Bis-Cyclometalated Iridium Complexes Containing 4,4′-Bis(phosphonomethyl)-2,2′-bipyridine Ligands: Photophysics, Electrochemistry, and High-Voltage Dye-Sensitized Solar Cells

In this report, the synthesis and characterization of two bis-cyclometalated iridium­(III) complexes are presented. Single-crystal X-ray diffraction shows that [Ir­(ppy)2(4,4′-bis­(diethylphosphonomethyl)-2,2′-bipyridine)]­PF6 adopts a pseudooctahedral geometry. The complexes have an absorption feature in the near-visible–UV region and emit green light with excited-state lifetimes in hundreds of nanoseconds. The redox properties of these complexes show reversible behavior for both oxidative and reductive events. [Ir­(ppy)2(4,4′-bis­(phosphonomethyl)-2,2′-bipyridine)]­PF6 readily binds to metal oxide supports, like nanostructured SnIV-doped In2O3 and TiO2, while still retaining reversible redox chemistry. When incorporated as the photoanode in dye-sensitized solar cells, the devices exhibit open-circuit voltages of >1 V, which is a testament to their strength of these iridium­(III) complexes as photochemical oxidants.