Time-dependent density functional theory study on the hydrogen bonding in electronic excited states of 6-amino-3-((thiophen-2-yl) methylene)-phthalide in methanol solution

In this work, we have applied the time-dependent density functional theory (TDDFT) method to investigate the excited-state hydrogen bonding dynamics of 6-amino-3-((thiophen-2-yl) methylene)-phthalide (6-ATMPH) in methanol (MeOH) solution. In the hydrogen-bonded complex, the intermolecular hydrogen bond (CO⋯HO) can be formed between the 6-ATMPH and the MeOH molecules. The 6-ATMPH monomer and hydrogen-bonded dimer can be photoexcited initially to the S1 state. We calculated the geometric structures and energies of the hydrogen-bonded complex and the isolated 6-ATMPH in different electronic states at the level of B3LYP with the TZVP basis sets. We found that the bond lengths of the CO and OH groups increased after formation of the intermolecular hydrogen bond in the ground state. Furthermore, the calculated hydrogen bond binding energy increased to 31.5 kJ/mol in the electronically excited state from 23.6 kJ/mol in the ground state. These results clearly indicate the intermolecular hydrogen bond of the hydrogen-bonded complex is strengthened in the electronically excited state. The hydrogen bond of the hydrogen-bonded 6-ATMPH-MeOH complex strengthening in the electronically excited state was also confirmed by theoretically monitoring the spectra shift of the stretching vibrational modes of the CO and OH groups. Our theoretical study results have clarified the dispute regarding the intermolecular hydrogen bond cleavage or strengthening in the electronically excited state.