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

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 ∼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 ∼16 and 62 ps. In addition to IR absorption lines of m-ABDI in its electronically excited state with a decay time of ∼16 ps, new features near 1513, 1532, 1554, and 1592 cm^–1 were observed to have a rise time of ∼19 ps and a decay constant of ∼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 τ 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 τ torsion and induce a new pathway that competes successfully with the τ 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.