Y. Ilan, M. Luria, and G. Stein
584
Photochemistry of Benzene in Aerated Aqueous Solutions in the Range of 214 to 265 nm Yigal Nan, Menahem Luria, and Gabriel Stein" Department of Physical Chemistry, The Hebrew University, Jerusalem, lsrael (Received February 24, 1975; Revised Manuscript Received August 4, 1975) Publication costs assisted by The Hebrew University
Benzene in aerated aqueous solution irradiated a t 214 nm (causing transition to the second excited singlet state) gives a similar major photoproduct, with similar yield, as irradiation at 229, 254, or 265 nm. The yield is nearly independent of temperature between 20 and 60 "C. The results in correlation with the wavelength dependence of fluorescence support the view that this photochemistry originates in nonrelaxed states prior to the formation of the thermalized fluorescent level. We report our results on the photochemistry of aerated aqueous solutions of benzene irradiated with X 214 nm and compare these with those of irradiations with X 229, 254, or 265 nm. Absorption of light at 214 nm causes transition to the second, probably lBIU,excited singlet state of benzene, while the absorption a t the three longer wavelengths results in excitation to the first, lBzU,singlet state (Figure 1). Symmetry considerations make either electronic transition unallowed from vibrational levels of the planar lAlg ground state to levels of either of the planar excited states, lBg, or lBlu, unless the transition is such that at least one nontotally symmetrical vibrational mode is aroused or eliminated in the process. In our case transitions originate in a symmetrical ground state and result in excited states in which one quantum of nontotally symmetrical vibration evolves, accompanied by one or more quanta of symmetrical vibrations. In the gas phase1 as well as in nonpolar solvent^^,^ deaerated benzene yields on illumination at 254 nm one major initial photoproduct only, benzvalene:
The reaction necessitates out-of-plane distortion for the formation of the valence-bond isomer. Recent detailed in~ e s t i g a t i o nsupported ~,~ the s ~ g g e s t i o n lthat * ~ such photochemistry originates in very short times, before vibrational relaxation to the fluorescent level. This photochemistry in nonpolar solvent was shown to be nearly independent of temperature3 while fluorescence4 depends on it. Fluorescence was also shown to be wavelength dependent: in the gas phase on excitation by X
