Excited-state N–H⋯S hydrogen bond between indole and dimethyl sulfide: time-dependent density functional theory study

Abstract

The intermolecular hydrogen bond N–H⋯S between indole and dimethyl sulfide is theoretically investigated. The formation of N–H⋯S hydrogen bonds between indole and dimethyl sulfide in ground and excited states is confirmed by the analysis of geometric structure, Mulliken charge, and infrared spectra. The result shows that the S₁ state of hydrogen bonded indole–Me₂S is mainly a charger transfer state, while the S₂ state is a local excited state and also the state corresponding to the experiment. More importantly, it is demonstrated that the intermolecular hydrogen bond N–H⋯S of indole–Me₂S is strengthened in the S₁ and S₂ states compared to that in ground state. Moreover, the strengthening of intermolecular N–H⋯S hydrogen bond in excited state induces the fluorescence emission peak of indole shifts to the red. These findings may provide insights for further study of N–H⋯S hydrogen bonds existing in many biomolecular systems.