The Journal of Physical Chemistry, Vol. 82, No. 23, 1978 2489
Photophysical Studies on Benzil
gramming the least-squares analysis.
References and Notes (1) M. A. A. Clyne, J. A. Coxon, and L. W. Townsend, J . Chem. SOC., Faraday Trans. 2, 68, 2134 (1972). (2) M. A. A. Clyne, J. A. Coxon, and H.W. Cruse, Chem. Phys. Lett., 6, 57 (1970). (3) M. J. Bina and S.G. Hadley, AFWL-TR-73-173, “Laser Digest”, 1973. (4) J. A. Coxon, Chem. Phys. Lett., 33, 136 (1975). (5) M. S. Child and R. B. Bernstein, J. Chem. Phys., 59, 5916 (1973). (6) J. W. Birks, S.D. Gabelnick, and H. S.Johnston, J. Mol. Spectrosc., 57. 23 (19751. (7) M . ’ A A . Clyne and I. S.McDermid, J. Chem. Soc., Faracky Trans. 2, 73, 1094 (1977). (8) A. L. Wahrhaftia, J. Chem. Phys., 10, 248 (1942). (9) H. J. SchumacKer, A. Schmitz,-and P. H. Broderson, Anal. Asoc. Quim Arg., 38, 98 (1950). (10) W. Stricker and L. Krauss, Z. Naturforsch. A , 23, 1116 (1968). \
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(11) A. H. Nielson and E. A. Jones, J. Chem. Phys., 19, 1117 (1951). I121 R. D. Coombe. A. T. Pritt. Jr.. and D. PiliPovich. “Electronlc Transition Lasers”, Vol. 11. L. E. wilson, S.N. Suchard, and J. I. Steinfeld, Ed., MIT Press, Boston, 1977, p 107. (13) J. B. Levy and B. K. Wesley Copeland, J . Phys. Chem., 69, 408 (1965). (14) T. J. Malone and H. A. McGee, Jr., J. Phys. Chem., 69, 4338 (1965). (15) J. N. Kieth, I. J. Solomon, I. Sheft, and H. H. Hyman, Inorg. Chem., 7, 230 (1968). (16) P. P. Chegodaev, V. I. Tupikov, and E. G. Strukov, Russ. J . Phys. Chem., 47, 746 (1973). (17) A. R. Young, 11, T. Hirata, and S.I. Morrow, J. Am. Chem. Soc., 86, 20 (1964). (18) J. A. Coxon and M. A. Wickramaaratchi, J . Mol. Spectrosc., 68, 372 119771 -(19) R I G . Derwent and B. A. Thrush, J. Chem. Soc., Faraday Trans. 2, 68, 720 (1972). (20) P. H. Broderson and J. E. Sicre, Z. Phys., 141, 34 (1955). (21) E. HulthBn, N. Jarlsater, and L. Koffman, Ark. Fys., 18, 479 (1961).
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Photophysical Studies on Benzil. Time Resolution of the Prompt and Delayed Emissions and a Photokinetic Study Indicating Deactivation of the Triplet by Reversible Exciplex Formation Tal-Shan Fang, Ronald E. Brown, Chi Lup Kwan, and Lawrence A. Singer* Department of Chemistty, Universlty of Southern California, Los Angsles, California 90007 (Received June 16, 1978)
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The prompt fluorescence and delayed emissions from benzil in benzene at room temperature were elucidated by laser time-resolved spectroscopy. The delayed emissions consist of thermal (Tl SI) (TDF) and P-type (T, + T1 S1 + So) delayed fluorescences and phosphorescence. From the temperature dependence of the TDF, the singlet-triplet splitting is estimated to be 5.4 f 0.4 kcal/mol. The extinction coefficients for absorption by the benzil T1were measured at several wavelengths relative to 6532.5 for benzophenone Tp From flash photolysis data, using an €460 of 7510 f 290 and an €480 of 8350 f 370 for the benzil T1, the rate constant for triplet-triplet annihilation is estimated to be (9.6 f 1.1)X lo9 M-l s?. The data for quenching of the benzil phosphorescence with anisole and benzonitrile from transient experiments give nonlinear Stern-Volmer plots which is shown to be consistent with deactivation by way of reversibly formed exciplexes. The transient data for the selfquenching of the benzil triplet similarly gives a nonlinear Stern-Volmer plot. Computer analyses of these data provide estimates for a number of the kinetic parameters.
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Benzil and other nonrigid 1,2-dicarbonyls have been intensely studied in recent years1 with the coupled goals of elucidating their numerous and interesting photophysical pathways and their photochemical reactionsa2 Both fluorescence and phosphorescence generally are observed in fluid media at room temperature which allows direct monitoring of the ( n , ~ * SI ) ~and T1 states. The fluorescence from benzil shows a large (5650 cm-l in 3methylpentane at 300 Kid) Stokes shift which is attributed to a rapid skewed trans-planar relaxation in the S1 state.lbl4 Our interest in these systems evolved out of our earlier studies on the benzophenone^.^ In that project, we elucidated the prompt and delayed fluorescences (thermal and P - t ~ p e ~from ~ - ~the ) benzophenones as well as the dual mechanisms for exciplex formation between the benzophenone triplets and benzene derivative^.'^ The two competing exciplexes, with partial charge transfer character, differ in the donor/acceptor roles assumed by TI and
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Q.
In this paper, we show that the photophysical pathways operating in the S1 and T1states of benzil are similar to those elucidated in the benzophenones. Through laser 0022-3654/78/2082-2489$0 1.OO/O
time-resolution spectroscopy, we sort out the prompt and delayed fluorescences (thermal and P-type) and in a flash photolysis experiment, we measure ha, the rate constant for triplet-triplet annihilation. The benzil phosphorescence quenching data (anisole and benzonitrile as quenchers) from transient experiments give nonlinear Stern-Volmer plots which is shown to be consistent with deactivation by way of reversibly formed exciplexes. The transient data for benzil triplet self-quenching in benzene similarly yields a nonlinear Stern-Volmer plot. A number of the important kinetic parameters for the quenching and self-quenching reactions are derived and compared.
Results and Discussion Time-Resolved Emission Spectra. The emission from benzil in benzene under deaerated conditions, following nitrogen laser excitation (337-nm ,- 10-ns pulse width), consists of fast (I several nanoseconds) and slow (1 microsecond) decaying components. Figure 1 shows a series of time-gated emission spectra (boxcar spectra) for 5.0 X M benzil in benzene at 25 OC. Similar results are obtained in hydrocarbon solvents. The spectrum of the fast emission, recorded a t Td = 0 ns, is identical with 0 1978 American Chemical Society
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The Journal of Physical Chemistty, Vol. 82, No. 23, 1978
Singer et al.
Flgure 3. Effect of llght intensity on P-type DF from 1.0 X IO-* M benzil in benzene. Spectra recorded at T,, = 2 ps, T, = i ps and at relative light intensities of (a) 1.0 and (b) 0.1.
Scheme J
,/ 450
, ,
,
,
,, ,
,
, , , ,
,
nm
600
550
600
660
Flgure 1. Boxcar spectra for 5.0 X M benzll in benzene at 25 OC: (a) r d = 0 ns, T, = 20 ns. Others with T, = 200 ns and (b) Td = 200 ns, (c) Td = 5 ps, (d) Td = 20 ps, and (e) T, = 40 ys.
il
Ib
I
403
MO
so
\
nm
S
600
I
700
Flgure 2. Total emission spectra for 5.0 X M benzil in benzene at 25 OC: (a) deaerated solution; (b) aerated solution.
the total emission observed from an aerated solution of benzil in benzene under steady state conditions (see Figure 2b) and is assigned to the prompt fluorescence. For dilute solution ( ki,,, k-isc. The quantity e-hE/RT is the fraction of triplets found at the level isoenergetic with so that aE is singlet-triplet splitting. Accordingly, the ratio of the intersystem crossing rates is N
The Journal of Physical Chemistry, Vol. 82, No. 23, 1978 2491
Photophysical Studies on Benzil
TABLE I: Extinction Coefficients for the Benzil Triplet Absorption at Several Wavelengths in Benzene at 25 O c a wavewavelength, length, nm f frel nm E Ere]
h
0.38 486 8350k 370b (1.0) 395 3170 0.77 0.43 500 6420 415 3630 0.38 0.67 510 3210 440 5630 0.23 460 7510k 290b 0.90 532 1910 a Determined relative t o benzophenone triplet, E 532 = 7630.'" This value was used to calculate the E'S at the other wavelengths for the benzo henone triplet using its relative absorption s p e ~ t r u r n . ' ~ Average of three determinations.
'
92"c
the excitation axis, as the probe beam. The solution to differential eq I1 is [T~IO/[TII= 11 + (ka/Ch)[T1101 exkt- (ka/Ck)[T110 (111) which when expressed in absorbance units becomes A-l = A 0.05 M benzil. The complications from triplet-triplet annihilation in the transient experiment were overcome by measuring the decays at long t's where they were exponential. The derived kinetic estimates for the benzil-anisole, -benzonitrile, and self-quenching systems appear in Table 111. The estimates of kQ with anisole and benzonitrile are known with reasonable accuracy from the photostationary experiment. The values for k, are more uncertain, but are clearly different. The k l / k 2 values, within the limits of our analysis, are the same and indicate that the dissociated and exciplex states for these systems are very nearly isoenergeticaZ6 The estimated range of hl/kp for selfquenching is significantly larger and indicates that this exciplex is stabilized relative to the dissociated state. At this time, we do not have an explanation for the relative stability difference between the exciplexes involving anisole or benzonitrile and the self-quenching exciplex. We do not know yet if the solvent difference (cyclohexane vs. benzene) is a key factor. Finally, we note that our inability to derive absolute values for hl and k 2 is a consequence of instrumental
limitations. We find that computer simulations of the emission decays are quite sensitive to the absolute values of kl and kz. When these parameters are ?lo6 in absolute magnitude, exponential decays (ae-xlt) are predicted by eq VI at times s. However,
