TD-DFT Study of the Double Excited-State Intramolecular Proton Transfer Mechanism of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole.

The 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPD) is chosen to investigate the excited-state double proton transfer process (ESDPT). The IR spectra, bond distance, and angle analyses show that the two intramolecular hydrogen bonds in the BPD molecule, formed between hydroxyl group and pyridine-type nitrogen atom, are significantly strengthened in the S1 state. The potential energy surfaces in both S0 and S1 states are scanned with varying O-H bond lengths to visually investigate the double proton transfer mechanism. Compared with previous investigations, the proton transfer process can be interpreted in more detail. The hydrogen bond strengthening promotes the proton transfer in the S1 state effectively. The large Stocks shift observed in the experiment can be explained more comprehensively according to the ESDPT mechanism.