Theoretical Investigations On The Color-Tuning Mechanism In Firefly Luminescence

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 (450nm), yellow-green (560nm), and red (620nm) correspond to neutral keto and enolic forms, the monoanion enolic form, and the monocation keto form respectively. Solvation effects on these species were investigated with reference to their dipole moments in their ground and first excited singlet states. A counter ion, 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 in the twisted structures but small in the planar ones. The differences between pK and pK* of various oxyluciferin species were estimated using a Forster cycle. A new possible light emitter namely the monocation keto form (keto+1) was considered. Population analysis was carried out and C14 was found important during excitation.