title: Ruthenium(II)-polyimine-coumarin light-harvesting molecular arrays: design rationale and application for triplet-triplet-annihilation-based upconversion.
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 %.
authors.affiliation: State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China.
authors.forename: Wanhua
authors.initials: W
authors.lastname: Wu
authors.forename: Shaomin
authors.initials: S
authors.lastname: Ji
authors.forename: Wenting
authors.initials: W
authors.lastname: Wu
authors.forename: Jingyin
authors.initials: J
authors.lastname: Shao
authors.forename: Huimin
authors.initials: H
authors.lastname: Guo
authors.forename: Tony D
authors.initials: TD
authors.lastname: James
authors.forename: Jianzhang
authors.initials: J
authors.lastname: Zhao
mesh_terms: D000042:Absorption; D003374:Coumarins / Q000737:chemistry*; D007097:Imines / Q000737:chemistry*; D008027:Light; D008163:Luminescent Measurements; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010777:Photochemistry; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*
chemical_list: D003374:Coumarins; D007097:Imines; D009942:Organometallic Compounds; D012428:Ruthenium; C030123:coumarin