Role of Solvent in Excited-State Proton Transfer in Hypericin

The excited-state proton transfer of hypericin is monitored by the rise time (-6-1 2 ps in the solvents investigated) of the component of stimulated emission corresponding to the formation of the long-lived (-5 ns) fluorescent tautomer. The assignment of this excited-state process to proton transfer has been verified by noting that a hypericin analog (mesonaphthobianthrone) lacking labile protons is not fluorescent unless its carbonyl groups are protonated. Recent experimental studies on other systems have suggested that three solvent properties play important roles in excited-state proton transfer: viscosity, hydrogen-bonding character, and dynamic solvation. We find that for hypericin, in a range of protic, aprotic, hydrogen-bonding, and nonhydrogen-bonding solvents in which the viscosity changes by a factor of 60 and the average solvation time changes by a factor of 100, the excited-state proton-transfer rate of hypericin is uncorrelated with these properties and varies not more than a factor of 2 (6-1 2 ps) at room temperature. The relative contribution of the bulk solvent polarity is considered, and the role of intramolecular vibrations of hypericin on the protontransfer rate is discussed.