Time-dependent density functional theory (TDDFT) study on the electronic spectroscopic blue-shift phenomenon and photoinduced charge transfer of firefly luciferin anion in aqueous solution: Insight into the excited-state hydrogen bond weakening mechanism

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In this work, the time-dependent density functional theory (TDDFT) method was performed to investigate the electronic spectra and charge transfer in excited states of firefly luciferin anion (NRO-) in aqueous solution. A novel electronic spectral blue-shift (usually red-shift) phenomenon of NRO- was observed due to the significant influence of intermolecular hydrogen bonds between firefly luciferin anion and water molecules. We theoretically demonstrated that the excited-state intermolecular hydrogen bond weakening is confirmed by the increase of hydrogen bond length and decrease of hydrogen bond binding energy from our optimized ground-state and excited-state geometric conformations of the hydrogen-bonded NRO-–H2O complex. The hydrogen bond weakening behavior could also be indicated by photoinduced charge transfer from the carboxyl group to the planar part of NRO- in the excited state. Furthermore, we pointed out that the electronic spectral blue-shift should be ascribed to the intermolecular hydrogen bond weakening in this electronic excited state. The weakened hydrogen bonding interaction in the excited state would induce smaller downshift for the excited-state energy level than that for ground-state energy level. As a result, the energy gap between the excited state and ground state becomes larger, which means that the spectral peak corresponding to the electronic excited state will shift to the blue induced by the excited-state hydrogen bond weakening dynamics.