TDDFT study of the electronic structure, absorption and emission spectra of the light emitters of the amazing firefly bioluminescence and solvation effects on the spectra

The ground and excited state properties of luciferin (LH2) and oxyluciferin (OxyLH(2)), the bioluminescent chemicals in the firefly, have been characterized using density functional theory (DFT) and time dependent DFT (TDDFT) methods. The effects of solvation on the electronic absorption and emission spectra of luciferin and oxyluciferin were predicted with a self-consistent isodensity polarized continuum model of the solvent using TDDFT. The S-0 -> S-1 vertical excitation energies in the gas phase and in water were obtained. Optimizations of the excited state geometries permitted the first predictions of the fluorescence spectra for these biologically important molecules. Shifts in both of the absorption and emission spectra on proceeding from the gas phase to aqueous solution were also predicted.