Towards modelling light processes of blue-light photoreceptors. Pyrene–isoalloxazine (flavin)–phenothiazine triad: electrochemical, photophysical, investigations and quantum chemical calculations

Abstract

The triad 6 containing the phenothiazine–isoalloxazine couple as donor–acceptor redox unit and pyrene as antenna absorbing in the UV-A region has been designed to mimic the light processes of natural photoreceptors. By cyclic voltammetry it is shown that the redox chemistry of the three subunits of triad 6 behave almost independently, indicating no electronic coupling between the subunits in the ground state. Triad 6 exhibits three accessible redox states with one oxidation and two reduction waves due to the formation of the phenothiazine radical cation and isoalloxazine and pyrene radical anions. UV/Vis/NIR spectroelectrochemistry reveals the generation of the protonated isoalloxazine dianion on reduction which is formed in the non-polar solvent in a reduction–protonation–reduction step (two-electron transfer process) and which is attributed to intermolecular proton transfer from the amide group to the electrochemically reduced isoalloxazine radical anion. Evidences for the photoinduced energy and electron transfer within the triad are provided by steady state and time-resolved absorption and fluorescence measurements. Spectroscopic studies displayed that upon excitation the pyrene emission was dramatically quenched in the dyad 4. This is most likely due to the energy transfer from pyrene to the isoalloxazine units as the absorption band of isoalloxazine overlaps with the pyrene emission band leading most likely to a CT state of the isoalloxazine/phenothiazine type. Quenching of the phenothiazine fluorescence in triad 6 was also ascribed to the spectroscopic overlap between the emission spectrum of phenothiazine and absorption spectrum of isoalloxazine. Again, photoinduced electron transfer from phenothiazine to isoalloxazine is expected to be the cause for the quenching of the isoalloxazine emission in the dyad 5. Molecular orbital calculations for compound 5 showed a complete electron transfer from phenothiazin to isoalloxazine.