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Ruthenium(II)–Polyimine–Coumarin Light‐Harvesting Molecular Arrays: Design Rationale and Application for Triplet–Triplet‐Annihilation‐Based Upconversion
Author(s) -
Wu Wanhua,
Ji Shaomin,
Wu Wenting,
Shao Jingyin,
Guo Huimin,
James Tony D.,
Zhao Jianzhang
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201101377
Subject(s) - chromophore , photon upconversion , photochemistry , phosphorescence , excited state , quantum yield , acceptor , ruthenium , chemistry , fluorescence , atomic physics , ion , physics , optics , biochemistry , organic chemistry , condensed matter physics , catalysis
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 ( M LCT) states. Lower energy levels of 1 IL/ 3 IL than the corresponding 1 M LCT/ 3 M LCT 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 %.