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 (C=O center dot center dot center dot H-O) 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 S(1) 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 C=O and O-H 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 C=O and O-H groups. Our theoretical study results have clarified the dispute regarding the intermolecular hydrogen bond cleavage or strengthening in the electronically excited state. (C) 2011 Elsevier B.V. All rights reserved.