Surface anchored self-assembled reaction centre mimics as photoanodes consisting of a secondary electron donor, aluminium(iii) porphyrin and TiO2 semiconductor

Abstract

A series of vertically assembled photoanodes, consisting of 5,10,15,20-tetrakis(3,4,5-trifluorophenyl)aluminum(III) porphyrin (AlPorF₃), a pyridine appended electron donor (PTZ-Py, PTZ = phenothiazine; TTF-Py, TTF = tetrathiafulvalene), and semiconductor TiO₂, have been fabricated by exploiting the unique axial properties of AlPorF₃. 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 (AlPorF₃–TiO₂ and AlPorF₃-Ph–TiO₂) exhibit electron injection from AlPorF₃ into the conduction band of TiO₂ and the injection efficiencies are strongly dependent on the linker. Faster electron injection and recombination is revealed when AlPorF₃ is directly bound to TiO₂. Although a secondary electron donor is coordinated to AlPorF₃ (viz., Donor-Py-AlPorF₃–TiO₂ and Donor-Py-AlPorF₃-Ph–TiO₂), the primary charge separation occurs in the form of electron injection from AlPorF₃ to TiO₂ followed by a secondary process involving photooxidation of the donor (PTZ and TTF) with AlPorF₃˙⁺. The estimated electron injection lifetimes and the AlPorF₃˙⁺ 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-AlPorF₃–TiO₂ and TTF-Py-AlPorF₃-Ph–TiO₂) show faster injection and shorter decay lifetimes of AlPorF₃˙⁺ over their PTZ counterparts (PTZ-Py-AlPorF₃–TiO₂ and PTZ-Py-AlPorF₃-Ph–TiO₂). 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.