Solvation effect on the ESIPT mechanism of 2-(4′-amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine

The excited-state intramolecular proton transfer (ESIPT) dynamics of 2-(4′-Amino-2′-hydroxyphenyl)-1H-imidazo-[4,5-c]pyridine (AHPIP-c) has been studied by using density-functional theory and time-dependent density-functional theory method. Three kinds of different polar aprotic solvents, including acetonitrile (strong polar), tetrahydrofuran (weak polar) and methylcyclohexane (non-polar) have been chosen to explore solvent effects on these molecules. The calculated absorption and fluorescence spectra agree well with the experimental results for the three solvents and the dual fluorescence emission mechanism is well explained. The electron density ρ(r) and Laplacian ∇2ρ(r) at the bond critical point (BCP) have been calculated using the Atoms-In-Molecule (AIM) theory, which prove that the intramolecular hydrogen bond (O1H2⋯N3) exists in the S0 state. The geometric parameters and the infrared vibrational spectra in the OH stretching vibrational region have been calculated, which manifests the hydrogen-bond is strengthened in the S1 state. The molecular electrostatic potential surface and frontier molecular orbitals analysis demonstrate that the proton transfer prefer occurring on excited state because of the charge redistribution upon photo-excitation. The results of potential energy curves, further confirm that the proton transfer process is more likely to conduct in the S1 state due to the lower potential energy barrier than that in the S0 state. In addition, we also find that ESIPT reaction is more easily to occur as the solvent polarity decreases. Therefore, we believe that solvent effect could play an important role in controlling excited state behaviors of AHPIP-c molecules.