net photochemical quantum yields are negligible for these compounds, both observations imply that a second S2 radiationless decay process exists which bypasses SI,and that this
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‘Institute of Physics. *Faculty of Chemistry.
0022-3654/88/2092-2485$01.50/0
process is in direct competition with Sz SI internal conversion. We have previously examined the decay dynamics of the Sz and TI states of a number of aromatic and other thiones using time-resolved emission spectroscopy13-~5~z2 and have also characterized their S, states via the observation of thermally activated delayed f l ~ o r e s c e n c e . ~In~ the present paper we examine the radiationless decay of these thiones under selective excitation of their S3,S2,SI,and TI states in a variety of solvents and provide quantitative data for the effects of intermolecular solvent perturbation on the decay of Sz. Experimental Section
The thiones examined in this work (Figure 1) were all prepared by reaction of the appropriate ketone with P4Sl0,following the methods of Arndt and Lorenzz4,Pedersen et al.,z5Scheermer et (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (2) Parker, C. A. Photoluminescence of Solutions; Elsevier: Amsterdam, 1968. (3) Hirayama, F.; Lipsky, S. J . Chem. Phys. 1975, 62, 576. (4) Easterly, C. E.; Christophoru, L. G. J. Chem. SOC.,Faraday Trans. 2 1974, 70, 267. ( 5 ) Lin, H-B.; Topp, M. R. Chem. Phys. Lett. 1977,47,442; 1977,48,251; Chem. Phys. 1979, 36, 365. (6) Plummer, B. F.; Al-Saigh, Z . Y. J . Phys. Chem. 1983, 87, 1579. (7) Griasser, H. J.; Wild, U. P. Chem. Phys. 1980,52, 117; J . Chem. Phys. 1980, 73, 4715. ( 8 ) Rentzepis, P. M.; Jortner, J.; Jones, R. P. Chem. Phys. Lett. 1970, 4, 599. (9) Beer, M.; Longuet-Higgins, H. C. J . Chem. Phys. 1955, 23, 1390. (10) Griesser, H. J.; Wild, U. P. J . Phorochem. 1980, 13, 309. (11) Olszowski, A. Chem. Phys. Lett. 1980, 73, 256. (12) Mahaney, M.; Huber, J. R. Chem. Phys. 1975, 9, 371. (13) Maciejewski, A.; Demmer, D. R.; James, D. R.; Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. J. Am. Chem. SOC.1985, 107, 2831. (14) Maciejewski, A.; Safarzadeh-Amiri, A.; Verrall, R. E.; Steer, R. P. Chem. Phys. 1984, 87, 295. (15) Maciejewski, A.; Steer, R. P. Chem. Phys. Lett. 1983, 100, 540; J . Am. Chem. SOC.1983, 105, 6738. (16) Mahaney, M.; Huber, J. R. Chem. Phys. Lett. 1984, 105, 395. (17) Maciejewski, A.; Steer, R. P. J . Photochem. 1984, 24, 303. (18) Freed, K. F.; Jortner, J. J . Chem. Phys. 1970, 52, 6272. (19) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145. (20) Kumar, C. V.; Qin, L.; Das, P. K. J. Chem. SOC.,Faraday Trans. 2 1984, 80, 783. (21) Bhattacharyya, K.; Das, P. K.; Ramumurthy, V.; Rao, V. P. J . Chem. SOC.,Faraday Trans. 2 1986, 82, 135. (22) Szymanski, M.; Steer, R. P.; Macieiewski,A. Chem. Phys. Lett. 1987. 135, 243. (23) Maciejewski, A,; Szymanski, M.; Steer, R. P. J. Phys. Chem. 1986, 90, 6314.
0 1988 American Chemical Society
2486
Szymanski et ai.
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
BPT
PT
TMIT
x1
S
S
O M BTPT
TXT
OMTBP
Figure 1. Structures of the thiones used in this work. TMIT, 2,2,3,3tetramethylindanethione; PT, 4H-pyran-4-thione; BPT, 4H- 1-benzo-
pyran-4-thione;XT, xanthione; TXT, thioxanthione;DMBTPT; 2,6-dimethyl-4H-l-benzothiopyran-4-thione; DMTBP, p,p'-dimethoxythiobenzophenone. a1.,26and Abrams et al.27 The crude thiones were purified by repeated fractional crystallization from ethanol and toluene and were stored in the dark at low temperature in the absence of air. Purity was monitored by IR and UV spectrophotometry and by gas chromatography using an HP Model 5880A instrument with a 50-m SE-52 capillary column or a Pye 104 instrument with a 3-m OV-17 column. Perfluoro- 1,3-dimethylcyclohexane (PF- 1,3-DMCH) and 3methylpentane (3-MP) were purified by fractional distillation followed by column chromatography. After purification these solvents showed no emission under the conditions used for thione excitation. All steady-state and dynamic measurements were performed on solutions which had been deoxygenated by repeated freeze-pumpthaw cycles or by air displacement using oxygen-free helium. Absorption spectra were taken on Cary 118C (Varian) or M-40 (Carl Zeiss Jena) spectrophotometers. Steady-state emission and excitation spectra were taken on a Spex Fluorolog 2 spectrofluorometer controlled by a Datamate computer, or on a sensitive home-made instrument built from SPM2 (Carl Zeiss Jena) and M3 (COBRABiD) monochromators. Emission lifetimes were measured with a Spectra-Physics mode-locked, synchronously pumped, cavity-dumped, frequency-doubled (for S2lifetimes only) argon ion/dye laser system with time-correlated single photon counting detection, as described previously.28 Absolute quantum yields of emission were measured by a relative method which has been described p r e v i o u ~ l y , ' ~using ~'~ 9,lO-diphenylanthracene in cyclohexane (& = 0.9329)as a reference. Accurate absolute values of the TI So phosphorescence quantum yields, &, were especially difficult to obtain for those thiones in which the phosphorescence spectra extended well to the red of 800 nm where the detection system exhibits low sensitivity. Therefore considerable effort was expended to obtain So accurate measurements of the relative values of the T I phosphorescence quantum yields under selective excitation to any two ( x , y ) of the S3, S2, SI,and TI states, q5px/&J'. The main experimental problem was to compare accurately the large intensities of phosphorescence obtained on So S2 excitation ( e lo4 M-I cm-l) with those of much weaker intensity obtained 1-10 M-' cm-l) on either So S, or So TI excitation (c under otherwise identical conditions. In order to solve this problem the measurements were carried out at all excitation wavelengths with the emission monochromator slits wide open (Ax = 8 nm) and high PMT gain to ensure that an accurately measurable signal was obtained for excitation in the weak absorption systems. (Data were taken under conditions which nevertheless minimized net photodecomposition.) In order to avoid errors due to the non-
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~
~
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(24) Amdt, F.; Lorenz, L. Berichte 1930, 63, 3121. ( 2 5 ) Pedersen, B. S.; Scheibye, S.; Nilsson, N. H.; Lawesson, S. 0. Bull. SOC.Chim. Belg. 1978, 87, 223. (26) Scheermer, J. W.; Ooms,P. H. J.; Nivard, R. J. F. Synthesis 1973, 149. (27) Abrams, S. R. J . Labelled Compd. Radiopharm. 1986, 24, 941. (28) James, D.R.; Demmer, D.R. M.; Verrall, R. E.; Steer, R . P. Rev. Sei. Instrum. 1983, 54, 1 1 21. (29) Meech, S.; Phillips, D.J . Photochem. 1983, 23, 193.
WAVELENGTH ( n m )
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Figure 2. Spectra of BPT in 3-MP at 293 K: (-) emission.
absorption, (---)
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110
WAVELENGTH (nm)Figure 3. Spectra of TXT in 3-MP at 293 K: (-)
absorption (---)
emission. linearity in the response of the detection system, the much higher emission intensity obtained when exciting the strong absorption systems or the reference under these conditions was attenuated to approximately the level of the weak emission by introducing an accurately calibrated neutral density filter into the emission beam. The relative absorbed intensities were measured independently by using the extinction coefficients obtained from the absorption spectra, and measurements of the incident intensities as a function of wavelength. The latter were accurately measured in the 300-610-nm range by using a rhodamine B quantum counter. Above 610 nm, where rhodamine B transmits significantly, corrections were applied to the quantum counter measurements by using data obtained with a bolometric power meter whose readings were interpreted in terms of photon flux (method A). Alternatively, the power meter was used to determine (laboriously) the entire profile of the incident intensity as a function of wavelength (method B). Results of experiments using both methods are reported and are identical within experimental error.
Results The UV-visible absorption spectra of most aromatic thiones consist of well-separated band systems, permitting clean, selective excitation to several excited electronic states. The intense, symmetry-allowed So S2 absorption is fully separated from the weak, symmetry-forbidden So SI band system because of the unusually large AE(S,-S,) in these molecules. In addition, the absorption spectra of most thiones (e.g., XT, BPT, PT, TMIT) exhibit a single band belonging to the So TIsystem which is relatively well-resolved from those of the So SI system (cf. Figure 2). On the other hand the So S3 absorption often
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2481
Radiationless Decay of Aromatic Thiones TABLE I: Phosphorescence Quantum Yield Ratios and S2
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SI Energy-Transfer Efficiencies, q(S,S1), Measured under Selective Excitation
4pS'/4pT1 thione TMIT
solvent PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP 3-MP 3-MP 3-MP
PTb BPT XT-do XT-d, TXT DMBTPT DMTBP
4pS2/4pT'
A
0.58 f 0.05 0.52 f 0.04 0.70 0.75 0.59 f 0.05 0.60 f 0.05
0.70 f 0.06 0.63 f 0.05 0.67 f 0.05 0.61 f 0.05
0.69 0.68 0.70 0.91 0.71 0.66
C
C
C
0.64 f 0.05 c 0.58 f 0.05 d d
0.85 f 0.07 0.64 f 0.05 d d
0.79 f 0.06 c 0.64 f 0.05 d d
d
d
d
C
-
-
4P2"
S(S2SI)
0.14 0.013 1 'X 10-4 2.3 X lo4 0.023 2.3 X lo-' 0.014 3 x lo-' 0.042 3.6 x 10-3 1.0 x 10-3 1.2 x 10-3 5.2 X lo4
0.97 f 0.08 0.81 f 0.08 1.00 f 0.09 0.82 f 0.08 0.88 f 0.09 0.95 f 0.09 0.85 f 0.09 0.78 f 0.08 0.99 f 0.09 0.91 f 0.09 0.86 f 0.09 0.89 f 0.07 0.76 f 0.06
B f 0.06
f 0.05 f 0.06
f 0.05
"From ref 13, 14, and 34. bQuantum yield ratios from selective laser excitation, ref 30. CInsufficient solubility in PF-1,3-DMCH to permit measurement. "So TIabsorption insufficiently resolved from So S , to permit clean excitation to T,. Values of q ( S 2 , S , ) determined by laser excitation, ref 30.
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'n:
overlaps the So S2absorption, and among the thiones studied here, relatively clean selective excitation of S3is only possible in TXT and XT (cf. Figure 3). Phosphorescence from T I is observed upon excitation to each of S3,S2,S1, and TI. The shapes of the phosphorescence spectra and the magnitudes of the lifetimes, T ~ of, the T1 states are independent of the state initially populated for a given thione at a given concentration in a given solvent at a given temperature. The phosphorescence quantum yields, +p, differ, however, indiS2 Sl TI occurs in all the cating that whereas S, thiones examined, other processes bypassing TI (and SI,vide infra) must also occur from the higher states. By comparing the phosphorescence quantum yields obtained following direct excitation to those electronic states accessible in the UV-visible (+:I, ,I+: ,2:+ , ) s : + one can determine the efficiencies of the various combinations of processes that are involved in the decay of the initially excited state. Excitation to lower states decreases the number of possible relaxation pathways. and +>/+:I, and these The crucial data are the ratios +>/+:I are reported in Table I. The errors quoted in the values of +/I+{ are the maximum additive errors in the measurements. Values outside the quoted limits are thus highly unlikely. The values of +~S,/C$;I were identical with those of +>/+:I for TXT, the only thione in which S3 decay was studied in detail. Measurement of the lifetimes, T ~and ~ the , quantum yields, +?, of S2 So fluorescence also permits an investigation of the dynamics of decay of the S2state. Some of these data have been reported previ~usly.'~-'~ Others, particularly those in 3-MP, were measured in the present work. A typical decay profile is given in Figure 4. All the S2 fluorescence lifetimes were single exponential and were determined with a typical precision of f10 ps. The data are presented in Table 11. In addition, for four of the thiones studied, PT, BPT, XT, and TMIT, information about the S1 state can be obtained from measurements of their thermally activated delayed fluorescence (TDF).23 Phosphorescence quantum yields were measured at three thione concentrations, 1.2 X 1.0 X loW5, and 8.0 X lo6 M, in 3-MP and PF-1,3-DMCH. Such small concentrations were chosen because self-quenching of these thione triplets is exceptionally efficient in fluid solution at room t e m p e r a t ~ r e(kselr ~ ~ , ~ ~1 O l o M-l SI), and because the triplet lifetimes are relatively long (up to ca. 40 Within each band system (So S3,So S2,So SI,So T,) measurements were performed at three different wavelengths separated by 5-30 nm. The results did not reveal any dependence on concentration or wavelength within a given band system. Thus, the +px/4: data in Table I are averages of nine measurements: three wavelengths within a band system for each of three concentrations. For those thiones in which the So T, and So S1 absorptions almost completely overlap, only +$, 42:, and '4: could be determined.
"Values of in perfluoroalkane solvents reported in ref 13 and 14. *Values of F62inter in perfluoroalkane solvents assumed to be 0.00 (see text). CCompare with value of 43 ps (in isooctane) in ref 38. eNot "Calculated from +.,(3-MP) = [@~(~-MP)/@(PF)]T~~(PF). determined.
Discussion First we consider the decay of S3. T X T has a value of AE-
(S&) = 5000 cm-', and its So S3and So S2absorptions are well separated. Because this is one of the largest S3-S2energy
>
-- -- --
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-
- -
- -
-
J
0
1 3. ... ,.
R
I
'
,
I
.
+ AC
-
Figure 4. S2 So fluorescence decay of ca. 1 X M XT in 3-MP at room temperature. The solid line is computed for T~~ = 38 ps for which x 2 = 1.13. IRF is the instrument response function, as determined by the excitation pulse shape mimic technique (ref 28). R is the distribution of weighted residuals and AC is the autocorrelation function. The horizontal axis show 512 channels at 5.9 ps per channel. TABLE 11: S2 Lifetimes for Selected Thiones in Perfluoroalkanes and in 3-MP, and Fractions of S2 States Decaying by Intermolecular Interaction in 3-MP thione TMIT BPT DMBTPT TXT XT-do XT-d, DMTBP
PT
solvent PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF-1,3-DMCH 3-MP PF- 1,3-DMCH 3-MP
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7S2' PS 880 f 10" 77 f 10 210 f 5 24 f 7 101 f 10 27 f 8 64 f 9 31 f 10 175 f 5 38 f I' 602 f 10 52 f 7" 35 f 10 30 f 10 120 120
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piinter
O.OO*
0.82 0.00 0.89 0.00 0.73 0.00 0.52 0.00 0.78 0.00 0.91 0.00 nd' 0.00 nd
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
Szymanski et al.
gaps among the thiones studied, the nonradiative S3 S2decay rate of TXT is e ~ p e c t e d ' ~to. ' ~ be among the slowest in the group. Nevertheless, no emission from S3of TXT could be observed, and was unity within an experimental error of ca. rt5% for all excitation wavelengths chosen in its So S3and So S, absorption systems. Because the corrected phosphorescence excitation spectra replicated the absorption spectra in the So S3 and So S2regions for all the other thiones as well, and because AE(S3-S,) for TXT is the largest among the group of thiones studied, we therefore assume that $:'/4$ = 1.OO generally. We S2is quanthus conclude that internal conversion from S3 titative. By contrast, the ratios $;~/C$~'I and $/+ :':I are both significantly smaller than unity for all the thiones studied in both PF- 1,3-DMCH and 3-MP. The fact that both methods (A and give excellent agreement lends B, Table I) of measuring $:I/+:L confidence to these results. The averages of the values of 42l/4:1 obtained by the two methods are employed in subsequent calculations. The observation that the yield of TI following excitation to S2 is less than quantititave is consistent with the reports of Das and co-workers20,21who examined the yields of triplet states of several thiones in laser flash photolysis experiments. For the aromatic thiones their values of the quantum yields of triplet formation, ca. 0.5-0.6 with error limits of f20%, are in agreement with the The present work dempresent comparable values of 4>/4:1. onstrates, however, that the inefficiency in the production of TI is due not only to bypassing the SI and TI states altogether, but TI efficiency which is less than 1.00. The latter also to an SI observation could not be made in the laser flash photolysis work because of the larger errors inherent in that method. The observation that $:l/$pTl is generally large but still significantly smaller than 1.00 for both solvents is in excellent agreement with recent experiments in which the intensities and quantum yields of phosphorescence were measured following clean selective excitation by a tunable dye laser into the SI and T, states of the same thionesS3O The present data are also consistent with the observation of weak thermally delayed fluorescence in these thiones at room temperat~re,~ and suggest that not only radiative but also nonradiative decay of SI to So must be taken into account in describing the dynamics of excited thione relaxation. Indeed, because even at room temperature the quantum yield of thermally delayed SI Sofluorescence is very small, the difference between the value of $J:I/~~'I and unity may be attributed, with good accuracy, to the fraction of SI states which decay nonradiatively So process should directly to So. Because the rate of the SI vary as ~ X ~ ( - A E ( S , - S ~it) )is~ perhaps ~, not suprising that a relatively small (ca. 15000-18000 an-')SI-Soseparation in these thiones should be associated with a relatively large value of the radiationless decay constant. This is unlike the first excited singlet states of the corresponding k e t ~ n e s , ~which ' - ~ ~ possess larger Sl-So energy gaps (25 000-28 000 cm-I) and which exhibit negligible internal conversion in solution at room temperature. The ratios of 4>/$:1 may be calculated from absolute values of 422 and 4'; or, better, from the more accurately measured values of the ratios 4:2/~$~~'and d;l/+:l given in Table I. Because the radiative decay (S, So) of some of these thiones is not negligible, especially in perfluoroalkane solvents, the @>/@21 ratios must be corrected before the efficiency of S2 SI radiationless decay, T $ ~ , , S , ) ,can be calculated. The appropriate values of 4fs2 are given in Table I and are used in the calculation 1 - 4fs2)I-l. This quantity represents the of q(S2,S1)= @$[@,"I( fraction of S2 decaying nonradiatively to S,, assuming (as is T, ( n > 1) So, SI does not occur. reasonable) that S2 The data show that a significant fraction of the S2states of several
thiones decay nonradiatively, even in inert perfluoroalkane solvents, by a path which bypasses SI and TI. The obvious candidate is direct nonradiative decay from S, So. We now examine the possible mechanisms of S2decay following two lines of evidence: the effect of solvent and the effect of thione deuteriation. We have previously s h ~ w n l ~that - ' ~ the nature of the solvent profoundly affects the quantum yields of S2 So fluorescence and the lifetimes of the S2 state by changing the apparent nonradiative rate of S2 decay. (The radiative rates remain approximately constant.) In most common solvents the nonradiative decay rate of S2 is dominated by intermolecular (solvent-excited solute) interactions. Among the solvents examined to date (all liquids at room temperature and 1 bar pressure), only the perfluoroalkanes appear to be sufficiently inert to permit the observation of intramolecular processes such as those illuminated by deuterium substitution in the t h i ~ n e .The ~ ~ present data permit us to describe the effect of intermolecular solvent induction of S2 nonradiative decay more quantitatively. We begin by assuming that we observe exclusively intramolecular S2decay in perfluoroalkane solvents. Using this assumption and using the T~~ and +fs1 data in Table I1 we attribute the ratio ss2(3-MP)/~,,(PF)or 4fs2(3-MP)/+;s'(PF) to the number of S2 states which decay intramolecularly compared to the number which decay by both intramolecular and intermolecular means = 1 - ( T ~ , ( ~ - M P ) / T ~ , ( Prepresents F)) the in 3-MP. Thus P2inte, fraction of S2 states which decay as a result of intermolecular are given Table 11. interaction with 3-MP; the values of Pzinter The fact that the lifetimes of the S2states and the quantum yields Sofluorescence are considerably larger in perfluoroalkanes of S, than in 3-MP does not prove that the same thiones would not exhibit still larger values of $9and ~~2 in even more weakly interacting environments or in a hypothetical ensemble of isolated S2 molecules exhibiting an equilibrium distribution of internal energies at room temperature.l8 Thus the values of p 2 1 n t e , given in Table I1 should be taken as minimum values. We simply note is large for most thiones in 3-MP (and at this stage that painter by extension, therefore, in most other common solvents), providing quantitative support for our earlier s u g g e s t i ~ n ' ~that - ~ ~the , ~ decay ~ of S2is dominated by intermolecular interactions in solvents other than perfluoroalkanes. The effect of selective deuteriation on the lifetimes and fluorescence quantum yields of the S2 states of XT34 in perfluoroalkanes suggest that large amplitude motion of H atoms fl to the thiocarbonyl group (perhaps leading to the reversible formation of chemically distinct unstable intermediates) is responsible for inducing at least part of the S2 So radiationless decay in these molecules. The present data for v(S2,Sl)support this suggestion and provide some semiquantitative indications of the magnitude of the effect. Consider those thiones which exhibit 17(S2,SI) close to 1.00 in perfluoroalkanes: TMIT, PT, and XT-d6 (cf. Table I). TMIT has no fl H atom close to the C=S group since the 5-membered ring of this molecule tilts the sulfur atom away from the aromatic ring hydrogen at position 7 . Similarly PT has no H atom in an accessible p position and, in any case, is not structurally rigid,3Sa fact which is undoubtedly responsible for its very rapid S2 radiationless decay.I4 XT possesses two hydrogen atoms in accessible p positions. However, when these are replaced by deuterium (as in XT-d6), this has the effect of dramatically slowing the rate of S2 decay,34 a process which appears to be induced by large amplitude motion of the p H(D) atoms. Analogies may be found in the decay of intramolecularly hydrogen-bonded aromatic hydroxyketones in which proton transfer in either the S, or TI state constitutes an important mode of r e l a ~ a t i o n . ~Thus, ~ there are good structural or dynamic reasons why these three molecules should exhibit nearly quantitative intramolecular S I S , internal conversion in per-
2488
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(30) Maciejewski, A.; Szymanski, M.; Steer, R. P. J . Phys. Chem., submitted for publication. (31) Lamola, A. A.; Hammond, G. S. J . Chem. Phys. 1965, 43, 2129. (32) Scaiano, J. C. J . A m . Chem. SOC.1980, 102, 7747. (33) Amrein, W.; Larsson, I.-M.; Schaffner, K. Helu. Chim. Acta 1974, 57, 2519.
(34) Abrams, S. R.; Green, M.; Steer, R. P.; Szymanski, M. Chem. Phys. Lett. 1987, 139, 182. (35) Mayer, R.; Bray, W.; Zahradnik, R. Adu. Heterocycl. Chem. 1967, 8, 219. ( 3 6 ) See, for example, Van Bentham, M. H.; Gillespie, G. D.; Haddon, R. C. J . Phys. Chem. 1982, 86, 4281.
J . Phys. Chem. 1988, 92, 2489-2492
--
fluoroalkane solvent, Le., potential competing S2 So decay is suppressed. The errors in q(S2,Sl)are just large enough to prevent a definitive statement to be made about the variation in the efficiency SI decay as the solvent is changed. Nevertheless, of net S2 the values of q(S2,S1) tend to be smaller in 3-MP than in perfluoroalkanes. (BPT may be exceptional for reasons which are unclear at present.) Moreover, the data of Das and co-workers20 apparently extend this trend because they report values of the quantum yields of TI formation following So S2 excitation which are 0.6 f 0.1 for XT and 0.5 f 0.1 for TXT both in benzene.37 (We note, however, that these values may be too low since they are unable to observe that the quantum yield of triplet formation on So SIexcitation is in fact significantly less than 1.OO.) The trend in q(S2,S1)is paralleled by clear differences in the measured S2 lifetimes of XT in different solvents; 175 ps in PF-1,3-DMCH,14 3815 (or 4338)ps in 3-MP and 1239(or 1838)ps in benzene. These data indicate that the relative strength of the interaction between XT in its S2 state and molecules of these three solvents increases in the order PF-1,3-DMCH < 3-MP < benzene, as would be expected on the basis of their physical proper tie^.^^^^^
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(37) An independent measurement gives $>+/I: = 0.65 i 0.04 for XT in benzene. Maciejewski, A., unpublished results. (38) Boens, N.; van den Zegel, M.; de Schryver, F. C. Chem. Phys. Lett. 1984. 111. 340. (39) Anderson, R. W. Jr.; Hochstrasser, R. M.; Pownall, H. J. Chem. Phys. Lett. 1976, 43, 224. (40) Morokuma, K. Acc. Chem. Res. 1977, 10, 294. (41) Brady, J. E.; Carr, P. W. J . Phys. Chem. 1982.82, 3053.
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Two alternative explanations of the means by which 3-MP (and other solvents except the perfluoroalkanes) cause an increase in the apparent rate of radiationless decay of S2would seem to be consistent with all the data. First, the more strongly interacting solvents could simply serve to increase the rate of nonradiative SI (and perhaps to Soor TI), without changing decay from Sz the mechanism of the decay process. In this model the solvent simply serves as an external perturber of the excited thione, causing more effective coupling between S2 and lower electronic states as the solute-solvent interaction energy increases.I8 Only minor variations in q(S2,S1) with solvent are to be expected if this model applies. An alternative model, which requires that q(S2,Sl)be significantly different in different solvents, involves the formation of weakly bound, but nevertheless chemically distinct, dark exciplexes between the S2thione and nonperfluoroalkane solvent molecules. The formation of such exciplexes is consistent with the results of bimolecular quenching experiment^'^.'^ which show that 3-MP and benzene quench S2 thiones such as XT and TMIT at nearly diffusion-limited rates. Measurable exciplex emission or transient absorption has not been observed but is to be expected in some favorable cases such as S2 XT in benzene. Attempts to obtain definitive evidence for such species are under way.
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Acknowledgment. W e acknowledge with gratitude the continuing support of the Natural Sciences and Engineering Research Council of Canada. Financial support under Polish Research Project CPBP 01.19 for part of this work is also gratefully acknowledged.
Supersaturated Solutions. 1, Equilibrium in the Decarbonylatlon of Formic Acid by Sulfuric Acid Peter G. Bowers* and Kara Hodest Department of Chemistry, Simmons College, Boston, Massachusetts 021 15 (Received: October 26, 1987)
In a dilute unstirred solution of formic acid in concentrated sulfuric acid, the carbon monoxide produced may reach a supersaturation concentration up to 0.105 M. Under these conditions a metastable equilibrium is established between formic acid, carbon monoxide, and water which prevents completion of the decarbonylation process. The equilibrium constant at 22 O C , based on concentrations, is 19 5. Formic acid decay resumes if carbon monoxide is removed by pumping or by sonication.
*
Introduction Recent studies in the decomposition of formic acid in sulfuric acid (the Morgan reaction) have chiefly centered around the oscillatory kinetics of carbon monoxide release that occurs at high formic acid c~ncentration.'-~ A detailed mechanism for the oscillations was first proposed by Smith and Noyes: based on periodic release of extremely high supersaturation concentrations of dissolved gas, up to almost 0.1 M. In measuring these concentrations, Smith5v6found that in a dilute unstirred solution the Morgan reaction does not in fact proceed to completion but reaches an equilibrium in accordance with eq 1. HCOOH(so1n) * CO(so1n) + HzO(soln) (1) This discovery was more or less incidental to Smith and Noyes' work on the gas release mechanism, and the reverse reaction plays no significant role in the oscillatory region. The equilibrium is metastable because the dissolved gas concentration is much higher Present address: Park Davis Research Division, Warner Lambert Company, 170 Tabor Road, Morris Plains, NJ 07950.
0022-3654/88/2092-2489$01.50/0
than the thermodynamic solubility of the gas under the conditions of the experiments. (True thermodynamic equilibrium, of course, would require that the high CO concentration be balanced by a correspondingly high external CO gas pressure, in accordance with Henry's law.) For example, starting with 0.046 M formic acid in 92% sulfuric acid at 40 "C, Smith found [CO], = 0.024 M, compared to the natural solubility, 9 X M. The evidence (1) Bowers, P. G.; Noyes, R. M. In Oscillations and Travelling Waues in Chemical Systems; Field, R. J., Burger, M., Eds.; Wiley: New York, 1985; pp 473-492. (2) Kaushik, S. M.; Rich, R. L.; Noyes, R. M. J . Phys. Chem. 1985,89, 5722-5725. (3) Yuan, Z.; Ruoff, P.; Noyes, R. M. J . Phys. Chem. 1985, 89, 5726-5732. (4) Smith, K. W.; Noyes, R. M. J . Phys. Chem. 1983, 87, 1520-1524. (5) Smith, K. W. Ph.D. Thesis, University of Oregon, Eugene, OR, 1981. Available from University Microfilms International, 300 N. Zeeb Road, Ann Arbor MI, 48106. (6) Smith, K. W.; Noyes, R. M.; Bowers, P. G. J . Phys. Chem. 1983, 87, 1514-1519.
0 1988 American Chemical Society
