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

The new heptanuclear ruthenium(ii) dendron, [Cl2Ru{(m-2,3-dpp)Ru[(m-2,3-dpp)Ru(bpy)2]2}2](PF6)12 (1; 2,3-dpp=2,3-bis(2’-pyridyl) pyrazine; bpy=2,2’-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(m-2,3-dpp)2} core, the {Ru(m-2,3- dpp)3}2+ intermediate, and the {(bpy)2Ru(m-2,3-dpp)}2+ peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (e value in the 105–106 m 1cm 1 range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (e value in the 104–105 m 1cm 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 1 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 of each absorption band and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the same family, namely [Cl2Ru{(m-2,3-dpp)Ru(bpy)2}2]4+ (2), [(bpy)2Ru(m-2,3-dpp)Ru(bpy)2]4+ (4), and [Ru{(m-2,3- dpp)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 (3MLCT) state involving the core {Cl2Ru(m-2,3-dpp)2} subunit. Interestingly, the 3MLCT levels involving the peripheral {(bpy)2Ru(m-2,3-dpp)}2+ subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy {Ru(m-2,3-dpp)3}2+ components, which, in other dendrimers, acted as “isolating” subunits 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.