Cyclometallated iridium-carbazole complexes for OLED applications

Abstract

Organic Light Emitting Diodes (OLEDs) are a relatively new multi-disciplinary research area. Much of the interest generated by OLEDs is due to their commercial potential as market leaders in display technology and ambient lighting solutions. The advancement of materials used in OLED devices has driven synthetic chemists to produce a great variety of structures for use in devices. Current phosphorescent triplet emitters are based on heavy metal complexes either as a single unit or incorporated into a polymer backbone. This work presents the synthesis of ligands via a Suzuki cross-coupling methodology and their subsequent homoleptic tris-cyclometalated iridium complexes. Ligands are based on a carbazole-pyridyl structure with a substituent group on the pyridyl aimed at tuning the complexes' HOMO- LUMO gap to produce different wavelengths of emission. Several new synthetic strategies were employed in an attempt to simplify the synthesis of the ligands although it was found that the most efficient method was the original synthesis of these ligands as performed in the Bryce group. An electron donating group (EDG) or an electron withdrawing group (EWG) was added in attempts to blue shift or red shift the emission, respectively. The complexes are based on two parent systems (shown below) which give two series of functionalised complexes. Investigations were carried out into the photophysical properties of these phosphorescent triplet emitters to determine the extent to which the functionalisation altered the emission properties. Solution state phosphorescence emission studies demonstrated that the substituent EDG and EWG groups had a significant effect over the emission of the complex with shifts up to 54 nm within a series and a total range of emission covering between 504 and 637 nm; green to red emission. Solution cyclic voltammetric data are also reported. TD-DFT computations concurred with the current theories regarding the excited state of such complexes, in that the emission was occurring from a mixed MLCT/π-π(_*) transition. Preliminary device investigations demonstrated relatively low external quantum efficiencies due to a number of factors. It was also determined that the substituent group had a significant effect in shifting electroluminescence emission.