Color-Tuning Mechanism in Firefly Luminescence: Theoretical Studies on Fluorescence of Oxyluciferin in Aqueous Solution Using Time Dependent Density Functional Theory

The first singlet excited state geometries of various isomers and tautomers of firefly oxyluciferin (OxyLH2), as well as their fluorescence spectra in aqueous solution, were studied using time dependent density functional theory (TDDFT). With changing pH in aqueous solution, three fluorescence peaks, blue (450 nm), yellow-green (560 nm), and red (620 nm) correspond to neutral keto and enolic forms, the monoanionic enolic form, and the monocationic keto form respectively. A counterion, Na+, was predicted to cause a blue shift in the fluorescence of anionic OxyLH2. The contributions of a charge transfer (CT) state upon electronic excitation of the planar and twisted structures were predicted. CT was large for the twisted structures but small for the planar ones. The differences between pK and pK* of various oxyluciferin species were predicted using a Forster cycle. A new possible light emitter, namely, the monocation keto form (keto+1), was considered.