Photon funnels for one-way energy transfer : multimetallic assemblies incorporating cyclometallated iridium or rhodium units accessed by sequential cross-coupling and bromination.

The generation of multimetallic assemblies is a widely explored theme, owing to the relevance of controlling energy and electron transfer between metal centres to many fields of contemporary importance. Boronic acid substituted coordination and organometallic complexes have been shown to be useful synthons in the formation of such structures through cross-coupling with halogenated complexes. In this work we used such a methodology to generate an octanuclear mixed-metal compound of composition Ir7Ru having a dendrimer wedge-like structure. The method combined cross-coupling with regiospecific bromination of phenylpyridine (ppy) ligands at the position para to the C–Ir bond. The propensity of Ir(ppy)2-based complexes to electrophilic bromination was found to be deactivated by the introduction of fluorine atoms. The coupling methodology was extended to rhodium-containing systems, exemplified by a tetranuclear system of composition Rh2Ir1Ru1. The synthesis required the use of boronic acid appended RhIII complexes, which could be accessed by the introduction of a neopentyl boronate ester appended bipyridine into the coordination sphere of RhIII. The excited-state energies of the constituent metal units in the resulting multinuclear complexes are such that unidirectional energy transfer occurs from the RhIII/IrIII branches to the RuII core. The luminescence thus resembles that of an isolated [Ru(bpy)3]2+ unit, but the ability of the structure to collect light is greatly enhanced