Abstract
In concert with the recent spectroscopic studies of the light‐color modulation mechanism of firefly (Hirano et al., J Am Chem Soc 2009, 131, 2385), quantum chemical calculations using complete active space SCF (CASSCF), multistate complete active space second order perturbation (MS‐CASPT2) theory as well as a time‐dependent density functional theory (TD‐DFT) approach with the Coulomb attenuated hybrid exchange‐correlation functional (CAM‐B3LYP) were performed on the excited state (S~1~) of the keto‐form oxyluciferin (keto‐OxyLH~2~). Benzene, DMSO, CH~3~CN, and H~2~O were chosen as polar solvents. The polarization effect of less polar solvent was considered by a simple model, complex of keto‐OxyLH~2,~ and NH~3~ with different covalent character. The calculated results supported the experimental conclusion: (1) the light emitter of bioluminescence is the S~1~ state of keto‐OxyLH~2~ anion [(keto‐1)*], and (2) light emission from (keto‐1)* is modulated by the polarity of surrounding environment and the degree of covalent character of hydrogen bond between (keto‐1)* and a protonated basic moiety. The mechanism of the multicolor bioluminescence was discussed from the theoretical viewpoint. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111:3371–3377, 2010