Synthesis and luminescence of iridium and rhodium complexes incorporating NCN-coordinating terdentate ligands

Abstract

The luminescent properties of transition metal complexes containing polypyridyl and cyclometalating ligands make them potential candidates for a range of applications; for example, as triplet-harvesting agents in organic light-emitting devices (OLEDs), owing to the potentially high quantum yields of triplet emission; in solar cells, converting light to electrical energy; and as sensors and probes in biological systems. The synthesis of a series of [M(NCN)(X^X^X)]’^n^+’ and [M(NCN)(X^X)C1](^n+) coordinated complexes (where M = Ir or Rh; X = heterocyclic N or cyclometalated aryl C; and n = 0-2) bearing pyridyl and pyrazolyl-based NCN-coordinating ligands (cyclometalating through the central phenyl ring) is reported, alongside their photophysical and electrochemical properties. Whilst luminescence was generally observed from the pyridyl-based iridium(III) complexes at ambient temperature, the charge-neutral Rh(III) complex [Rh(NCN)(NC)Cl] was the only complex amongst those containing a rhodium centre to be emissive under these conditions. Similarly, the pyrazolyl Ir(III) complexes exhibit lower luminescence intensities than their pyridyl analogues, owing to the poorer Ti-accepting ability of the pyrazole rings which results in a blue-shift in the emission profile and more ligand-based character. In addition to the synthesis of these complexes, a sequential cross-coupling - bromination - cross-coupling strategy has been developed for the linear stepwise expansion of an [Ir(NCN)(NNC)(^+) coordinated complex, incorporating a pendant bromophenyl group on the central pyridyl ring of the NNC ligand, via in situ palladium- catalysed Suzuki cross-coupling reactions with aryl boronic acids. This strategy has-been further extended to the controlled synthesis of linear multimetallic assemblies using boronic acid appended Ir(III) and Ru(II) complexes. A heterometallic trinuclear [Ir-ɸ-Ir- ɸ (_2)-Ru](^4+) assembly was prepared, where the phenylene bridges between the metal centres do not contribute to the excited state of the trimetallic system, and efficient energy-transfer occurs to the lower energy ruthenium terminus. The emissive and energy-channelling properties of such multimetallic assemblies can be tailored by the careful choice of the constituent "building blocks”.