Thermally Stable Donor-Acceptor Type (Alkynyl)Gold(III) TADF Emitters Achieved EQEs and Luminance of up to 23.4% and 70 300 cd m-2 in Vacuum-Deposited OLEDs

Abstract

Thermally stable, strongly luminescent gold-TADF emitters are the clue to realize practical applications of gold metal in next generation display and lighting technology, a scarce example of which is herein described. A series of donor-acceptor type cyclometalated gold(III) alkynyl complexes with some of them displaying highly efficient thermally activated delayed fluorescence (TADF) with Φ up to 88% in thin films and emission lifetimes of ≈1-2 µs at room temperature are developed. The emission color of these complexes is readily tunable from green to red by varying the donor unit and cyclometalating ligand. Vacuum-deposited organic light-emitting diodes (OLEDs) with these complexes as emissive dopants achieve external quantum efficiencies (EQEs) and luminance of up to 23.4% and 70 300 cd m^-2, respectively.

Keywords: gold; ligand‐to‐ligand charge transfer; organic light‐emitting devices; thermally activated delayed fluorescence.

Figure 1

Chemical structures of complexes 1–8.

Figure 2

a) Absorption spectra of 1, 2, 4, and 5 in toluene, b) emission spectra of 1, 2, 4, and 5 in degassed toluene at room temperature, c) emission spectra of 1, 2, 4, and 5 in PMMA thin films (4 wt%), d) absorption spectra of 3, 6, 7, and 8 in toluene, e) emission spectra of 3, 6, 7, and 8 in degassed toluene at room temperature, and f) emission spectra of 2 in different solvents at room temperature.

Figure 3

Emission spectra and photograph of 8 (concentration = 2 × 10⁻⁵ mol dm⁻³) in degassed hexane and toluene.

Figure 4

Emission spectra of 1 and 3–5 (4 wt%) in PMMA thin films at 77 K.

Figure 5

Electron density difference map (eddm), molecular orbital (MO) surfaces of the optimized S₁ and T₁ excited states with δ at their respective optimized geometries. Color code for the eddm: green, increase in electron density; magenta, decrease in electron density.

Figure 6

a–c) EQE–luminance characteristics of OLEDs based on 2, 5, and 6 with dopant concentration of 2, 4, and 8 wt%, and d) normalized EL spectra of devices with dopant concentration of 4 wt%.