Temperature dependence of the excited-state proton-transfer reaction of quinone-cyanine-7

Steady-state and time-resolved fluorescence techniques were used to study the temperature dependence of the photoprotolytic process of quinone-cyanine-7 (QCy7), a very strong photoacid, in H₂O and D₂O ice, over a wide temperature range, 85-270 K. We found that the excited-state proton-transfer (ESPT) rate to the solvent decreases as the temperature is lowered with a very low activation energy of 10.5 ± 1 kJ/mol. The low activation energy is in accord with free-energy-correlation theories that predict correlation between ΔG of reaction and the activation energy. At very low temperatures (T < 150 K), we find that the emission band of the RO^-, the deprotonated form of QCy7, is blue-shifted by ∼1000 cm^-1. We attributed this band to the RO ^-···H₃O⁺ ion pair that was suggested to be an intermediate in the photoprotolytic process but has not yet been identified spectroscopically.