In this thesis a pyrene-flavin dyad, a phenothiazine-flavin dyad, a pyrene-flavin-phenothiazine triad, and their constituents -the phenyl-isoalloxazine, bromo-phenyl-isoalloxazine, pyrene and 1-methylpyrene, heptyl-phenothiazine and heptyl-phenyl-phenothiazine- dissolved in either dichloromethane or acetonitrile have been characterized by absorption and emission spectroscopy. These dyads are model compounds for the flavin based blue-light photoreceptors phototropin and BLUF proteins. The triad was designed to mimic the dye-based functions of cryptochromes. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes of dyes were determined. The photo-stability of the dyes has been investigated by observation of absorption spectral changes due to prolonged blue-light excitation. The absorption spectra of the dyads and the triad resemble the superposition of the absorption spectra of their constituents. Photo-excitation of the flavin moiety causes fluorescence quenching by reductive electron transfer in thermodynamic equilibrium with the exited flavin subunit. The charge-separated states recover by charge recombination. Photo-excitation of the pyrene or phenothiazine moiety causes oxidative electron transfer with successive recombination
