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

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journal contribution
posted on 19.10.2009 by Bryan D. Koivisto, Kiyoshi C. D. Robson, Curtis P. Berlinguette
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-R1)(tpy-R2)](PF6)2 (16; tpy = 2,2′:6′,2′′-terpyridine; R1 = −H, −2-furyl, or −OMe; R2 = −H, −2-furyl, or −CO2H) and [Ru(tpy-R2)(dpb-R1)]PF6 (710; Hdpb = 1,3-di(pyridin-2-yl)benzene; R2 = −H or −2-furyl; R1 = −H or −OMe). Absorption spectra for the [Ru(tpy-R1)(tpy-R2)]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-R2)(dpb-R1)]+ series, where a single Ru−N dative bond is replaced by a Ru−C σ-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 (R1 = −OMe; R2 = −2-furyl), where a strong electron-donating group is situated para to the Ru−C bond (λmax = 523 nm; ε = 2.6 × 104 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.