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.
