PHOTOREACTIVITIES OF CONTACT CHARGE-TRANSFER COMPLEXES BETWEEN 1,1-DIARYLETHENES AND OXYGEN MOLECULES - DIMERIZATION AND OXYGENATION ACCELERATED IN MEDIUM POLAR-SOLVENT

The selective excitation of contact charge-transfer (CCT) bands of 1,1 -diarylethenes [Ar = 4-MeOC6H4 (1a); 4-MeC6H4 (1b); Ph (1c)] with mole cular oxygen in CH2Cl2 and MeCN resulted in the formation of the corre sponding 3,3,6,6-tetraaryl-1,2-dioxanes (2) as a primary product, toge ther with diaryl ketones (3). The reaction mechanism and intermediates for the production of 2 and 3 were studied in terms of the effects of the solvent polarity, additives, substituents on the aromatic rings, and the excitation wavelength on the product distribution, as well as in terms of the result of the photolysis of 2. On the basis of these r esults, it was shown that 2 was produced through dimer cation radicals of 1, whereas 3 was formed through the photolysis of 2 and the autoxi dation of 1 initiated by neutral radical species, which must have been generated by the reaction of monomer cation radicals of 1 (1(.+)) wit h a superoxide anion radical. In particular, the formation of 2 depend ed to a large degree on the solvent polarity; namely, 2 was produced m ore efficiently in CH2Cl2 with moderate polarity rather than in MeCN w ith high polarity. Moreover, the reactivities of monomer and dimer cat ion radicals of 1 were investigated by gamma-radiolyis and pulse radio lysis. For 1a and 1b, the transient-absorption spectra of their dimer cation radicals trapped by oxygen molecules were directly observed at 365 ns after pulse irradiation. The reactivities of 1(.+) are also dis cussed based on the optimum structure, charge density, and spin densit y, obtained by semi-empirical molecular orbital calculations (PM3 meth od).