Direct Interfacial Electron Transfer from High-Potential Porphyrins into Semiconductor Surfaces: A Comparison of Linkers and Anchoring Groups

This study probes a series of linkers and anchoring groups for direct interfacial electron transfer (IET) from high-potential porphyrins into semiconductor surfaces. Eight different linker-anchor combinations of CF₃-substituted, high-potential porphyrins were designed, synthesized, and characterized. Specifically, a series of four anchors was examined (carboxylate, hydroxamate, phosphonate, and silatrane), along with two different linkers (phenylene and benzanilidylene), which differ in terms of their electronic conjugation and overall length. The electrochemical and photophysical properties of the porphyrins were evaluated by steady-state and transient spectroscopies in solution and on mesoporous SnO₂ substrates for use as dye photosensitizers in aqueous photoelectrochemical cells. IET dynamics were measured using time-resolved terahertz (TRTS) and transient absorption spectroscopies. From TRTS measurements, injection yields were determined relative to a commonly used phosphonated ruthenium(II) polypyridyl complex, which is reported to have near quantitative injection yield. We find that IET occurs through space rather than through the linkers, due to the tilted orientation of the adsorbed porphyrins in direct contact with the metal oxide surface. As a result, the anchoring groups have a less significant effect on IET dynamics than for adsorbates exhibiting through-linker injection. Experiments are supported by DFT calculations, including the analysis of different electron-injection pathways. Direct IET offers the advantage of the selection of anchoring groups based solely on chemical/photoelectrochemical stability and synthetic viability, irrespective of the electronic coupling of the anchoring group to the metal oxide surface.