Photoreactivities of Contact Charge-Transfer Complexes between 1,1-Diarylethenes and Oxygen Molecules. Dimerization and Oxygenation Accelerated in Medium Polar Solvent

Article abstract of DOI:10.1246/bcsj.71.2211

The selective excitation of contact charge-transfer (CCT) bands of 1,1-diarylethenes [Ar = 4-MeOC₆H₄ (1a); 4-MeC₆H₄ (1b); Ph (1c)] with molecular oxygen in CH₂Cl₂ and MeCN resulted in the formation of the corresponding 3,3,6,6-tetraaryl-1,2-dioxanes (2) as a primary product, together 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 results, it was shown that 2 was produced through dimer cation radicals of 1, whereas 3 was formed through the photolysis of 2 and the autoxidation of 1 initiated by neutral radical species, which must have been generated by the reaction of monomer cation radicals of 1 (1^.+) with a Superoxide anion radical. In particular, the formation of 2 depended to a large degree on the solvent polarity; namely, 2 was produced more efficiently in CH₂Cl₂ with moderate polarity rather than in MeCN with high polarity. Moreover, the reactivities of monomer and dimer cation radicals of 1 were investigated by γ-radiolyis and pulse radiolysis. 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 discussed based on the optimum structure, charge density, and spin density, obtained by semi-empirical molecular orbital calculations (PM3 method).