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  • Title: Ruthenium(II)-polyimine-coumarin light-harvesting molecular arrays: design rationale and application for triplet-triplet-annihilation-based upconversion.
    Author: Wu W, Ji S, Wu W, Shao J, Guo H, James TD, Zhao J.
    Journal: Chemistry; 2012 Apr 16; 18(16):4953-64. PubMed ID: 22407570.
    Abstract:
    Ru(II)-bis-pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16,000  M(-1)  cm(-1)). Thus, Ru(II)-polyimine complexes that show intense visible-light absorptions are of great interest. However, no effective light-harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible-light-harvesting Ru(II)-coumarin arrays, which absorb at 475 nm (ε up to 63,300  M(-1)  cm(-1), 4-fold higher than typical Ru(II)-polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy-transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady-state and time-resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru(II)-polypyridine-chromophore light-harvesting arrays, which states that the (1)IL energy level of the ligand must be close to the respective energy level of the metal-to-ligand charge-transfer (MLCT) states. Lower energy levels of (1)IL/(3)IL than the corresponding (1)MLCT/(3)MLCT states frustrate the cascade energy-transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light-harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10-diphenylanthracene as a triplet-acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.
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