Temperature dependence of the excited-state intramolecular proton transferreaction in hypericin and hypocrellin A

The excited-state intramolecular proton-transfer reactions of hypericin and hypocrellin A are measured as a function of temperature in an ethanol/meth anol mixture. The data yield activation energies of 0.044 +/- 0.008 and 2.1 2 +/- 0.070 kcal/mol for hypericin and hypocrellin A, respectively. The neg ligible activation energy of hypericin is consistent with previous suggesti ons that the proton-transfer reaction is adiabatic (K. Das et al., J. Phys. Chem. 1997, 101A, 3241.) and that a very low-amplitude displacement in at least one other coordinate be taken into account in order to describe the r eaction dynamics. The proton transfer for hypocrellin is also considered to occur in the adiabatic regime, but the significant activation energy sugge sts that a larger amplitude motion than that for the case of hypericin comp rises part of the reaction coordinate. Much of the barrier cited above for hypocrellin A results from the temperature dependence of the viscosity of t he solvent mixture. The viscosity independent part of the activation barrie r is 0.41 +/- 0.088 kcal/mol.