Theoretical investigation of intermolecular hydrogen bond induces fluorescence quenching phenomenon for Coumarin-1

In this work, density functional theory and time-dependent density functional theory were performed to investigate intermolecular hydrogen bond (H-bond) effect on Coumarin-1 (C1). The optimized geometric structures of C1 in acetonitrile and methanol (MeOH) revealed that C1 exhibits planar configuration in ground state and locally excited (LE) state. Due to intermolecular H-bond interaction between C1 and MeOH, twisted intramolecular charge transfer (TICT) state with twist configuration exists in excited state for C1-MeOH. The apparent variation of characteristic group frequencies and intensities for IR spectra is ascribed to dramatic charge density changes. Compared with molecular orbitals of LE state, the low charge coupling degree of C1-MeOH in TICT state is induced by twist configuration in TICT state. Indeed, such low charge coupling degree induced TICT state is nonluminous that responsible for low fluorescence quantum yield of C1-MeOH. We not only provide a comprehensive understanding of TICT state deactivation process but also benefits to design new environment-sensitive probe.