Chromophoric dipyrrin complexes capable of binding to TiO₂: Synthesis, structure and spectroscopy

The synthesis, characterisation, and TiO₂ binding studies of a series of chromophoric complexes of 5-(4-carboxyphenyl)-4,6-dipyrrin (L^b) are presented. The synthesis of [Ru(bipy)(L^b)₂] (bipy = 2,2′-bipyridine), [Rh(L^b)₃], and [Pd(L^b)₂] was achieved by initial coordination of 5-(4-methoxycarbonylphenyl)-4,6-dipyrrin (L^a) followed by hydrolysis of the ester group. The carboxyl groups that are located on the peripheries of these complexes are able to engage in intermolecular hydrogen bonding interactions in the solid state, as revealed by X-ray crystallography. These groups also allow the complexes to anchor to the surface of TiO₂ nanoparticles, as evidenced by colouration of the TiO₂ and FT-IR spectroscopy. The ability of these complexes to capture a significant fraction of sunlight and to adhere to TiO₂ surfaces renders them viable dyes for photochemical devices such as dye sensitised solar cells.