Effects of Hydrogen Bonding on Internal Conversion of GFP-like Chromophores. II. The meta-Amino Systems

Abstract

To rationalize the efficient quenching of the fluorescence and the Z -> E photoisomerization of m-ABDI, the meta-amino analogue of the green fluorescent protein (GFP) chromophore, in protic solvents, the femtosecond time-resolved fluorescence and transient infrared (TRIR) spectra of m-ABDI in CD3CN, CH3OH, and CD3OD are determined. For solutions in CD3CN, the fluorescence decay lifetime is similar to 7.9 ns and IR absorption lines near 1513, 1531, 1557, and 1613 cm(-1) of m-ABDI in its electronically excited state were observed with a decay time >5 ns. For solutions in CH3OH, the fluorescence decay is double exponential with time constants of similar to 16 and 62 ps. In addition to IR absorption lines of m-ABDI in its electronically excited state with a decay time of similar to 16 ps, new features near 1513, 1532, 1554, and 1592 cm(-1) were observed to have a rise time of similar to 19 ps and a decay constant of similar to 58 ps, indicating formation of an intermediate. The assignments for the IR spectra of the ground and excited states were assisted with DFT and TDDFT calculations, respectively. We conclude that the torsion of the exocyclic C=C bond (the tau torsion) is responsible for the nonradiative decay of electronically excited m-ABDI in CD3CN. However, in CH3OH and CD3OD, the solute-solvent hydrogen bonding (SSHB) interactions diminish significantly the barrier of the tau torsion and induce a new pathway that competes successfully with the tau torsion, consistent with the efficient fluorescence quenching and the diminished yield for Z -> E photoisomerization. The new pathway is likely associated with excited state proton transfer (ESPT) from the solvent to m-ABDI, particularly the carbonyl group, and generates an intermediate (ESPT*) that is weakly fluorescent.