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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry