Excited-state dynamics of phenol–pyridinium biaryl

The excited-state dynamics of a donor–acceptor phenol–pyridinium biaryl cation was investigated in various solvents by femtosecond transient absorption spectroscopy and temperature dependent steady-state emission measurements. After excitation to a near-planar Franck–Condon delocalized excited S₁(DE) state with mesomeric character, three fast relaxation processes are well resolved: solvation, intramolecular rearrangement leading to a twisted charge-shift (CSh) S₁ state with localized character, and excited-state proton transfer (ESPT) to the solvent leading to the phenoxide–pyridinium zwitterion. The proton transfer kinetics depends on the proton accepting character of the solvent whereas the interring torsional kinetics depends on the solvent polarity and viscosity. In nitriles, ESPT does not occur and interring twisting arises with no significant intrinsic barrier, but still slower than solvation. The CSh state is notably fluorescent. In alcohols and water, ESPT is faster than the solvation and DE → CSh relaxation processes and yields the zwitterion hot ground state, which strongly quenches the fluorescence. In THF, solvation and interring twisting occur first, leading to the fully relaxed, weakly fluorescent CSh state, followed by slow ESPT towards the zwitterion. At low temperature (77 K), the large viscous barrier of the solvent inhibits the torsional relaxation but ESPT still arises to some extent. Strong emission from the DE geometry and planar zwitterion is thus observed. Finally, quantum chemical calculations were performed on the ground and excited state of model phenol–pyridinium and phenoxide–pyridinium compounds. Strong S₁ state energy stabilization is predicted upon twisting in both cases, consistent with a fast relaxation towards the perpendicular geometry. A substantial S₀–S₁ energy gap is still present for the twisted cationic species, which can explain the long-lived emission of the CSh state in nitriles. A quite different situation arises with the zwitterion for which the S₀–S₁ energy gap predicted at the twisted geometry is very small. This suggests a close-lying conical intersection and can account for the strong fluorescence quenching observed in solvents where the zwitterion is produced by ESPT.