Luminescent Transition Metal Complexes of Terdentate Quinoline-Based Ligands for Light-Emitting Devices

Abstract

Luminescent transition metal complexes are of interest due to their applications in areas such as organic light-emitting devices (OLEDs), solar cells and biological probes and sensors. The use of heavy transition metals promotes phosphorescent emission from the triplet state. In OLEDs, this enables the quantum efficiency of devices to reach a theoretical maximum of 100 % through harvesting photons from both singlet and triplet excited states.

The synthesis of cyclometallated N^C^N-coordinated iridium(III) complexes and two representative rhodium(III) analogues, based upon a series of isomeric 1,3-(diquinolyl)benzene ligands, is presented. The d6 complexes take the form M(N^C^N)(X^Y)Z, where X^Y denotes a bidentate ligand binding through N^C- or N^N-coordination, and Z denotes a monodentate ligand, such as chloride. Platinum(II) complexes containing terdentate ligands, of the form Pt(N^C^N)Cl, have also been investigated. All complexes are luminescent at room temperature and at 77 K, emitting in the red-to-yellow region of the visible spectrum. The photophysical properties are interpreted with the aid of electrochemical and computational analyses. Preliminary results from incorporation of three of the complexes into OLEDs are described. In addition, an iridium phosphor used in conjunction with a blue emitter produces a white device which displays excellent white light characteristics.

The platinum(II) complex of 1,3-di(8-quinolyl)benzene features six-membered chelate rings, a rarer feature than the usual five-membered chelate rings common to the majority of metal complexes studied to date. The related N^N^N-coordinating ligand, 2,6-di(8-quinolyl)pyridine, has also been used to prepare a complex of the form [Pt(N^N^N)Cl]+, which, unlike [Pt(tpy)Cl]+, displays moderately intense luminescence in solution, attributed to a more favourable bite angle. The corresponding [Ir(N^N^N)2]3+ complex is also reported.