Ruthenium(II)-polyimine-coumarin light-harvesting molecular arrays: Design rationale and application for triplet-triplet-annihilation-based upconversion

Ru II-bis-pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (Îμ<16000M -1cm -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 (Îμ up to 63300M -1cm -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 1IL 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 1IL/ 3IL than the corresponding 1MLCT/ 3MLCT 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%.