Spectroscopy and photophysics of lumiflavins and lumichromes

Solvent effects on the spectroscopic properties of lumichromes and lumiflavins are presented. Fluorescence yields for lumiflavins are an order of magnitude larger than those for lumichromes, due to their lower nonradiative rate constants. Solvent effects on the absorption and emission band positions are explained on the basis of hydrogen-bonding interactions. TD-DFT calculations predicted that the lowest energy states are n,π* in the case of lumichromes, but π,π* in the case of the lumiflavins. The overall consistency of the predicted singlet-singlet and triplet-triplet transitions obtained for the various compounds studied, and the good correspondence between the predicted and measured transitions, indicate that the techniques applied provide an accurate description of the spectral properties of lumiflavins and lumichromes. The measured singlet oxygen yields have shown the lumichromes to be efficient singlet oxygen sensitizers.