Systematic Manipulation of the Light-Harvesting Properties for Tridentate Cyclometalated Ruthenium(II) Complexes

The response of the metal-to-ligand charge-transfer (MLCT) band to variability in terminal substituents within a related set of tridentate polypyridyl and cyclometalated Ru(II) complexes is reported. These complexes are formulated as [Ru(tpy-R-1)(tpy-R-2)](PF6)(2)(1-6; tpy=2,2':6',2 ''-terpyridine; R-1=-H, -2-furyl, or -OMe; R-2 = -H, -2-furyl, or -CO2H) and [Ru(tpy-R-2)(dpb-R-1)]PF6(7-10; Hdpb=1,3-di(pyridin-2-yl)benzene; R-2 = -H or -2-furyl; R-1 = -H or -OMe). Absorption spectra for the [Ru(tpy-R-1)(tpy-R-2)](2+) series highlight the sensitivity of the MLCT band to the indicated substituents at the 4' position of one or both tpy ligands (e.g., a bathochromic shift up to 24 nm coupled with a 2-fold increase in absorpfion intensity). Similar observations are made for the [Ru(tpy-R-2)(dpb-R-1)](+) selies, where a single Ru-N dative bond is replaced bya Ru-C sigma-bond to form a cyclometalated complex. The reduced symmetry at the metal centerwithin this series results in a broadening of the lowest-energy MLCT band, while an additional set of transitions at higher energies emerges that involves an excited state localized on the cyclometalating ligand. These MLCT transitions collectively render a broad absorption envelope of substantial intensity at wavelengths longer than ca. 525 nm. Optimal results are obtained for compound 10 (R-1=-OMe; R-2=-2-furyl), where a strong electron-donating group is situated para to the Ru-C bond (lambda(max)=523 nm; epsilon=2.6 x 10(4) M-1 cm(-1)). This approach irripVs substantial polarization within the molecule, which should benefit excited-state electron-transfer reactions for photosensitizing applications (e.g., dye-sensitized solar cells). Spectroscopic data are corroborated by electrochemical and TD-DFT measurements for all compounds.