Surface anchored self-assembled reaction centre mimics as photoanodes consisting of a secondary electron donor, aluminium(iii) porphyrin and TiO 2 semiconductor.

A series of vertically assembled photoanodes, consisting of 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(iii) porphyrin (AlPorF ₃ ), a pyridine appended electron donor (PTZ-Py, PTZ = phenothiazine; TTF-Py, TTF = tetrathiafulvalene), and semiconductor TiO ₂ , have been fabricated by exploiting the unique axial properties of AlPorF ₃ . The new photoanodes were characterized by steady-state and transient spectroscopic techniques. Transient-absorption studies show that in the absence of a donor, both the photoanodes (AlPorF ₃ -TiO ₂ and AlPorF ₃ -Ph-TiO ₂ ) exhibit electron injection from AlPorF ₃ into the conduction band of TiO ₂ and the injection efficiencies are strongly dependent on the linker. Faster electron injection and recombination is revealed when AlPorF ₃ is directly bound to TiO ₂ . Although a secondary electron donor is coordinated to AlPorF ₃ (viz., Donor-Py-AlPorF ₃ -TiO ₂ and Donor-Py-AlPorF ₃ -Ph-TiO ₂ ), the primary charge separation occurs in the form of electron injection from AlPorF ₃ to TiO ₂ followed by a secondary process involving photooxidation of the donor (PTZ and TTF) with AlPorF ₃ ˙ ⁺ . The estimated electron injection lifetimes and the AlPorF ₃ ˙ ⁺ decay lifetimes strongly depend on the electron richness of the donor; the higher the electron density of the donor, the faster the electron injection and photooxidation witnessed. The photoanodes with TTF (TTF-Py-AlPorF ₃ -TiO ₂ and TTF-Py-AlPorF ₃ -Ph-TiO ₂ ) show faster injection and shorter decay lifetimes of AlPorF ₃ ˙ ⁺ over their PTZ counterparts (PTZ-Py-AlPorF ₃ -TiO ₂ and PTZ-Py-AlPorF ₃ -Ph-TiO ₂ ). The observed trends suggest that the strong secondary electron donor enhances the injection and the subsequent photooxidation processes in the investigated photoanodes. The successful mimicking of a sequential charge-separation process makes aluminum(iii) porphyrins potential sensitizers for the construction of photoanodes, especially for photocatalytic and dye-sensitized solar cells for conversion and storage of solar energy.