A DFT/TDDFT study on the excited-state hydrogen bonding dynamics of 6-aminocoumarin in water solution.

Intermolecular hydrogen bonding, as a site-specific solute-solvent interaction, plays an extremely important role on the spectra of chemical and biological molecules. A detailed theoretical study on the hydrogen bonds is extremely necessary for understanding the unusual spectra blue-shift for some molecules. In the present paper, the hydrogen bonding dynamics of 6-aminocoumarin (6AC) in the water was investigated by the time-dependent density functional theory (TDDFT) and density functional theory (DFT) methods. According to our results, it is demonstrated that C6-N1⋯H9 bond (AHB) is the strongest hydrogen bonds among three ones formed between 6AC and water molecules in the ground state. Upon the photoexcitation, due the photo-induced charge transfer from N1 to C6, AHB is proved to be extremely weakened which is significantly greater than the strengthening of C9=O2⋯H11 bond (BHB) and N1-H6⋯O5 bond (CHB) in S1 states. This is expected to be theoretical explanation for the unusual blue-shift of a long-wavelength band in absorption spectra for 6AC in the water observed in the experiment.