Photodynamics of pyrene-flavin and phenothiazine-flavin dyads

Abstract

A pyrene-flavin (isoalloxazine) dyad (PFD) and a phenothiazine-phenylene-isoalloxazine dyad (PPF), dissolved in dichloromethane are characterized by absorption and emission spectroscopy. These dyads are model compounds for flavin based blue-light photoreceptors. Absorption cross-section spectra, fluorescence quantum distributions, fluorescence quantum yields, and fluorescence decay times are determined. The absorption spectra of the dyads resemble the superposition of the absorption spectra of the constituents (1-methylpyrene, isoalloxazine, and phenylphenothiazine). 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 phenylphenothiazine moiety causes oxidative electron transfer with successive recombination, and additionally Förster-type energy transfer and Dexter-type energy transfer. For PFD in dichloromethane the rates of reductive electron transfer and oxidative electron transfer were determined to be (5 ps)^-1 and (77 ps)^-1, respectively, and a charge recombination time of about 16 ps was found. For PPF in dichloromethane the rates of reductive electron transfer and oxidative electron transfer were determined to be (700 fs)^-1 and (100 ps)^-1, respectively. The HOMO level position of the pyrene radical cation subunit relative to the HOMO level of the excited isoalloxazine subunit is determined from the delayed fluorescence emission of the PFD sample.