Design and Development of Functionalized Cyclometalated Ruthenium Chromophores for Light-Harvesting Applications

The syntheses and the electrochemical spectroscopic properties of a suite of asymmetrical bistridentate cyclometalated Ru(II) complexes bearing terminal triphenylamine (TPA) substituents are reported. These complexes, which contain structural design elements common to both inorganic and organic dyes that exhibit superior power conversion efficiencies in the dye-sensitized solar cell (DSSC), are broadly formulated as [RuII(L-2,5′-thiophene-TPA-R1)(L-R2)]+ [L = tridentate chelating ligand (e.g., 2,2′:6′,2′′-terpyridine (tpy); deprotonated forms of 1,3-di(pyridin-2-yl)benzene (Hdpb) or 6-phenyl-2,2′-bipyridine (Hpbpy)); R1 = −H, −Me, −OMe; R2 = −H, −CO2Me, −CO2H]. The following structural attributes were systematically modified for the series: (i) electron-donating character of the terminal substituents (e.g., R1 = −H, −Me, −OMe) placed para to the amine of the “L-2,5′-thiophene-TPA-R1” ligand framework; (ii) electron-withdrawing character of the tridentate chelate distal to the TPA-substituted ligand (e.g., R2 = −H, −CO2Me, −CO2H); and (iii) position of the organometallic bond about the Ru(II) center. UV–vis spectra reveal intense and broad absorption bands arising from a collection of metal-to-ligand charge-transfer (MLCT) and TPA-based intraligand charge-transfer (ILCT) transitions that, in certain cases, extend beyond 800 nm. Electrochemical data indicate that the oxidative behavior of the TPA and metal chelate units can be independently modulated except in cases where the anionic phenyl ring is in direct conjugation with the TPA unit. In most cases, the anionic character of the cyclometalating ligands renders a metal-based oxidation event prior to the oxidation of the TPA unit. This situation can, however, be reversed with an appropriately positioned Ru–C bond and electron-rich R1 group. This finding is important in that this arrangement confines the highest occupied molecular orbital (HOMO) to the TPA unit rather than the metal, which is optimal for sensitizing TiO2; indeed, a remarkably high power conversion efficiency (η) in the DSSC (i.e., 8.02%) is measured for the TPA-substituted pbpy– chelate where R1 = −OMe. These results provide a comprehensive strategy for improving the performance of bistridentate Ru sensitizers devoid of NCS– groups for the DSSC.