Time-dependent density functional theory study on the hydrogen bonding-induced twisted intramolecular charge-transfer excited states of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine.

In this work, the time-dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen-bonded intramolecular charge-transfer excited state of 2-(4'-N,N-dimethylaminophenyl)imidazo[4,5-b]pyridine (DMAPIP) in methanol (MeOH) solvent. All the geometric conformations of the ground state and locally excited (LE) state and the twisted intramolecular charge-transfer (TICT) state for isolated DMAPIP and its hydrogen-bonded complexes have been optimized. At the same time, the absorption and fluorescence spectra of DMAPIP and the hydrogen-bonded complexes in different electronic states are also calculated. We theoretically demonstrated for the first time that the intermolecular hydrogen bond formed between DMAPIP and MeOH can induce the formation of the TICT state for DMAPIP in MeOH solvent. Therefore, the two components at 414 and 506 nm observed in the fluorescence spectra of DMAPIP in MeOH solvent were reassigned in this work. The fluorescence peak at 414 nm is confirmed to be the LE state. Furthermore, the red-shifted shoulder at 506 nm should be originated from the hydrogen-bonded TICT excited state.