Excited-State Proton Transfer of 5,8-Dicyano-2-naphthol in High-Temperature and High-Pressure Methanol: Effect of Solvent Polarity and Hydrogen Bonding Ability

Excited-state proton transfer (ESPT) of 5,8-dicyano-2-naphthol (DCN2) in methanol at 30 MPa isobar between 294 and 543 K was studied using time-resolved fluorescence spectroscopy. From room temperature up to 513 K, a fluorescence band from an anionic form (RO–*, a proton dissociated form of DCN2) was observed, which indicates that the ESPT occurred under these thermal conditions. The time profiles of fluorescence intensity of the normal form of DCN2 (ROH*) (proton-associated form of DCN2) and RO–* were analyzed, considering the diffusion process of the contact ion pair RO–*···H in the Coulomb field based on the Debye–Smoluchowski theory. Proton dissociation rate was slower than the solvent reorganization rate estimated from the dynamic Stokes shift, indicating that the proton transfer (PT) is not influenced by the solvent dynamic factor but by the solvation free energy. The proton dissociation rate constants were discussed from the change of the activation free energy of PT controlled by the solvent characteristics. It was found that the PT dissociation rate constants for various alcohols under different thermal conditions could be explained by the competing effects of hydrogen bonding and dipolarity/polarizability that controlled the energy state of ROH* and RO–*···H, respectively