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In this study, within the framework of density functional theory (DFT) and time-dependent DFT (TDDFT) methods, we theoretically investigated the novel system 2-phenylimidazo[4,5-b]pyridine (PIP) with respect to the dynamical behavior of its excited state in methanol (MeOH) solvents. Herein, two hydrogen-bonded networks have been discussed between PIP and MeOH, and it has been found that two MeOH connected to PIP (PIP–2MeOH) should be the best arrangement in both S0 and S1 states. Investigations on the electronic spectra of PIP–2MeOH have verified this point. Via analysis of hydrogen bond wires and corresponding infrared (IR) vibrational spectra, we have found that N1–H2⋯O3 of PIP–2MeOH undergoes the biggest change upon photoexcitation that reflects the tendency of the excited state intermolecular proton transfer (ESIPT) process. According to the results of our theoretical potential energy curves along different coordinates, we confirmed that ESIPT reaction should occur along the hydrogen bond wire N1–H2⋯O3 first. After the ESIPT reaction, proton transfer of PIP–2MeOH-PT* could proceed via intersystem crossing (ISC) process from S1 state to T1 state with a negligible energy gap 0.031 eV. Due to this non-radiation process, the fluorescence peak of PIP–2MeOH-PT* could be quenched. Our study not only explains previous successful experiment, but also proposes a new excited state dynamical mechanism for the PIP system.

A theoretical study about the excited state intermolecular proton transfer mechanisms for 2-phenylimidazo[4,5-b]pyridine in methanol solvent