Excited-State Proton Transfer of Phenol Cyanine Picolinium Photoacid

Steady-state and time-resolved fluorescence techniques as well as quantum-mechanical calculations were used to study the photophysics and photochemistry of a newly synthesized photoacid—the phenol cyanine picolinium salt. We found that the nonradiative rate constant knr of the excited protonated form of the photoacid is larger than that of the excited-state proton transfer (ESPT) to the solvent, kESPT. We estimate that the quantum efficiency of the ESPT process is about 0.16. The nonradiative process is explained by a partial trans–cis isomerization reaction, which leads to the formation of a “dark” excited state that can cross to the ground state by nonadiabatic coupling. Moreover, the ESPT process is coupled to the photo-isomerization reaction, as this latter reaction enhances the photoacidity of the studied compound, as a result of photoinduced charge transfer. To prevent trans–cis isomerization of the cyanine bridge, we conducted experiments of PCyP adsorbed on cellulose in the presence of water. We f...