A DFT/TDDFT study of hydrogen bonding interactions between resorufin anion and water molecules in the excited state

In order to explore the hydrogen bonding interactions between resorufin anion (denoted as Res− in this paper) and water molecules, we have calculated the geometric structures as well as the total energies of the various hydrogen-bonded Res−–water complexes composed based on the atomic charges obtained from the NBO analysis. We found that there are four sites (O1–3, N1) within the Res− molecule through which intermolecular hydrogen bonds can be formed with water molecule. Furthermore, by comparing the hydrogen bond lengths and the relative energies of the four singly hydrogen-bonded complexes Res−–H2O, we found that hydrogen bonds formed at the two carbonyl oxygen atoms (O2 and O3) are stronger than those formed at heteroatom O1 and N1 which can be reasonably predicted by the highest negative charge localized on the two former atoms. Moreover, as the number of the water molecules hydrogen-bonded to the Res− increases, the hydrogen-bonded complex Res−–water becomes more stable although the strength of each hydrogen bond decreases. In addition, the electronic spectra of the various hydrogen-bonded Res−–water complexes as well as the Res− monomer are also calculated. We found that, compared with that of the Res− monomer, the excitation spectra of the hydrogen-bonded Res−–water complexes are mostly blue-shifted in the S2 and S3 states while they are all red-shifted in the higher excited states, especially in states S10, S11 and S12. According to the relationship between electronic spectral shifts and electronic excited-state hydrogen-bonding dynamics first clarified by Zhao and coworkers [51], we demonstrate that, for all the hydrogen-bonded Res−–water complexes, most of the hydrogen bonds are weakened in the S2 and S3 states while they are all strengthened in excited states S10, S11 and S12.