Electronically excited state of neutral/protonated, indole/5-hydroxyinodole–water clusters: a theoretical study

Abstract

The second-order approximate coupled-cluster (RI-CC2) method was employed to investigate photoinduced hydrogen-bond weakening or strengthening in neutral and protonated indole–, 5-hydroxyindole–water clusters. In addition to the protonation effect on the electronic structure of 5-hydroxyindole, the intermolecular H-bond weakening or strengthening of selected systems in the S₁ excited state has been investigated. According to our calculated results, it has been predicted that the electronic excitation effect on the hydrogen-bond strength in protonated clusters is essentially more pronounced than neutral analogues. Also, a charge transfer character of the excited state over the chromophore moiety can be suggested for interpreting the excited state dynamics of H-bonds in protonated complexes. Moreover, it has been predicted that protonation is accompanied by a strong red-shift effect (∼1.10 eV) on the S₁–S₀ transition energy of 5-hydroxyindole.