J. Phys. Chem. 1981, 85, 459-462
459
Concentration Dependence of Photoracemization of 1,l’-Binaphthyl. Evidence for the Involvement of Chain Transfer Reaction via Triplet State Takehiko Yorozu, Kikuo Yoshida, Koichlro Hayashl, and Masahiro Irie” The Institute of Scientific and Industrial Research, Osaka University, Suita Osaka, Japan (Recelved: June 13, 1980; In Final Farm: September 23, 1980)
The concentration dependence of photoracemization of optically active 1,l’-binaphthyl was studied in tetrahydrofuran. It was found that the quantum yield of the photoracemization increases as the concentration of binaphthyl increases. This fact is interpreted as due to a chain reaction of intermolecular triplet-triplet energy transfer between (E)-and (8)-binaphthyls. Triplet sensitizing and quenching studies also confirmed the chain transfer mechanism via a triplet excited state. The rate constants of racemization and energy transfer were estimated by a quantitative quenching experiment.
Introduction Recent work on photo- and radiation-induced racemization of 1,l’-binaphthyl in this laboratory has proved that triplet excited and radical anion states are responsible for the racemization reaction.’V2 In the course of this study, we found that the racemization quantum yield depends on the concentration of 1,l’-binaphthyl in the case of photoracemization and that it exceeds unity. For radiation-induced racemization, the G value of racemization was found to exceed 30. These results were interpreted by assuming chain transfer reaction contributions. A chain transfer mechanism in an isomerization reaction was first presented to explain the large G value of radiation-induced cis trans isomerization of stilbene. A G value as high as 210 was interpreted as due to chain transfer reactions via the radical anion state of tilb bene.^ A chain reaction via a radical cation state has been proposed for the isomerization of diene derivative^.^ Although the radical anion mechanism can well explain the large G value observed in the radiation-induced racemization of l,l’-binaphthyl,2 it is not applicable to the photoracemization reaction. A chain reaction involving the triplet state was once proposed by Hurley and Testa to account for the enhanced cis trans isomerization quantum yield in benzophenone-sensitized photoisomerization of &-1,3-~entadiene.~Saltiel et al. have confirmed that the mechanism is also applicable for the photoisomerization of cis,cis-2,4-hexadiene but not for trans-1,3pentadiene.6 In these cases an isomerization quantum yield exceeding unity was observed only with very high diene concentrations (1.0-10 M) because of large endothermicity of the excitation triplet energy transfer process. In this report we focus our attention on the concentration dependence of the quantum yield of photoracemization of 1,l’-binaphthyl to reveal the contribution of the chain transfer reaction to photoracemization. We also carefully examined the chain transfer mechanism, i.e., the nature of the chain carrier and its kinetic behavior, by the effect of the addition of quenchers or sensitizers to the system.
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(1)M. hie, K. Yoshida, and K. Hayashi, J . Phys. Chem., 81, 969 (1977). (2)M. I&, T.Yorozu, K. Yoshida, and K. Hayashi, J. Phys. Chem., 81,973 (1977). (3)R.R. Hentz, K. Shima, and M. Burton, J. Phys. Chem., 71,461 (1967). (4)Y. Harata, M. Matsui, and M. Imamura, Chem. Lett., 199 (1977). (5) R. Hurley and A. C. Testa, J. Am. Chem. Soc., 92,211 (1970). (6)J. Saltiel, D. E. Townsend, and A. Sykes, J. Am. Chem. SOC.,95, 5968 (1973).
TABLE I : Relative Rate of the Photoracemization of 1,l’-Binaphthyl in the Presence of the Additives at 0 “ C
additivesa
relative rate
acetophenone benzophenone
2.0 1.4 0.5 0.04 1.0
1,3-pentadiene
oxygen none
a The concentrations of 1,l’-binaphthyl and the additives were 3.2 x and 1.2 X mol dm-3, respectively.
Experimental Section Optically active (+)-(S)-l,l’-binaphthyl was prepared by the method that we used previ~usly.~The optical activity thus obtained had [ctI2’D of 156’ with an enantiomer excess of 64%. Tetrahydrofuran was distilled twice over metal sodium. The sensitizers and the quenchers used were the same as described in our previous paper.2 Photoillumination was carried out by using the mercury line of 313 nm passed through a monochromator (Model MC20, Ritsu Applied Optics). The change of optical rotation was followed by measuring [aI2OD with a polarimeter (Hitachi Type PO-B). The quantum yield of the racemization was defied as dividing the number of binaphthyl molecules which convert from one enantiomer to the other by the number of photons which were absorbed by the excess (+)-(S)-binaphthyl molecules. The number of molecules (N) which convert from one enantiomer to the other is defined in eq 1, where [Ctl2’Di
and [(rI2ODf are the optical rotation before and after irradiation by the 313-nm light, respectively, C is the number of binaphthyl molecules in the sample cell, and 245’ is the maximum optical rotation of optically active l,l’-bin a ~ h t h y l .The ~ number of photons absorbed by the sample molecules was determined by potassium trioxalatoferrate(II1) chemical actinometry.s All samples were degassed by freeze-pump-thaw cycles under vacuum (
