Design and syntheses of gold(III) complexes with tridentate and tetradentate ligands for organic light-emitting device application

Abstract

The present work represents the design and syntheses of novel classes of phosphorescent gold(III) complexes bearing tridentate and tetradentate cyclometallating ligands for organic light-emitting device (OLED) applications. Through special design of the ligand frameworks to manipulate the electronic structures as well as the intermolecular interactions, the emission colour has been tuned all the way from sky-blue to orange-red. The overall device performances and robustness of the phosphorescent emitters have also been enhanced through the incorporation of functional main-group moieties and tetradentate ligands, respectively. To overcome the undesirable excimeric emission in the solid state, a series of alkynylgold(III) complexes bearing bulky dendrons has been designed and synthesised. Through the incorporation of the higher-generation dendrimer, the first sky-blue-emitting dendritic alkynylgold(III) complex has been successfully realised. The thermal stability, photophysical and electrochemical properties of this class of complexes have been studied by thermal gravimetric analysis, electronic spectroscopies and cyclic voltammetry, respectively. The high solubility of this class of complexes has allowed the use of solution-processing method for device fabrication and satisfactory device performances with external quantum efficiencies (EQEs) up to 7.1 % have also been realised. On the other hand, through fusing different heterocycles into the same alkynyl ligand to alter the electronic structure of the ligand, the emission colour of the complexes has also been fine-tuned. High photoluminescence quantum yields (PLQYs) of up to 59 % in the solid-state thin film have also been realised. In order to enhance the device performance, the introduction of main group moieties, such as phosphine oxide and silane units, into the tridentate ligand-containing gold(III) complexes has been carried out. In particular, through the incorporation of electron-transporting phosphine oxide moiety and hole-transporting triarylamine moiety, bipolar gold(III) complexes that showed more balanced charge transport have been synthesised. This class of complexes showed extremely high PLQYs of up to 77 %. Highly efficient OLEDs with reduced efficiency roll-offs of only 1 % and 8 % at the practical brightness of 500 cd m–2 and 1000 cd m–2 have been realised respectively based on this class of complexes. Exceptionally high EQE and current efficiency (CE) of 15.3 % and 51.6 cd A–1 have been achieved, respectively. On the other hand, the incorporation of bulky silane moieties was shown to disfavour the close packing of chromophores to reduce concentration quenching in the solid state, and devices based on this class of complexes exhibited maximum EQEs and CEs of up to 12.2 % and 42.5 cd A–1. For practical applications to be realised, robust emitters are highly desirable. In this work, a highly robust class of gold(III) complexes with tetradentate ligands has been designed and synthesised by employing a simple one-pot two bond-forming reaction. Two isomers were isolated which showed drastically different photophysical behaviours. The effect of positional isomers on their device performances was investigated and it was found that the devices based on the C^N^C^C gold(III) complex showed much better performances (EQE of 6.6 %) than the C^N^C^N counterpart (EQE of 3.0 %).