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.y Electronic supplementary information (ESI) available: Colour representations of Scheme 1 and Figs. 17 and 18. See

Absorption and emission spectroscopic characterization of 10-phenyl-isoalloxazine derivatives

“…For the understanding of the complex electron transfer and energy transfer processes in these dyads and the triad, a detailed knowledge of the photo-physical behaviour of the constituents is necessary. The photo-physical behaviour of pyrene is reviewed in [29] and of phenothiazine is reviewed in [20,30]. For the 10-phenyl-isoalloxazines, an absorption spectroscopic, fluorescence spectroscopic, and mass spectroscopic characterization is undertaken here.…”

“…Some investigation of N10-phenyl-substituted isoalloxazine derivatives is reported in [27,28]. The studied 10-phenyl-isoalloxazine dyes are constituents of pyrene-isoalloxazine, phenothiazine-isoalloxazine dyads, and of a pyrene-isoalloxazinephenothiazine triad which are model compounds for blue-light photoreceptors [20]. For the understanding of the complex electron transfer and energy transfer processes in these dyads and the triad, a detailed knowledge of the photo-physical behaviour of the constituents is necessary.…”

“…The optical spectroscopic behaviour of flavins (isoalloxazine dyes) is reviewed in [9,11,17,18]. Isoalloxazine dyes covalently linked to other dyes in donor acceptor systems with bridges, antennas, and mediators are artificial model systems for their biological counterparts [19][20][21][22] and gain importance in photo-voltaic, molecular switching, and molecular logics applications [23][24][25].…”

Synthesis and fluorescence emission properties of 1,3,6,8-tetraarylpyrenes

“…In this study, pyrene was chosen as a -conjugation center core because of its excellent fluorescence properties [33], its electron-acceptor nature [34,35], and its good performance in solution [36]. Substituted fluorenyl groups were introduced into pyrene molecules to invoke high solution and solid-state photoluminescence (PL) quantum yields [35], and in order to block the inter-chain interactions.…”

“…Substituted fluorenyl groups were introduced into pyrene molecules to invoke high solution and solid-state photoluminescence (PL) quantum yields [35], and in order to block the inter-chain interactions. To improve solubility and thermal stability, two long alkyl groups were introduced at the C9 position of the fluorene moiety [37,38].…”