Synthesis and characterization of rhenium(I) 4,4′-dicarboxy-2,2′-bipyridine tricarbonyl complexes for solar energy conversion

The synthesis of a series of rhenium(I) tricarbonyl complexes containing 4,4 '-dicarboxy-2,2 '-bipyridine (dcbpy) and various halide or pseudohalide ligands is reported. Deprotonation of the carboxylates occurs in basic medium resulting in upfield H-1 NMR shifts for the protons of the carboxyl groups attached to 2,2 '-bipyridine. Metal-to-ligand charge transfer (MLCT) due to d pi -> pi* transitions systematically shift in the halide series from 382 nm for Re(dcbpy)(CO)(3)F to 436 nm for Re(dcbpy)(CO)(3)I. The compounds emit in the 600-650 nm region and have emission lifetimes greater than 50 ns. The solar energy conversion efficiency is less than 1% when TiO2- Re(dcbpy)(CO)(3)X, (X = F, CN, SCN, Br, I), photoanodes are used in Gratzel cells.