821
SOLVENT EFFECTS ON ACETOPHENONE PHOTOREDUCTION
Solvent Effects on Acetophenone Photoreduction Studied by Laser Photolysis by Hanspeter Lutz, Marie-Christine Duval, Emilienne BrChCret, and Lars Lindqvist* Laboratoire de Photophysique MolBculaire,’ UniversitB de Paris-Sud, Centre d’orsay, Ql-Orsay, France (Received August 23, 1.971) Publication costs assisted by the Centre National de la Recherche Scientifique
The influence of the nature of the solvent on the rate constant of hydrogen abstraction, k,, by acetophenone and benzophenone in the triplet state was studied by direct triplet lifetime measurements in mixed solvents using the 265- and 353-nm harmonics of a &-switched Nd glass laser as excitation light source. A significant decrease in k , with increasing solvent polarity was observed for acetophenone in 2-propanol-benzene and 2-propanolwater solutions; this effect was not shown by benzophenone. The results are discussed in terms of mixing of n,n* and T,T* triplet states.
An interesting feature in the photoreduction of acetophenone by 2-propanol is its reported dependence on the alcohol concentration. The photoreduction quantum yield of acetophenone in benzene increases with the concentra,tion of added 2-propanol up to 1 M of alcohol and decreases at higher alcohol contents.2 A similar effect has been observed for ben~ophenone.~ The concentration dependence has been related to the formation of an intermediate which may act as light filter or as quencher of triplet k e t ~ n e . ~ ~ I~n- a’ recent investigation of the acetophenone photoreduction, Lewis8 eliminated the influence of a possible intermediate by working at very low conversions and light intensities. The quantum yields were nevertheless low a t high alcohol concentrations, and Lewis attributed this to the presence of a quenching impurity in commercial 2-propanol. The primary process in these photoreductions is known to be the reaction of triplet ketone to form the ketyl radical. We have studied the influence of the nature of the solvent on this process by determining directly the lifetime of triplet acetophenone and benzophenone in mixed solvents using the laser photolysis technique and by relating this lifetime to the reactivity of the triplet state. The sample to be studied, contained in a 1 cm square silica cell, was exposed to the third (353 nm) or fourth (265 nm) harmonic (pulse half-width, 30 nsec) of an Nd glass laser (Compagnie GBn6rale d’Electricit6). Transient optical densities were measured photoelectrically in a crossed-beam arrangement using a xenon flash lamp as monitoring light source. Acetophenone (Eastman) was purified by vacuum distillation. Benzophenone (Hoplkins) was sublimed after recrystallization from ethanol-water. Benzene and 2-propanol (Rlerck) were spectroscopic grade. The solutions were degassed thoroughly. On laser excitation at 353 nm of solutions containing acetophenone (0.1 M ) or benzophenone (0.007 M ) in 2-propanol-benzene, transient absorption due, to triplet
formation was observed. A slo~vlydisappearing residual absorption assigned to the ketyl radical remained a t the end of the triplet decay. For acetophenone, the triplet decay was obtained from measurements at 380 nm, after correction for absorption due to radical formation during the triplet decay.g This correction was based on the assumption that the residual absorption due to the ketyl radical is formed at the same rate as the triplet disappears. For benzophenone, two different monitoring wavelengths were chosen: 525 nm, the maximum of the triplet absorption, and 600 nm, a wavelength at which the ketyl absorption may be neglected compared to that of the triplet. lo In Figure 1the overall decay rate constants, k , are plotted for acetophenone (curve a) and for benzophenone (curve b) as a function of the 2-propanpl concentration in benzene. Acetophenone (0.0005-0.0015 M ) was similarly studied in 2-propanol-water solution. The triplet, produced by excitation at 265 nm, was monitored a t 350 nm. Curve c, Figure 1 shows the corresponding variations in the value of k . The rate constants for hydrogen abstraction from 2-propanol, IC,, as represented in Figure 2, were obtained from the expression k = kd k, [RH] where RH represents 2-propanol and k d is the rate constant of triplet decay including all decay processes except hydrogen
+
(1) Groupe de recherche du C.N.R.S. (2) S. G. Cohen and B. Green, J . Amer. Chem. Sac., 91, 6824 (1969). (3) (a) S. G. Cohen, D. A. Laufer, and W.V. Sherman, ibid., 86, 3060 (1964) ; (b) S. G. Cohen and R. J. Baumgarten, ibid., 89, 3471 (1967). (4) J. N. Pitts, R. L. Letsinger, R. P. Taylor, J. M. Patterson, G. Recktenwald, and R. B. Martin, ibid., 81, 1068 (1959). (5) H. L. J. Backstrom, K. L. Appelgren, and R. J. V. Niklasson, Acta Chem. Scand., 19, 1555 (1965). (6) N. Filipescu and F. L. Minn, J . A m e r . Chem. SOC., 90,1544 (1968). (7) P. J. Wagner, Mol. Photochem., 1,71 (1969). ( 8 ) F. D. Lewis, J . Phys. Chem., 74, 3332 (1970). (9) H. Lutz and L. Lindqvist, Chem. Commun., 493 (1971). (10) H. Tsubomura, N. Yamamoto, and S. Tanaka, Chem. Phys. Lett., 1, 309 (1967).
The Journal of Physical Chemistry, Vol. 76, N o . 6,1973
822
H. LUTZ,M. DUVAL,E. B R ~ H ~ R E AND T , L. LINDQVIST 20 I
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2-PROPANOL CONC. (M)
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I
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I
2
4
6
8
10
I
1
NEAT
2-PROPANOL CONC. (M)
Figure 1. Triplet decay rate constant as a function of the concentration of 2-propanol added to : (a) acetophenone in benzene; (b) benzophenone in benzene; and (c) acetophenone in water.
Figure 2. Rate constant of hydrogen abstraction as a function of the concentration of 2-propanol added to: (a) acetophenone in benzene; (b) benzophenone in benzene; and (c) acetophenone in water.
abstraction from 2-propanol. The value of k d was 3.3 X lo6sec-' for acetophenone in benzene and 2.3 X lo5 sec-' in water; for benzophenone in benzene, kd was 2.5 X l o 5 sec-l. The values of k d may be slightly affected by a second-order triplet-triplet annihilation decay due to the high triplet concentration required to observe the triplet. In our calculation, k d was assumed to be independent of [RH]. The full line curves in Figure 2 show that k, decreases with increasing alcohol concentration in the case of acetophenone but remains constant in the case of benzophenone. This difference in behavior may be due to variations in energy of the lowest-lying n,a* and n,a* triplet states. Lamolal' showed in a phosphorescence study that acetophenone in a nonpolar medium exhibits a ala*triplet state situated only slightly above the n,a* triplet state. In solutions of high alcohol content, the n electrons in the ground state become hydrogen bonded, and the energy of the n,a* triplet state is raised with respect to the ala*triplet state. As the n,a* triplet becomes more energetic, it assumes more ala* character, leading to decreased photochemical reactivity. This effect is expected to occur to a much higher extent in acetophenone than in benzophenone, due to the larger energy gap between the n,a* and T,T* triplets in the latter compound.12 The influence of solvent polarity on the reactivity of acetophenone triplet is particularily evident in the photoreduction in 2-propanol-water solutions as shown by curves c in the Figures 1 and 2. In water solution containing 2 M 2propanol k, is only 0.5 X lo6 M-l sec-l. It has been postulated that the polar solvent effects may arise from solvation or hydrogen bonding with the ketone triplet.2 The fact that the triplet reactivity of benzophenone is not altered in alcoholic solvents sug-
gests that hydrogen bonding does not exist or is very weak in the n,a* triplet state. The same is, of course, expected to be true for acetophenone. Since the carbonyl oxygen becomes electron deficient i.t1 the n,a* triplet state, the hydrogen-bonding interaction is supposed to be weak. The values of the rate constants for hydrogen abstraction obtained by nanosecond laser photolysis may be compared with results from indirect measurements. For benzophenone, our value for k, agrees reasonably well with the results of Beckett and Porter13 (1.28 X lo6 M-l sec-l) and those of Yang and Dusenbery14 (1.25 X 106 M-' see-l). For acetophenone however, the rate constants for hydrogen abstraction in benzene solutions containing 2 M 2-propanol reported by Yang and Dusenberylb (4.33 X lo5 M-' sec-l) and Lewis8 (7.5 X 105 M-' sec-1) are significantly lower than our value of 1.2 X 108 M-1 sec-' at the same alcohol concentration. It may be noted that the indirect methods include solvent effects on all secondary reactions involved in the photoreduction. We conclude from the present study that a substantial part of the low efficiency of acetophenone photoreduction in polar solvents may be attributed to a decreased reactivity of the acetophenone triplet state. This reduced reactivity is supposed to be due to the higher a,a* character of the acetophenone triplet in polar solvents.
The Journal of Physical Chemistry, Vol. 76, N o . 6,1078
(11) A.A.Lamola, J . Chem. Phys., 47,4810 (1967). (12) M.Batley and D. R. Kearns, Chem. Phys. Lett., 2, 423 (1968). (13) A.Beckett and G . Porter, Trans. Faraday Soc., 59,2038 (1963). (14) N. C. Yang and R. L. Dusenbery, Mol. Photochem., 1, 169 (1969). (15) N. C. Yang and R. L. Dusenbery, J . Amer. Chem. SOC.,90, 5899 (1968).
