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.

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Hydrogen bonds were shown to play an important role in the lumichrome photophysics and photochemistry both in solutions and in the solid state. In solutions, lumichrome can form hydrogen-bonded complexes with a variety of molecules, such as acetic acid or methanol, as supported by spectral and equilibrium studies. Photoexcitation of some hydrogen-bonded complexes, having appropriate configuration, as in the case of acetic acid, may lead to excited-state proton transfer, resulting in formation of the isoalloxazinic structure, detectable by its characteristic emission, distinct from that of the intrinsically alloxazinic lumichrome. Theoretical calculations confirmed the role of the hydrogen-bonded complexes, yielding several stable eight-membered cyclic structures of such complexes characterized by spectral changes similar to those observed experimentally. Hydrogen bonds play an essential role in the formation of the lumichrome crystal structure, as follows from the X-ray diffraction results. Interestingly, the crystals studied included molecules of methanol used as solvent in crystal growth. The emission studies of polycrystalline samples, similar to the processes occurring in solutions, point to the importance of hydrogen-bonding interactions in crystal packing allowed by the symmetry of the hydrogen-bonded dimers.

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