Adiabatic Triplet State Tautomerization of p-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.