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

Abstract

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 (Phi(f)) and the isomerization Z -> E (Phi(ZE)) and the femtosecond fluorescence and transient infrared absorption in selected solvents. The behavior that Phi(ZE) congruent to 45 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 tau torsion) to form a perpendicular species (tau similar to 90 degrees) 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 Phi(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 tau torsion in a way that the IC occurs also for the twisted intermediates with a tau-torsion angle smaller than 90 degrees, which favors the formation of the Z isomers. The possibility of IC induced by torsion of the exocyclic C-C bond (the phi torsion) is also considered but excluded.