Time-resolved ESR studies on the excited triplet states and photoenolization of 2-methylacetophenone and related molecules

Nonphosphorescent excited triplet (³ππ*) states of photoenols (³E) generated from the intramolecular hydrogen transfer of o-methylacetophenone (OMAP) and related compounds were measured, together with their phosphorescent excited triplet state (³K), by time-resolved ESR techniques at low temperatures. The triplet ESR spectrum with zero-field splitting parameters, \D\ = 0.060 and \E\ = 0.0025 cm^-1, clearly different from those of the parent molecules (³K), was assigned to ³E. The observed small \D\ value suggests the biradical character of ³E. It was pointed out from the examination of substituent and matrix effects that the ease of photoenol generation clearly depends on the character of the T₁ state of the parent molecules and also on the coplanarity of the carbonyl group with the aromatic ring. The presence of water induces a striking change in the character of the T₁ state of OMAP; i.e., nπ* of T₁ in nonpolar or absolute ethanol glassy matrices is altered by ππ* in 0.5% H₂O/ethanol. CIDEP spectra observed from the quenching reaction with methylviologen proved that ³E behaves as an excellent electron donor.