photosensitizers: effect of varied linker conjugation on interfacial electron transfer†

The efficient capture and conversion of solar energy into a usable form is a significant research target. Dye-sensitized systems, such as dye-sensitized solar cells (DSSCs) and watersplitting dye-sensitized photoelectrochemical cells (WSDSPECs), utilize high surface area metal oxide scaffolds sensitized with a molecular light-absorbing dye to harvest sunlight and convert it into an electrical potential via an electrontransfer event between the sensitizer and a metal oxide. A solution-phase redox shuttle quenches the sensitizer radical in DSSCs, while in WS-DSPECs it is quenched by a water oxidation. Unfortunately, in WS-DSPECs and p-type DSSCs, rapid back electron transfer (recombination) significantly limits the power conversion efficiencies that can be obtained. Molecular design offers a promising strategy for tuning the interfacial electron-transfer kinetics in dye-sensitized energyconversion systems. In DSSCs, a substantial research effort has explored systems of donor–acceptor dyes where the donor– acceptor pair is bridged by a p-conjugated system and has led to significant improvements in power-conversion efficiencies. Alternatively, positioning saturated alkyl linkers between chromophores and anchoring groups is effective at retarding detrimental recombination. The intermediate case, where the chromophore and anchor are connected by a partially conjugated bridge, is largely unexplored. Recently, we demonstrated a phenyl-amide structure that functions as a molecular rectifier based on the spatial asymmetry of the amide moiety. The amide partially breaks the conjugation within the molecule and depending on its orientation, moves the LUMO closer or further from the Fermi level of the system, making it sensitive to the bias potential and thereby imparting rectifying character. Inspired by these rectifiers, we developed a novel series of dyes (Ru1–Ru3, Chart 1), as a platform for exploring partially conjugated linking strategies using spectroscopic techniques to characterize the interfacial electron-transfer dynamics in conjunction with quantum chemistry calculations. The dyes, based on the well studied ruthenium tris(bipyridine) motif, were prepared by a standard synthetic strategy, starting with the preparation of the functionalized bipyridine intermediates bearing an ethyl phosphonate group and incorporating amide or ethylene units. The amide bonds were then formed by coupling an amine unit to the carboxylic acid, pre-activated with thionyl chloride for Amide1 or carbodiimide for Amide2. The alkene was formed by condensation of a phosphonate and aldehyde in a Horner–Wadsworth–Emmons reaction. In the next step, the ruthenium tris(bipyridine) complex was synthesized and subjected to chloride-to-PF6 anion exchange for better solubility. Finally, removal of the ethyl groups unveiled the phosphonate anchoring groups. Each target compound was 1