A new heptanuclear dendritic ruthenium(II) complex featuring photoinduced energy transfer across high-energy subunits

The new heptanuclear ruthenium(II) dendron, [Cl2Ru{(mu-2,3-dpp)Ru[(mu -2,3-dpp)Ru(bpy)(2)](2)](2)](PF6)(12) (1; 2,3-dpp = 2,3-bis(2'-pyridyl)pyrazine; bpy=ZZ-bipyridine), was prepared by means of the "complexes as ligands/complexes as metals" synthetic strategy, and its absorption spectrum, redox behavior and luminescence properties were investigated. Compound 1 is a multicomponent species, which contains three different types of chromophores (namely, the {Cl2Ru(mu-2,3-dpp)(2)} core, the (Ru(mu-2,3-dpp)(3)}(2+) intermediate, and the {(bpy)(2)Ru(mu-2,3-dpp)}(2+) peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (F value in the 10(5)- 10(6) M-1 cm(-1) range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (epsilon value in the 10(4)-10(5) M-1 cm(-1) range) as a result of the various spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. The redox investigation (accomplished by cyclic and differential pulse voltammetry) indicates that I undergoes a series of reversible metal-centered oxidation and ligand-centered reduction processes within the potential window investigated (+ 1.90/-1.40 V vs. the standard calomel electrode, SCE). The assignment oil each absorption band and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the some family, namely [Cl2Ru{(u-2,3-dpp)Ru(bpy)(2))(2)](4+) (2), [(bpy)(2)Ru(u-2,3-dpp)Ru(bpy)(2)](4+) (4), and [Ru{(u-2,3dpp)Ru(bpy)(2)}(3))(4+) (5). The title compound exhibits luminescence both at room temperature in acetonitrile fluid solution and at 77 K in butyronitrile rigid matrix. The emission is attributed to the triplet MLCT ((MLCT)-M-3) state involving the core {Cl2Ru(u-2,3-dpp)(2)} subunit. Interestingly, the (MLCT)-M-3 levels involving the peripheral {(bpy)(2)Ru(u-2,3-dpp)}(2+) subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy {Ru(u-2,3-dpp)}(2+) components, which, in other dendrimers, acted as "isolating' subunit's toward energy transfer processes. Ultrafast experiments on 1 and on the parent species 2 and 5 allowed us to rationalize this behavior and highlight that a sequential two-step electron-transfer process; can be held responsible for the efficient overall energy transfer, which offers a way to overcome a limitation in antenna metal dendrimers.