Photoreactions of 2,6-Diphenyl-s-Hydrindacene-l,3,5,7-Tetrone and its Derivatives in Polar and Nonpolar Solvents

Continuous UV-irradiation as well as flash-photolysis of derivatives of 2,6-diphenyl-s-hydrindacene-l,3,5,7-tetrone (bisindandione — BID), substituted in the p and p ' positions of the phenyl rings, was found to result in the formation of stable photoproducts, of which the absorption, emission and excitation maxima coincide with those of the corresponding diarylidene-benzodifurandiones (bis-benzylidenephthalides BBPh). Studies of light intensity effects gave evidence that this isomerization proceeds via a semi-isomerized intermediate. Additionally, flash-photolysis of BID showed the formation of short lived transients, which, however, are not the precursors of the stable photoproducts. These transients were assigned to 2,6-diphenyl-shydrindacene-l,3,5,7-tetrone-2-yl free radicals. Their UV-absorption maxima are dependent on the polarity of the solvent and show a bathochromic shift upon substitution.