Photo-induced dynamics in a pyrene-isoalloxazine(flavin)-phenothiazine triad

(αR)-α-[[[10-[4-(10-heptyl-10H-phenothiazin-3-yl)phenyl]-4,10-dihydro-2,4-dioxobenzo[g]pteridin-3(2H)-yl]acetyl]amino]pyrenepropionic acid phenylmethyl ester, a pyrene-isoalloxazine(flavin)-phenothiazine triad (abbreviated as PYFPT), was designed to mimic the dye-based functions of cryptochromes. PYFPT dissolved in dichloromethane and acetonitrile is characterized by absorption and fluorescence spectroscopy. Absorption cross-section spectra, fluorescence quantum distributions and quantum yields, degrees of fluorescence polarisation, and fluorescence lifetimes are determined. The triad is highly photo-stable as has been shown by monitoring absorption spectra under prolonged blue-light exposure. The absorption spectrum of PYFPT resembles the superposition of the absorption of the isoalloxazine (flavin), pyrene, and phenothiazine subchromophores. Blue light (λexc = 428 nm) excites exclusively the isoalloxazine subunit causing reductive electron transfer from the phenothiazine subchromophore to the locally excited isoalloxazine with a time constant of approximately 600 fs under formation of a charge transfer state which subsequently undergoes charge recombination. Near ultraviolet radiation (λexc = 311 nm) causes simultaneous excitation of the isoalloxazine, pyrene, and phenothiazine chromophores. Both, the locally excited pyrene and phenothiazine moieties cause oxidative electron transfer to the flavin moiety with time constants of about 80 ps and 50 ps followed by subsequent charge recombination. Förster-type energy transfer processes have been found to be one to two orders of magnitude slower.