Abstract
We studied intermolecular excited-state proton transfer (ESPT) dynamics in hydrogen-bonded complexes of 3-cyano-4-methyl-7-hydroxycoumarin (3CN4MU) by using steady-state absorption/fluorescence spectroscopy and ultrafast time-resolved optical Kerr gating fluorescence spectroscopy. Two different proton acceptors, 1-methyl imidazole (1MI) and triethylamine (TEA), were studied, and the experiments were carried out in strong- and weak-polarity solvents. We also carried out density-functional-theory (DFT) calculations to help us interpreting the experimental results. We found that 3CN4MU-TEA complex undergoes proton transfer in its ground state in both polar and non-polar solvents. In 3CN4MU-1MI complex, the proton transfer does not occur in ground state, as confirmed by experiments and DFT calculations. Due to the lack of solvation, ESPT in 3CN4MU-1MI complex can only partially occur in non-polar solvent. However, the ESPT of 3CN4MU-1MI complex occurs in polar solvent. We used a tri-exponential decay function to fit the total fluorescence intensity function (P(t)) and the time constants are 0.6±0.3ps, 28±3ps and 2290±50ps.The fastest component is assigned to the first step of ESPT, which is a solvation-controlled step. The second component is assigned to a structural relaxation which stabilizes the ion pair. The last one is the lifetime of the ESPT complex.