The excited triplet states of 1,2-naphthoquinone and 9,10-phenanthrenequinone were observed in several glassy matrices at low temperatures by time-resolved ESR method. Notable solvent effects on the zfs parameters were observed, indicating the small energy separation between 3nπ* and 3ππ* states in these o-quinones. It was found that the T1 state has dominantly ππ* character in ethanol while it is an nπ* in nonpolar solvents. CIDEP spectra of the semiquinone radicals produced by photoinduced hydrogen abstraction from the solvent reflect the electronic character of the T1 states.
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