Carbene-stabilized enantiopure heterometallic clusters featuring EQE of 20.8% in circularly-polarized OLED

Abstract

Bright and efficient chiral coinage metal clusters show promise for use in emerging circularly polarized light-emitting materials and diodes. To date, highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs) with enantiopure metal clusters have not been reported. Herein, through rational design of a multidentate chiral N-heterocyclic carbene (NHC) ligand and a modular building strategy, we synthesize a series of enantiopure Au(I)-Cu(I) clusters with exceptional stability. Modulation of the ligands stabilize the chiral excited states of clusters to allow thermally activated delayed fluorescence, resulting in the highest orange-red photoluminescence quantum yields over 93.0% in the solid state, which is accompanied by circularly polarized luminescence. Based on the solution process, a prototypical orange-red CP-OLED with a considerably high external quantum efficiency of 20.8% is prepared. These results demonstrate the extensive designability of chiral NHC ligands to stabilize polymetallic clusters for high performance in chiroptical applications.

Fig. 1. Ligand design and crystal structures.

a Ligand structures of R/S-NHC^py/ql-H·PF₆. b Structure of the enantiomers of R/S-py/ql-X (X = Cl, Br, and I). Au, yellow; Cu, brown; N, dark blue; C, gray; X (Cl, Br, and I), purple. The H atoms were omitted for clarity.

Fig. 2. Photoluminescence properties and thermal stability.

a, b Solid-state photoluminescence spectra of complexes R-py-X (X = Br and I) and R-ql-X (X = Cl, Br, and I) excited at 400 nm. c PLQYs of five R-type clusters in the solid state. d Images of a single crystal of R-ql-I in the range of 83–473 K under ambient and UV light. e PXRD patterns of R-NHC^ql-AuCu₄-I in the range of 293–473 K.

Fig. 3. Temperature-dependent emission spectra of R-ql-I.

a Normalized temperature-dependent emission spectra in the range of 93 to 383 K (λ_ex = 400 nm). b Three-dimensional excitation-emission matrix (3D-EX-EM) luminescence spectra in the solid state at 93 K. c Temperature-dependent low-energy and the high-energy emission peak intensities in the range of 93 to 383 K. d Plot of transient decay lifetimes against temperature (93 to 303 K); the black line represents the fit according to the equation accounting for TADF.

Fig. 4. Natural transition orbital analysis.

a–c The hole and electron pairs for S₁/S₀ and T₁/S₀ transitions were obtained by natural transition orbital analysis at optimized S₁ and T₁ geometries of R-ql-Cl (a), R-ql-Br (b), and R-ql-I (c) (isovalue of 0.02). Au, yellow; Cu, brown; N, dark blue; C, gray; Cl, blue; Br, green; I, light, purple; H, white.

Fig. 5. TADF and phosphorescence emission processes and transient absorption.

a–c Energy diagram of R-ql-Cl (a), R-ql-Br (b), R-ql-I (c) indicating TADF and phosphorescence emission processes. d–f Global fitting results for the femtosecond probe TA spectra of R-ql-Cl (d), R-ql-Br (e), R-ql-I (f).

Fig. 6. Functional modules in the metal cluster.

Color codes: Au, yellow; Cu, brown; N, dark blue; C, gray; X (Cl, Br, and I), purple. One ligand and the H atoms were omitted for clarity.

Fig. 7. Chiroptical properties.

CPL spectra of R/S-py-X (X = Br and I) (a) and R/S-ql-X (X = Cl, Br, and I) (b) in CH₂Cl₂ (1 × 10⁻⁵mol/L) under ambient conditions.

Fig. 8. CP-OLED performance.

a Device architecture of CP-OLED. b Current density–voltage–luminance characteristics of S-OLEDs. Inset: EL spectra. c CE, PE, and EQE vs. current density curves of S-OLEDs. d CPEL spectra of the devices.