Adiabatic Triplet State Tautomerization of <i>p</i>-Hydroxyacetophenone in Aqueous Solution

The primary photophysical processes of p-hydroxyacetophenone
(HA) and the ensuing proton transfer reactions in aqueous solution
were investigated by picosecond pump–probe spectroscopy and
nanosecond laser flash photolysis. Previous studies have led to mutually
inconsistent conclusions. The combined data allow us to rationalize
the excited-state proton transfer processes of HA in terms of a comprehensive,
well-established reaction scheme. Following fast and quantitative
ISC to the triplet state, 3HA*, adiabatic proton transfer
through solvent water simultaneously forms both the anion, 3A–*, and the quinoid triplet enol tautomer, 3Q*. The latter subsequently equilibrates with its anion 3A–*. Ionization and tautomerization are
likely to compete with the desired release reactions of p-hydroxyphenacyl photoremovable protecting groups.