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

Abstract

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))](PF(6))(2) (1-6; tpy = 2,2':6',2''-terpyridine; R(1) = -H, -2-furyl, or -OMe; R(2) = -H, -2-furyl, or -CO(2)H) and [Ru(tpy-R(2))(dpb-R(1))]PF(6) (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 absorption intensity). Similar observations are made for the [Ru(tpy-R(2))(dpb-R(1))](+) series, where a single Ru-N dative bond is replaced by a Ru-C sigma-bond to form a cyclometalated complex. The reduced symmetry at the metal center within 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 imparts 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.