Effects of Hydrogen Bonding on Internal Conversion of GFP-like Chromophores. I. The para-Amino Systems

To understand the effects of solvent–solute hydrogen bonding (SSHB) on the excited-state dynamics of two GFP-like chromophores, p-ABDI and p-CFABDI, we have determined the quantum yields for fluorescence (Φ_f) and the isomerization Z → E (Φ_ZE) and the femtosecond fluorescence and transient infrared absorption in selected solvents. The behavior that Φ_ZE ≅ 0.50 in aprotic solvents, such as CH₃CN, indicates that the E–Z photoisomerization adopts a one-bond-flip mechanism through the torsion of the exocyclic CC bond (the τ torsion) to form a perpendicular species (τ ∼90°) in the singlet excited state followed by internal conversion (IC) to the ground state and partition to form the E and Z isomers with equal probabilities. The observed Φ_ZE decreased from 0.50 to 0.15–0.28 when CH₃CN was replaced with the protic solvents CH₃OH and CF₃CH₂OH. In conjunction with the solvent-independent rapid (<1 ps) kinetics for the fluorescence decay and the solvent-dependent slow (7–20 ps) kinetics for the ground-state recovery, we conclude that the SSHB modifies the potential energy surface for the τ torsion in a way that the IC occurs also for the twisted intermediates with a τ-torsion angle smaller than 90°, which favors the formation of the Z isomers. The possibility of IC induced by torsion of the exocyclic C–C bond (the φ torsion) is also considered but excluded.