CCC-pincer-NHC osmium complexes: New types of blue-green emissive neutral compounds for organic lIght-emitting devices

Abstract

An important class of N-heterocyclic carbene ligands is that introducing a donor atom between two carbene edges. This type of ligands provide pincer coordination, and although a diverse array of pincer ligands bearing the NHC motif have been employed, those based on an NHC-aryl-NHC framework are attracting special interest, since the transition metal complexes bearing these ligands, mainly iridium and platinum, have interesting photophysical properties. Compared to Ir and Pt, cyclometallatated osmium complexes with pincer type carbene ligands are very scarce. These Os(II) salts, [Os(CCC)(CO)(NN)]X, are emissive in the red-spectral region with quantum yields between 10-4 and 10-2. In the search for blue emitting neutral complexes with a flat backbone we have investigated the reactivity of the complex OsH6(PiPr3)2 towards CbenzimidazoliumCarylCbenzimidazolium and CimidazoliumCarylCimidazolium salts. We show here a general entry to neutral Os(CCC)-pincer derivatives including Os(IV)- and Os(II)-hydride species and homoleptic and heteroleptic bis(tridentate) Os(II) complexes. These homoleptic and heteroleptic bis(tridentate) Os(II) derivatives show promising photophysical properties, and moderate to high photoluminescence quantum yields are observed in the solid state. Since the photoluminescent properties can be tuned by selecting two proper ligands, the heteroleptic complexes offer a great flexibility for materials design.