Stepwise Photoinduced Electron Transfer in a Tetrathiafulvalene-Phenothiazine-Ruthenium Triad

A molecular triad comprising a [Ru(bpy)(3)](2+) (bpy = 2,2 '-bipyridine) photosensitizer, a primary phenothiazine (PTZ) donor and a secondary (extended) tetrathiafulvalene (exTTF) donor was synthesized and explored by UV/Vis transient absorption spectroscopy. Initial photoinduced electron transfer from PTZ to the (MLCT)-M-3-excited [Ru(bpy)(3)](2+) occurs within less than 60 ps, and subsequently PTZ is regenerated by electron transfer from exTTF with a time constant of 300 ps. The resulting photoproduct comprising exTTF(center dot+) and [Ru(bpy)(3)](+) has a lifetime of 6100 ps in de-aerated CH3CN at room temperature. Additional one- and two-pulse laser flash photolysis studies of the triad were performed in the presence of excess methyl viologen (MV2+), to explore the possibility of light-driven charge accumulation on exTTF. MV2+ clearly oxidized [Ru(bpy)(3)](+) and thereby re-instated ground-state [Ru(bpy)(3)](2+) in triads in which exTTF had been oxidized to exTTF(center dot+), but further excitation of the solution containing the exTTF(center dot+)-PTZ-[Ru(bpy)(3)](2+) photoproduct did not provide evidence for exTTF(2+). Nevertheless, it seems that the design principle of a covalent donor-donor-sensitizer triad (as opposed to simpler donor-sensitizer dyads) is beneficial for light-driven accumulation of oxidation equivalents. These investigations are relevant in the greater context of multi-electron photoredox chemistry and artificial photosynthesis.