A Tetranuclear Ruthenium(II) Complex Containing both Electron-Rich and Electron-Poor Bridging Ligands. Absorption Spectrum, Luminescence, Redox Behavior, and Intercomponent Energy Transfer

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Abstract
The first luminescent and redox active multinuclear Ru(II) compound containing both electron-poor (2,3-bis(2-pyridyl)pyrazine, 2,3-dpp) and electron-rich (3,5-bis(pyridyn-2-yl)-1,2,4-triazole, Hbpt) polypyridine bridging ligands has been synthesized. The novel compound is [(bpy)2Ru(μ-bpt)Ru{(μ-2,3-dpp)Ru(bpy)2}2]7+ (1; bpy = 2,2‘-bipyridine). Its absorption spectrum, luminescence properties, and redox behavior have been studied and are compared with the properties of the parent complexes [Ru{(μ-2,3-dpp)Ru(bpy)2}3]8+ (2) and [(bpy)2Ru(μ-bpt)Ru(bpy)2]3+ (3). The absorption spectrum of 1 is dominated by ligand-centered bands in the UV region and by metal-to-ligand charge transfer bands in the visible region. Excited states and oxidation and reduction processes are localized in specific sites of the multicomponent structure. However, perturbations of each component on the redox and excited states of the others, as well as electronic interactions between the chromophores, can be observed. Intercomponent energy transfer from the upper-lying (μ-bpt)(bpy)Ru→bpy CT excited state of the Ru(bpy)2(μ-bpt)+ component to the lower-lying (bpy)2Ru→μ-2,3-dpp CT excited state of the Ru(bpy)2(μ−2,3-dpp)2+ subunit(s) is efficient in 1 in fluid solution at room temperature, whereas this process is not observed in a rigid matrix at 77 K. A two-step energy transfer mechanism is proposed to explain the photophysical properties of the new compound.

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