4573
J. Phys. Chem. 1993,97, 4573-4576
ARTICLES Definition of the Nature of Ketone Triplet States on the Basis of Singlet Oxygen Generation Efficiency Alexander P. Darmanyan and Christopher S. Foote’ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 Received: December 9, 1992
The efficiency of singlet oxygen generation (SA) was measured for acetophenone, p-aminobenzophenone, and Michler’s ketone by the time-resolved 02(*Ag) phosphorescence method in various solvents. The SAvalue for acetophenone is 0.3-0.4 in benzene, cyclohexane, acetonitrile, and alcohols (practically independent of solvent); the lowest triplet state of acetophenone is therefore probably pure n,r* in all solvents. The values for Michler’s ketone and p-aminobenzophenone are 0.24-0.4 in all solvents investigated (except in ethanol for Michler’s ketone) and are also typical for ketones with n,?r* triplet states. For Michler’s ketone in ethanol, SA= 0.65; the triplet state therefore probably has substantial C T character. Amino-substituted benzophenones do not undergo photoreduction in alcohols, which is explained partly by a low quantum yield of triplet and mainly by the considerable negative charge at the carbonyl oxygen because of intramolecular charge transfer, especially in alcohols. As a result, the oxygen atom in the 3n,r*state is considerably less electrophilic than in benzophenone. Internal conversion is the main deactivation channel for the excited singlet state of Michler’s ketone and p-aminobenzophenone and probably occurs by fast back electron transfer with deactivation to the ground state; the rate of this process increases strongly with decreasing energy of the C T state.
Introducti0n The efficiencyof singlet oxygen generation (SA) is very sensitive to the electron density distribution in the lowest state of ketones. For T,T* triplet states, SA 0.8-1.0, while for n,r* triplets SA 0.3-0.4.1-12 Intramolecular triplet charge-transfer states are similar in character to 3r,r*statesI3J4and would therefore also be expected to have SAvalues near 1. For example, Chattophadhyay et al. showed that that the efficiency of 02(lAs) generation from oxygen quenching of the metal-to-ligand CT triplet state of a tris(bipyridine)ruthenium(II) complex is approximately These results suggest that it should be possible to define the nature of the lowest triplet state of ketones by measuring the SA values. On the other hand, introduction of different substituentsor changing the medium can lead toinversion of the ordering of the n,r* and r,r*or CT triplet states in ketones (see ref 13 for a review). For acetophenone, analysis of spectra, phosphorescencelifetimes,lS and triplet-triplet absorption spectra in various solventsI6 led to the conclusion that the 3n,?r*and 3 ~ , states ~ * have very similar energies and that, in nonpolar hydrocarbons, the 3n,T*is the lowest state, in cyclohexane they are isoenergetic, but in acetonitrile or hydrogen-bonding solvents such as alcohols, the lowest state is %r,r*.Porter and Suppan (and many later investigators) have shown that 3n,?r* ketones characteristically abstract hydrogen atoms from donors very efficiently and have phosphorescence spectra with characteristic vibrationalstructure, shortphosphorescencelifetimes,and a small SI*-T~ p l i t t i n g . ~Moreover, ~J~ the dipole moment in the 3n,?r* state is lower than in the ground state.18J9 In comparison with benzophenone, amino-substituted benzophenones have an additional strong CT absorption band in their electronic14 and T-T absorption20 spectra. Analysis of the photochemical data led to the conclusion that in p-aminobenzophenone and Michler’s ketone the lowest state in benzene and cyclohexane is 3n,r* but is CT or T,T* in alcohols.~*~2l However, Brown and Porter,Z2Shimamoriet a1.,19and Hoshino and KogureZ0 concluded from measurement of the triplet dipole moment that
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Michler’s ketone triplet state is CT even in benzene and cyclohexane. Brown and Porter explained the efficient reduction of Michler’s ketone by cyclohexane as resulting from thermal population of the 3n,r* state.22 Thus, the definition of the nature of the lowest triplet state of ketones is not yet unambiguous and new independent methods for probing this question are important. In the present work we measured the efficiency of 02(lAg) generation by acetophenone, paminobemophenone, and Michler’s ketone in various solvents and define the nature of the lowest triplet states on this basis.
Experimental Section 1-Methylnaphthalene and the ketones were used as purchased from Aldrich. Solvents from Fisher Scientific were purified by distillation. Experiments were carried out at 23 f 1 OC. Absorption spectra were recorded on a Beckman DU-25 spectrophotometer. The decay kinetics of the sensitizer triplet state were measured by nanosecond laser photolysis, and the yield and decay kinetics of 02(lA,) luminescence at 1.27 pm were determined with an IR laser fluorimeter, as described elsewhere.IOJI The solutions were excited at 355 nm by 1-5-mJ 7 4 s pulses from a Quanta-Ray Nd:YAG laser. The decay kinetics were averaged over 1-50 laser pulses. Optical densities of the solutions were -0.3 at 355 nm, corresponding to acetophenone concentrations of (2-5) X 1 t 2 M, Michler’s ketone concentrations of 4 X M in cyclohexane and (0.7-2.0) X l t 5 M in other solvents, and M,1 X 10-4 M, p-aminobenzophenone concentrations 3 X le5 and 8 X 10-4 M in 2-propanol, acetonitrile, and cyclohexane, respectively. Singlet oxygen quantum yields (@A*) were measured in solutions that were air-saturated or bubbled with different 02/ N 2 mixtures. The standard was benzophenone in air-saturated benzene @Ast = 0.3.3968 The method, oxygen solubilities, and relative phosphorescence Oz(IA,) rate constants were reported previously.
0022-3654/93/2091-4513~04.Q0/0 0 1993 American Chemical Society
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4574 The Journal of Physical Chemistry, Vol. 97, No. 18,19'93
Darmanyan and Foote concentration range 0.06-0.13 M in cyclohexaneand acetonitrile and 0.3-0.5 M in 2-propanol. The @T value for Michler's ketone in benzene (table) is in excellent agreement with the values 1-OZ4 and 1.0l2I obtained by the method of Lamola and H a m m ~ n d . ~ ~ The value aT= 1.02 for acetophenonein acetonitrile agrees well with the expected value of 1.Oand confirms that the T-T energytransfer method works correctly for ketones. The koT values for Michler's ketone in benzene, cyclohexane, acetonitrile, and ethanol (Table I) are slightly higher than the limiting values extrapolatedto zero ketone concentrations reported in ref 21. This fact is probably caused by some self-quenching of the triplet states in the present work, which does not affect the measurements.
L
0
200
400 14021, M-1
Figure 1. Dependence of quantum yield of 02(lAg) generation by acetophenoneon oxygen concentrationin (a) 2-propanol, (b) ethanol, (c) cyclohexane, and (d) acetonitrile.
Triplet decay rate constants in degassed (koT)and air-saturated (k,T) solutionwere measured in the region of the T-T absorption maximum (-450 nm (shoulder) for acetophenone,l6 -600 nm for Michler's ketone,20and 500 nm forp-aminobenzophenone). The quenching rate constants of the triplet states by molecular oxygen (kqT)were estimated for acetophenonein acetonitrile and substituted benzophenones in all solvents from eq 1.
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Results The quantum yield of 02(lAg) generation at [O,] (*A") was evaluated from the dependenceof @hob"values on the oxygen concentration in solution (Figure 1). Plots of l/QA0Obb vs 1/[02] were linear and the intercept on the ordinate axis is l / @ ~ " . l O By d e f i n i t i ~ nS, ~A ~values were obtained from eq 2:
where @T is the quantum yield of the sensitizer triplet. Since acetophenone in acetonitrile and amino-substituted benzophenones in all solvents (except p-aminobenzophenonein cyclohexane) have long-lived triplet states (Table I), @ A values ~ ~ were ~ practically independent of the oxygen concentration. In these cases, the @A" values were obtained from averaged (PAobsvalues in solutions with different oxygen concentrations. According to ref 10, the slope a of the linear plot of the figure equals (3) The kqTvalues for acetophenone in cyclohexaneand alcohols were estimated from eq 3, using literature data for koT and the aA"values of the present work (Table I). The quantum yields of the triplet were obtained by the T-T energy-transfermethod, using 1-methylnaphthaleneas the triplet acceptor.23 The standard was benzophenone, assuming that the aTvalue is 1.O in benzene24 and other solvents. We measured the optical density of T-T absorption of 1-methylnaphthaleneat 425 nm by independently exciting benzophenone and ketone under conditions of total quenching of sensitizer triplets. The ketone triplets have higher energies than 1-methylnaphthalene (ET = 20 850 cm-1)25in all solvents.14J7 The quenching rate constant of the benzophenone triplet by 1-methylnaphthalene in benzene is 9.4 X lo8 M-1 s - I , ~ ~ and we evaluated this constant only for p-aminobenzophenone in 2-propanol ( 5 X 108 M-1 s-I) and Michler's ketone in acetonitrile (6.5 X lo9 M-I s-l). The @T values obtained (*lo%) are presented in the table. They do not depend on the 1-methylnaphthalene concentration over the
Discussion The SA value for acetophenone is constant in benzene, cyclohexane, and acetonitrile (Table I), which implies that the lowest triplet state of this ketone is n,r* in these solvents. In alcohols, the SAvalue is slightly higher (-0.4). The same regularity was observed for benzophenoneand was explained by specific solvation of the ketone by hydrogen-bonding alcohols.I2 Mixing with the higher-energy 3a,a*state is absent in benzophenone even in strongly hydrogen-bonding alcohols (see refs 13,30, and 3 1for review) because of the large energy gap between the 3n,r* and 3a,r*states.32-33 Specific solvation of the ketone triplet state by alcohols has been observed by several group^.^^-^^ Thus theconclusion of earlier ~orkersl5-1~J3 that 3n,7r*and 37r,7r* states of acetophenone are very similar in energy and that the %,r*state is the lowest state in polar solvents is not reflected in the SAvalues, and we suggest that the lowest triplet state of acetophenone is pure n,r* in all solvents. In all solvents,thevaluesof kgT/kdirare-0.1 for acetophenone, while the SAvalues are -0.3-0.4 (Table I). Thus, as in ref 12, we conclude that the usual approachS6of using kqT I/gkdir for the estimation of the sensitizer triplet state quenching by oxygen by the energy-transfer mechanism cannot be used. The same conclusion about lack of the triplet-state inversion can be made for p-methoxyacetophenone. Kearns et al.37and Yang et al.38measured phosphorescence excitation spectra for p-methoxy- and p-hydroxyacetophenonein glass matrixes at 77 K and concluded that the lowest triplet state is %,7r* for both ketones and that the 3n,7r*state lies -2000 and 2500 cm-I higher for p-methoxy- and p-hydroxyacetophenone,respectively. If so, SA should be 1 for both ketones. However, SA = 0.27 and 0.42 for p-methoxyacetophenonein benzene and acetonitrile, respectively.5 Thus, as for acetophenone, the lowest triplet state of p-methoxyacetophenone is most probably n,r*. According to the well-known El-Sayed mechanism, spin-orbit coupling is very large and intersystem crossing is very rapid (lolo10lI SKI)only between h , a * and %,7r* states of heteroaromatic However, experimental evidence for an intermediate 37r,7r* state between the In,** and 3n,a* states in acetophenone and benzophenone is lacking in spite of the large rate of intersystem crossing in these molecules. In particular, the So 3u,~* transition was not observed in phosphorescence excitation spectra in a glass matrix at 77 K32,37or in a supersonic jet for acetophenone and benzophenone4I and pyrazine.42 Thus, as in ref 4 1, we concludethat it is necessary to reconsider the mechanism of fast intersystem crossing in ketones. The fluorescence quenching of organic molecules by oxygen occurs at a diffusioncontrolled rate,23.29and, when the SI*-Tsplitting is smaller than 8000 cm-*, quenching occurs solely via enhanced intersystem crossing in the excited molecule. Probably the presence of the oxygen atom near the a*orbital sharply increases the spin-orbital coupling and leads to fast intersystem crossing. The Sa values lie between 0.24 and 0.42 (Table I) for aminosubstituted benzophenones in all solvents, with the exception of Michler's ketone in ethanol, and are typical for ketones with
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Nature of Ketone Triplet States
The Journal of Physical Chemistry, Vol. 97, NO. 18, 1993 4515
TABLE I: Photophysical Properties of Ketones sensitizer acetophenone
Michler's ketone
p-aminobenzophenone
solvent cyclohexane benzene acetonitrile 2-propanol ethanol cyclohexane benzene acetonitrile 2-propanol ethanol cyclohexane acetonitrile 2-propanol
X,,,(CT),'
335 347 352 360 368 302' 320' 334'
nm
@T
l.Od 1.08 1.02 l.Od l.Od 0.911 0.96 0.19 0.24, 0.08, 0.92 0.067 0.05
0.33 0.33h 0.29 0.41 0.38 0.29 0.23 0.059 0.10 0.052 0.22 0.023 0.019
SA
koT,bX 10" s-I
0.33 0.33h 0.29 0.41 0.38 0.32 0.24 0.31 0.42 0.65 0.24 0.34 0.39
4.4' 0.33'
0.50 9.0' 7.1' 0.05 0.07 0.07 0.03 0.06 6.0 0.15 0.6
kqT,X
M-I s-I 2.Y 4.1 4h 4.1b 1.9 2.Y 11.0b 12.3b 11.4k 5.0b 6.0b 6.0, 11.06
kqT/kd,f 0.08 0.14 0.1 1 0.08 0.10 0.37 0.44 0.31 0.22 0.24 0.20 0.30
Error f 2 nm. Error *20%. The values of diffusion-controlled rate constants were taken from ref 29 (see also ref 12). Assumed. e Reference 27. /Error f30%. g Reference 24. Reference 7. Reference 28.1 Reference 21. Reference 5 .
*
3n,1r*states.I-l2 Fessenden et al.ls and Shimamori et al.I9 showed that amino-substituted benzophenones have considerable intramolecular charge separation in both ground and triplet states. Our experimental data suggest that the **-electron density distribution in the region of the excitation is the same for aminosubstituted ketones and benzophenone. Only for Michler's ketone in ethanol does it appear that the large value of SA is caused by mixingof 3 n , ~and * CT states. Amino-substituted benzophenones are photoreduced very efficiently by benzene and cyclohexane and not at all (quantum yield < by 2-propan01.~~3~8 This fact can be explained by the low triplet yield and mainly by the large negative charge a t the oxygen atom caused by electron transfer from amino group and aromatic ring, especially in alcohols. As a result, the electrophilicity of the oxygen atom in the 3n,1r*state and its ability to abstract an H atom is much lower than for benzophenone. This suggestion is confirmed by thevalues of the dipole moment in the triplet state:I9 2.1,3.4, and 7.8 D for benzophenone, p-aminobenzophenone and Michler's ketone, respectively. For amino-substituted benzophenones the value of kqT/kdif l / 3 - I/5 (Table I), while the SAvalue, with the exception of Michler's ketone in ethanol, is practically the same as for acetophenone. Thus for substituted benzophenones the kqT/kdif parameter also allows no conclusion about the mechanism of triplet-state quenching by molecular oxygen. For amino-substituted benzophenones, there is a tendency for the quantum yield of the triplet to decrease with displacement of the intense long-wavelength CT band in the absorption spectra (Table I). Since these ketones fluorescence only weakly in alcohols a t r c " temperature (yield 10-3),44the main deactivation channel of the excited SI*(CT) state is internal conversion. Probably, the internal conversion occurs by fast back electron transfer with deactivation of the molecule to the ground state. The rate of this process increases sharply and the quantum yield of triplet decreases with decreasing energy of the CT state. The conclusions of this work about the nature of the lowest triplet state of ketones contradict those made from analysis of the spectra and lifetimes of phosphorescence in the literature (see, for example, ref 14). However, according to ref 45, the assignment of short- and long-lived components of phosphorescence to the 3n,7r* and ~ T , A *states is uncertain. It is also known that the phosphorescence decay of acetophenone is nonexponential at all stages in various frozen glasses, but the reason for this effect is ~nknown.~~.~~ Rusakowicz et al. reached the important conclusion that the carbonyl oxygen is protonated in strongly hydrogen-bonding solvents and acidic media from a study of the phosphorescence of carbonyl compounds at 77 K.34 For example, the phosphorescence lifetime of benzophenone in methylcyclohexane is 0.005 s, while in 98% HzS04 it is 1.03 s.34 Recently a temperaturedependent spectral shift of the phosphorescence spectrum of Michler's ketone in ethanol was observed below 130 K.35 This
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shift was explained by solvation and conformational changes of ketone in the triplet state. In summary, we conclude that it is necessary to reinvestigate the spectra and decay kinetics of ketone phosphorescence as was done by Hoshino and Koizumi for Michler's ketone.35
Conclusion In the present work we use a new method for the analysis of the nature of the lowest triplet state of ketones in liquid solvents based on the efficiency of singlet oxygen generation. We conclude that, except for Michler's ketone in ethanol, acetophenone, p-aminobenzophenone, and Michler's ketone have lowest n,** triplet states in nonpolar hydrocarbons, polar acetonitrile, and hydrogen-bonding alcohols. We found no evidence for a 37r,1r* intermediate state between In,** and 3n,7r*states in acetophenone and conclude that it is necessary to reconsider the mechanism of fast intersystem crossing in ketones.
Acknowledgment. Supported by NSF Grant CHE89-11916. References and Notes (1) Gorman, A. A.; Lovering, G.; Rodgers, M. A. J. J . Am. Chem. SOC. 1978, 100, 4527. (2) Darmanvan. A. P. Chem. Phvs. Lett. 1983. 96. 383. (3) Gorman; A. A.; Hamblett, I.;kodgers, M. A. J.'J. Am. Chem. SOC. 1984, 106, 4679. (4) Chattopadhyay, S. K.; Kumar, C. V.; Das, P. K. J . Photochem. 1984, 24, 1. (5) Chattopadhyay, S. K.; Kumar, C. V.; Das, P. K. J . Photochem. 1985, 30, 81. (6) Gorman, A. A.; Hamblett, I.; Lambert, C.; Prescott, A. L.; Rodgers, M. A. J.; Spence, H. M. J. Am. Chem. SOC.1987, 109, 3091. (7) Redmond, R. W.; Braslavsky, S. E. Chem. Phys. Lett. 1988, 148, 523. (8) McLean, A. J.; McGarvey, D. J.; Truscott, T. G.; Lambert, C. R.; Land, E. 1.J. Chem. SOC.,Faraday Trans. 1990,86, 3075. (9) Terazima, M.; Tonooka, M.; Azumi, T. Photochem. Photobiol. 1991, 54. 59. (10) Darmanyan, A. P.; Arbogast, J. W.; Foote, C. S. J . Phys. Chem. 1991, 95, 7308. (1 1) Darmanyan, A. P.; Foote, C. S. J . Phys. Chem. 1992.96.3723.6317. (12) Darmanyan, A. P.; Foote, C. S. J. Phys. Chem., in press. (1 3) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park, 1978; Chapter 10. (14) Porter, G.; Suppan, P. Trans. Faraday SOC.1965, 61, 1664. (15) Lamola, A. A. J. Chem. Phys. 1967, 47,4810. (16) Lutz, H.; Breheret, E.; Lindqvist, L. J . Phys. Chem. 1973,77, 1758. (17) Porter, G.; Suppan, P. Trans. Faraday SOC.1966,62, 3375. (18) Fessenden, R. W.; Carton, P. M.; Shimamori, H.; Scaiano, J. C. J . Phys. Chem. 1982, 86, 3803. (19) Shimamori, H.; Uegaito, H.; Houdo, K. J . Phys. Chem. 1991, 95, 7664. (20) Hoshino, M.; Kogure, M. J . Phys. Chem. 1988, 92, 417. (21) Schuster, D. 1.; Goldstein, M. D.; Bane, P. J . Am. Chem. SOC.1977, 99, 187. (22) Brown, R. G.; Porter, G. J. Chem. Soc., Faraday Trans. 1 1971, 73, 1569. (23) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: New York, 1970. (24) Lamola, A. A,; Hammond, G. S. J . Chem. Phys. 1965, 43, 2129.
4516 The Journal of Physical Chemistry, Vol. 97, No. 18, 1993 (25) McClure, D. S.J. Chem. Phys. 1949, 17,905. (26) Urriti, E. H.; Kilp, T. Macromolecules 1984, 17, 50. (27) Lutz, H.; Lindqvist, L. Chem. Commun. 1971,493. (28) Cohen, S.G.; Cohen, J. I. J. Phys. Chem. 1968, 72, 3782. (29) Ware, W. R. J. Phys. Chem. 1962,66,455. (30) Scaiano. J. C. J . Photochem. 1973, 2, 81. (31) Cohen, S.G.; Parda, A.; Parsons, G. H.,Jr. Chem. Rev. 1973, 73, 141. (32) Batley, M.; Kearns, D. R. Chem. Phys. Lerr. 1968, 2, 423. (33) Lutz, H.; Duval, M. C.; Breheret, E.; Lindqvist, L. J. Phys. Chem. 1972, 76, 82 1. (34) Rusakowicz. R.; Byers, G. W.; Leermakers, P. A. J . Am. Chem. Soc. 1971, 93, 3263. (35) Hoshino, M.; Koizumi, M. Bull. Chem. Soc. Jpn. 1972, 45, 2731. (36) Gijzeman, 0.L. J.; Kaufman, K.; Porter, G. J. Chem. Soc., Faraday Trans. 2 1913, 69, 708.
Darmanyan and Foote (37) Kearns, D. R.; Case, W. A. J. Am. Chem. Soc. 1966, 88, 5087. (38) Yang, N. C.; McClure, D. S.;Murov,S. L.; Houser, J. J.; Dusenberry, R. J. Am. Chem. Soc. 1%7,89, 5466. (39) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834. (40) Lower, S.K.; El-Sayed, M. A. Chem. Reu. 1966,66, 199. (41) Ohmori, N.; Suzuki, T.; Ito, M. J. Phys. Chem. 1988, 92, 1086. (42) Tomer, J. L.; Holtzclaw. K. W.; Pratt, D. W.; Spangler, L. H. J . Chem. Phys. 1988,88, 1528. (43) Potashnik, R.; Goldschmidt, C. R.;Ottolenghi, M. Chem. Phys. Lerr. 1971, 9, 424. (44) Parker, C. A. Photoluminescence of Solurions; Elsevier Publishing Com.: Amsterdam, 1968; Chapter I. (45) Wagner, P. J.; May, M. J.; Haug, A.; Graber, D. R. J . Am. Chem. SOC.1970, 92, 5269. (46) Yang, N. C.; Murov, S.L. J . Chem. Phys. 1966,45,4358. (47) Griffin, R. N. Phorochem. Phorobiol. 1968, 7, 175.
