Coupling synthetic antenna and electron donor species: A tetranuclear mixed-metal Os(II)–Ru(II) dendrimer containing six phenothiazine donor subunits at the periphery

A new dendritic-shaped tetranuclear complex, [Os{(μ-,3-dpp)Ru(bpy-PTZ)2}3]8+ (1), containing an {Os(μ-2,3-dpp)3}2+ chromophore as the core and three {(μ-2,3-dpp)Ru(bpy-PTZ)2}2+ polypyridine building blocks as peripheral subunits, has been prepared (2,3-dpp = 2,3-bis(2′-pyridyl)pyrazine; bpy-PTZ = 10-(2,2′-bipyridin-4-yl)methyl)phenothiazine). Compound 1 can be viewed as an integrated light-harvesting antenna–reaction center species, in that the {Os{(μ-2,3-dpp)Ru(bpy)2}3}8+ subunit is an efficient antenna system, coupled to well-known electron donor subunits, the phenothiazine (PTZ) moieties. The absorption spectrum, redox behavior, and luminescence behavior (both at room temperature in fluid solution and at 77 K in rigid matrix) of 1 have been investigated and compared with the properties of the parent [Os{(μ-2,3-dpp)Ru(bpy)2}3]8+ complex (2). The absorption spectrum and redox behavior confirm that 1 behaves as a supramolecular, multicomponent species, in that each subunit (including the phenothiazine donor groups) is only slightly perturbed by the others. Analogously to what happens in the model complex 2, the light absorbed by the peripheral Ru(II) chromophores is transferred to the Os(II) core: however, the luminescence of the Os(II) core is efficiently quenched in 1 via moderately exoergonic reductive electron transfer by the peripheral phenothiazine donors, in spite of the presence of the intermediate Ru(II) centers. The quenching process becomes inefficient at 77 K.