Modular Synthesis of Elongated Phosphonate Bipyridines
The synthesis and the photophysical properties of a series of bpy-R2 derivatives L1–L3 (bpy = 2,2′-bipyridine, R represents the substitution at the 4- and 4′-positions of the bpy) are described. R includes phosphonic ester groups as precursors for potent phosphonate anchoring groups, which enable immobilization on transition metal oxide semiconductor surfaces for applications like dye-sensitized solar cells (DSSCs) or dye-sensitized photoelectrosynthesis cells (DSPECs). The ligands L1–L3 differ in the length of conjugated linker units between bpy core and anchoring groups. Phenylene and triazole moieties serve as building blocks for linker elongation. The resulting adjustability of the distance between semiconductor and chromophore represents a viable route to improve cell efficiency, as it will allow tuning of charge carrier recombination and dye aggregation. Furthermore the photophysical studies of the free ligands reveal a pronounced effect of the aryl substitution. The solid-state structures of L1 and L2 are reported within this contribution, enabling the determination of distances between bipyridine nitrogen donor and anchoring group.