Isotope and temperature effects in ultrafast proton-transfer from a strong excited-state acid

The introduction of naphthol derivatives that exhibit excited-state acidities comparable to strong mineral acids, has opened the way for direct investigation of ultrafast proton-transfer in aqueous solutions and alcohols. We report on ihe direct measurement of the isotope and temperature effects on 5-cyano-1-naphthol excited-state proton dissociation in the full composition range of water-methanol mixtures. The results indicate that in pure water the proton-transfer rate is almost activationless in the proton coordinate and is mainly controlled by the solvent. In pure methanol the reaction shows down considerably and is mainly controlled by activation along the proton coordinate.