A time-dependent density functional theory investigation on the origin of red chemiluminescence

Is the resonance-based anionic keto form of oxyluciferin the chemical origin of multicolor bioluminescence? Can it modulate green into red luminescence? There is as yet no definitive answer from experiment or theory. The resonance-based anionic keto forms of oxyluciferin have been proposed as a cause of multicolor bioluminescence in the firefly. We model the possible structures by adding sodium or ammonium cations and investigating the ground- and excited-state geometries as well as the electronic absorption and emission spectra. A role for the resonance structures is obvious in the gas phase. The absorption and emission spectra of the two structures are quite different--one in the blue and another in the red. The differences in the spectra of the models are small in aqueous solution, with all the absorption and emission spectra in the yellow-green region. The resonance-based anionic keto form of oxyluciferin may be one origin of the red-shifted luminescence but is not the exclusive explanation for the variation from green (approximately 530 nm) to red (approximately 635 nm). We study the geometries, absorption, and emission spectra of the possible protonated compounds of keto(-1) in the excited states. A new emitter keto(-1)'-H is considered.