Design and syntheses of gold(III) and gold(I) complexes : from photophysics to organic light-emitting devices and the control of molecular assembly and orientation

Abstract

The present work describes the design and syntheses of novel classes of luminescent gold(III) and gold(I) complexes for organic light-emitting device (OLED) applications and the control of molecular assembly and orientation. Through judicious design of the ligand frameworks and manipulation of intermolecular interactions, the emission colour was tuned from sky-blue to near-infrared (NIR). The luminescence properties and robustness of the gold emitters were enhanced through rational design strategies, as demonstrated in the overall increase in the device efficiency and operational stability.

To realise red to NIR gold(III) emitters, a series of alkynylgold(III) complexes was designed and synthesised. Through incorporating strong σ-donating alkynyl ligands, the emission band maxima of the complexes were found to extend to 823 nm in solution and 730 nm in thin films. A large bathochromic shift of ~1370 cm−1 was obtained upon fusing the N-heterocyclic moieties into the triphenylamine-based alkynyl ligand. The high solubility of this class of complexes rendered the realisation of red- to NIR-emitting solution-processable OLEDs, in which a peak maximum at 720 nm and Commission International de l’Éclairage coordinates of (0.67, 0.33) were obtained.

In order to enhance the device performance, the factors governing the stability of emitters, such as the trans effect, the trans influence, the hard-soft acid-base relationship between the gold metal centre and the coordinating atom of the ligands, and the facile non-radiative decay processes, were investigated. Through rational design based on careful considerations, a series of gold(III) complexes with tunable emission colours spanning from sky-blue to red was realised. On top of the classical pyridine ring, the effect of incorporating a diversity of N-heterocyclic moieties into the cyclometalating ligand on the photophysical and electrochemical properties was investigated. These complexes exhibited high quantum yields of up to 90 % in thin films, excellent solubility and high thermal stability. High maximum external quantum efficiencies (EQEs) of 11.9 % and 21.6 % were achieved in the solution-processable and vacuum-deposited OLEDs, respectively, with long operational half-lifetimes of ~138,000 h at 100 cd m−2.

On the other hand, an approach to enhance the higher robustness and rigidity of the ligand framework so as to facilitate a facile control of molecular alignment through various non-covalent interactions has been adopted. Through restricting the carbazole moiety in the tetradentate ligand of the gold(III) complexes and systematically attaching the bulky groups onto the ligand framework, the effect on the orientation of gold(III) emitters in doped films was investigated. This class of complexes demonstrated a horizontal dipole ratio of 0.87 in doped films, while a maximum EQE of 20.6 % with an estimated out-coupling efficiency of around 30 % was realised in the vacuum-deposited device. These devices were found to operate with half-lifetimes of ~37,500 h at 100 cd m−2. Moreover, the molecular packing of rigid-rod alkynylgold(I) complexes was studied. Based on the rigid oligo(p-phenylene ethynylene) backbone, the effect of different phosphine ligands on the photophysical and self-assembly properties of the resulting complexes was investigated. Sky-blue-emitting gold(I)-based devices were also realised, with a peak maximum at 432 nm.