1148
GEORGE S. HAMMOSD A N D PETER 9. LEERMAKERS
T-ol. 66
IliPECHANISMS OF PHOTOREACTIONS IS SOL UTIOS"'. X. RELATIVE EFFICIENCIES G F VARIOUS GUESCHERS IY THE PHOTOREDUCTION OF BENZOPHEYOXE BY GEORGE S. HAMJIOSD AKD PETER A. LEERMAKERS' Contribution No. BY97 from the Gates and Crellin Laboratoizes of Chemistrzj, Calzfornia Institute of Technology, Pasadena, California Received Januarlj 3, 1963
A number of organic compounds quench the photoreduction of benzophenone by benzhydrol. Quantitative study of the effect shows the efficiencies of many active quenchers are very similar. From this fact we infer that transfer of energy from benzophenone triplets to quenchers probably occurs a t every collision if transfer of triplet excitation is exothermic.
Chemicals. 1-Naphthaldehyde (Eastman Rodak >Vhite Label) was vacuum distilled and the fraction boiling in the range 110-112° a t 3 mm. was collected for use. 2-Acetonaphthone (Eastman Kodak White Label)owas recrystalBenzhydrol lized from petroleum ether, m.p. 55-56 (Matheson Coleman & Bell, Reagent Grade) was twice recrystallized from petroleum ether. Benzophenone (m.p. 47-48'), biacetyl, b e n d (m.p. 94-95'), naphthalene (m.p. 80-81°), and maleic anhydride (m.p. 53-55'), all Reagent Grade, mwe used as furnished by Matlieson Coleman dz
Bell. Azulene (Aldrich Chemical Corp., m.p. 101-102') and benzene (Mallinckrodt Analytical Reagent Grade) were used as furnished. cis-Piperylene was obtained from mixed piperylene furnished by K and I< Laboratories. The mixture was heated under reflux with maleic anhydride to remove the trans-isomer. The cis-isomer then was distilled and a fraction boiling a t 43-44' a t atmospheric pressure was collected for use. Purified cyclohexene mas obtained from Dr. Karl Kopecky . 9-Anthraldehyde (8ldrich Chemical Corp.) mas recrystallized from acetic acid, m.p. 104'. Cyclooctatetraene (Aldrich Chemical Corp.) was used as furnished. The preparation of tributylstannane has been described in a previous paper.O Apparatus.-The light source, optical system, reaction cells, etc., for the quantum yield determinations have been described p r e v i ~ u s l y . ~The filter system, consisting of Cornipg glass filters 7-60 and 0-52 in series, isolated the 3660 A. line from the mercury arc. Actinometry.-h benzophenone-benzhydrol actinometer was employed. Solutions 0.05 M in benzophenone and 0.10 Jf in benzhydrol were degassed and irradiated for measured periods of time. The residual benzophenone then was determined spectrophotometrically. A quantum yield of 0.74 for the process is assumed.7 The results obtained by this method n-ere consistent with one determination using the uranyl oxalate actinometer. A solution 0.10 M in oxalic acid and 0.03 M in uranium acetate was irradiated. After 11,160 sec. the residual oxalic acid was titrated with 0.100 N permanganate. h quantum yield of 0.49 is assumed for the uranyl oxalate system.s Photoreduction of Benzophenone.-Benzene solutions usually containing 0.05 M benzophenone, 0.10 M benzhyM) drol, and a quencher in low concentration (2-10 X were placed in quartz cells, degassed, and irradiated. After a measured length of time the reaction was stopped and the residual benzophenone was determined spectrophotometrically after appropriate dilution. Photoreduction of 9-Anthra1dehyde.-Benzene solutions containing 1.0 X 10-3 M 9-anthraldehyde and 0.148 M butylstannane were degassed and irradiated. The cells then were opened and residual anthraldehyde was determined by 25-fold dilution and measurement of its absorbance a t 4000-4400 A. In one experiment azulene (5.0 X 10-3 M ) was included. In an experiment to identify products a solution containing 7.5 mmoles of tributylstannane in 130 ml. of benzene was irradiated for 8 hr. The solvent then was stripped off and petroleum ether wa9 added. A white solid (0.4 g., 27%) separated, m.p. 220-230". Chromatography of the entire reaction mixture on alumina yielded this material pure, m.p. 228-232". The product was shown to be the pinacol, 1,2-(9-anthryl)-glycol by cleavage to 9-anthraldehyde with lead tetraacetate. Treatment Of 18 mg. of the compound with lead tetraacetate in acetic acid for 3 hr., followed by the addition of 2,4-dinitrophenylhydrazine reagent, y.ielded a brick-red solid (not Tveighed) which was recrystallized from ethanol-ethyl acetate, m.p.
(1) National Science Foundation Predoctoral Fellow, 1968-1961. (2) G. S. Hammond, N. 3 . Turro, and P. A. Leermakers, J. P h y s . Chem., 6 6 , 1144 (1962). (3) €1. L. J. Rackstrom and II. Moore and M. Ketchum, ibid., 84, 1366 (1962).
(6) G. S. Hammond and P. A. Leermakers, ibzd., 84, 207 (1962). ( 7 ) This assumption is based on the extensive investigation of the system b y W, M. hloore, Ph.D. thesis, Iowa State College, 1969, and ref. 4; arid on the slight revision of the data by Mr. R. P. Foss of these Laboratories. (8) C. R. Masson, V. Boekelheide, and W. A. n'oyes, Jr., "Technique of Organic Chemistry," ed., A. Weissberger, Vol. 11, 2nd Ed., Interscience Publishers, New York, N. Y., 1956.
In the accompanying paper2 it was shown that energy transfer from the triplet staLe of a ketone or aldehyde to an acceptor molecule with cvncomitant production of the triplet state of the latter can indeed be an efficient process. Such an energy transfer process was employed to promote chemical reaction of the acceptor molecule. The present paper reports the results of an investigation to determine quantitatively the relative rates of energy transfer (or quenching) from benzophenone triplet,s to other organic molecules with relatively lom-lying triplet states. Backstrom and Sandros3 previously have observed, using the flash photolytic technique, that certain polynuclezr hydrocarbons quench biacetyl phosphorescence with rate constants of the order of IO9 1. mole-' set.-'. Presumably the mechanism of quenching is triplet excitation transfer to the quencher. It previously has been shown that the chemically active excited state of benzophenone in photoreduction reactions is a triplet.4 If triplettriplet energy transfer is an efficient process in solution, then suitable acceptor molecules ought to compete with a hydrogen donor for benzophenone triplets and thus reduce the quantum yield for phot)oreduction of the ketone. In the present investigation, the quantum yields for photoreduction of benzophenone by henzhydrol in the presence of various quenchers have been measured. The data have been analyzed to yield quantitative information concerning the relative rates of quenching by the various quencher (acceptor) molecules. Similar observations have been reported by RIoore and Ketchum, who used naphthalene as a q ~ e n c h e r . ~ Experimental
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RELATIVE EFFICIEKCIES OF QUENCHERS IS BENZOPHENONE PHOTOREDUCTION 1149
June, 1962
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271-275 * Authentic 9-anthraldehyde;2,4-dinitrophenylhydrazone was prepared, m.p. 277-282 ; mixed m.p. 272280".
Results and Discussion From previous work. in these Laboratories4 it is known that the photoreduction of benzophenone by benzhydrol in the presence of a quencher follows the rate law where kd, k,, and k , are, respectively, the rate constants for thermal deactivation, hydrogen abstraction, and quenching of the triplet state; [DH] is hydrogen donor concentration; is quencher concentration; and ip' is the quantum yield of triplets (unit8y for the case of benzophenone). Measurement of the quantum yield, a, for the photoreduction as a function of quencher concentration a t fixed benzhydrol concentration yielded values of kq/kr, the ratio of the rates of quenching to hydrogen abstraction. The dat'a are given in Table I. From recent observations in t'hese Laboratories2JJO it is clear that the quenching mechanism
[a]
TABLE I" PHOTOREDUCTION OF BENZOPHENONE BY BENXHYDROL IN THE PRESEXCE OF QUENCHERS Quencher 2-Acetonaphthone 2-Acetonaphthone Naphthalene Naphthalene cis-Piperylene cis-Piperylene Benzil Cyclohexene Azulene Azulene Azulene 1-Naphthaldehyde 1-Naphthaldehyde Cyclo6ctatetraene Cycloactatetraene Biacetyl Maleic anhydride None
[Bena[Quencher] hydro11 Q. flrb 59 5 X 10-4 0 . 2 0 0.30 ' 790 .20 730 .10 5 X 10-4 750 61 .lo .20 5 X lO--4 2 x 10-4 .io .34 5 X lO--4 .10 .21 750 55-60 1 . 0 X lo-.' .IO .lo4 4 . 4 X lo-.' .10 .31 436 62 .lo .67 30 ? 5 X lO-.4 .10 .15 5 X 10-4 2 x 10-4 .IO .25 1100 ? 3 X 10-4 .10 .20 5 X 10-'4 .IO .17 880 57 2 X 10-'4 . l o .34 5 X 10-4 .lo .20 630 ? 1.0 X lo-'* . 1 0 .13 5 X lO-'4 .10 .41 56 5 X lO-'4 .10 .58 ? .. .10 .74
?!
'
.. ..
..
Ref. 9
9
* 9
9
9
..
Concentration of benzophenone = 0.05 M . Energy of lowest triplet state (kcal.). Quencher consumed in the reaction. Quencher consumed, probably by the Schenck reaction." e Reference 12. a
must involve energy transfer from the benzophenone triplet to the acceptor (quencher) wit,h production of the triplet state of the latter. Transfer from benzophenone triplets to naphthalene a t low temperature has been observed directly by Terenin and Ermolaev13 and by Farmer, Gardner, and 114cDo~e11.~~ Porter and Wilkinson15also have (9) G. S. Hammond, P. A. Leermakers, and N. J. Turro, J . Am, Chem. SOC..83, 2395. 2396 (1961). (10) K. R. Kopecky, G. S.Iiarnmond, and P. A. Leermakers, ibid., 83, 2397 (1961). (11) G. 0. Slohenck and R. Steinmetz, Tetrahedron Letters, 21, 1 (1960). (12) G. S. Hammond, P. A. Leermakers, and N. J. Turro, J . Am. Chem. SOC.,83,2396 (13) A. Terenin and V. Ermolnev, Dokladu Akad. N a u k S.S.S.R., 85, 547 (1952); Truns. Faraday Soc., 63, 1042 (1966). (14) J. B. F'azmer, C . 1;. Gardner, and C . A. MoDowell, J . Chem. Phys., 34, 1058 (1961). (15) G. Porter and F. Wilkinaon, Proc. Roy. Soc. (London), 826.4, 1 (1961).
(1961).
shown that benzophenone triplets transfer excitation to naphthalene, with production of the triplet state of the latter, in a diffusion controlled process in solution. I n every case in which the energy of the lowest triplet state of the acceptor is known, the transfer would be exothermic, &ice the lowest triplet of benzophenone lies 70 kcal. above the ground state.I6 No information is available concerning the triplet states of azulene, cyclooctatetraene, cyclohexene, and maleic anhydride. However, since the first singlet-singlet transition of azulene is found at 7000 A. (42 kcal.) the compound must have one, and probably more,17 triplet states lying below the lowest triplet of benzophenone. It also is highly reasonable that cyclooctatetraeiie has a t least one triplet state lower in energy than that of benzophenone. If the molecule is considered to be two independent butadiene 'systems, the T1-So transition energy would be around 60 kcal.18 The assumption of any con jugation between the butadiene systems would presumably further lower the transition energy. However, it seems very probable that energy transfer from benzophenone to cyclohexene would be endothermic. Evans17 observed the S,-T1 transition of ethylene at 29.000 cm.-l (83 kcal.). Incorporation of the ethylenic function in a cyclic system might lower the energy of the triplet state somewhat, but it seems very doubtful that it could be as low as 70 kcal. We believe that the relatively inefficient quenching by cyclohexene is probably characteristic of an energy transfer which is endothermic by a few kcal. If the argument is turned around, we can infer that the S,-TI transition of maleic anhydride, a reasonably efficient quencher, must require 70 kcal. or less. The quenching efficiencies of most of the compounds tested are grouped remarkably close together. Naphthalene, 1-naphthaldehyde, 2-acetonaphthone, piperylene, and cyclooctatetraene all have values of k,/k, around 800; the most efficient quencher of all tested was azulene, in which k,/kr was 1100. The remarkable similarity in the rates of quenching by compounds as different as piperylene (1,3pentadiene), acetonaphthone, and azulene (rates well within a factor of two of each other) is compelling evidence for the existence of one common, rate-limiting factor. A diffusion controlled process is implied. Backstrom and Sandros3J9previously have assumed that energy transfers of this sort are diffusion controlled. The rate constants which they obtained for quenching of the phosphorescence of biacetyl by certain polynuclear aromatic lo9 1. mole-' sec.-l) are of the hydrocarbons ( k , same order of magnitude as the rate constants of diffusion controlled reactions. Since this process must involve triplet-triplet transfer, it probably is safe to assume that quenching of benzophenone triplets by energy transfer will be a t least close to
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(16) G. N. Lewis a n d M. Kasha, J . Am. Chem. Soo., 66, 2100 (1944). (17) R. Pariser, J . Chem. Phys., 25, 1112 (1956). (18) D. F. Evans, J. Chem. Soc., 1735 (1960). (19) 13. L. J. Backstrom and I
