Ultrastrong circularly polarized electroluminescence achieved by chiral co-assembly of a liquid crystal-functionalized carbazole derivative

Abstract

Circularly polarized organic light-emitting diodes (CP-OLEDs) exhibiting circularly polarized electroluminescence (CP-EL) properties hold significant promise for future display technologies. However, enhancing the electroluminescence dissymmetry factor (g _EL) remains a substantial challenge. Herein, ultrastrong CP-EL emissions are achieved using a liquid crystal (LC)-functionalization strategy under the regulation of chiral co-assembly. The LC molecule 3CzPCH containing carbazole and mesogenic units could be readily synthesized using Suzuki coupling reactions. When doping chiral inducers R/S-D into 3CzPCH, the chiral co-assemblies formed helical nanofibers upon 120 °C annealing treatment and emitted strong deep-blue circularly polarized luminescence (CPL) with |g _lum| of 0.13 (λ _em = 422 nm, Φ _FL = 34%). Most importantly, these chiral co-assemblies served as the emitting layers (EMLs) of CP-OLEDs, reaching an ultrastrong deep-blue CP-EL with a |g _EL| value up to 0.47 (λ _EL = 440 nm). The corresponding Q-factor (EQE × |g _EL|) of 1.12 × 10^-2 represents one of the highest values reported for CP-OLEDs. This study demonstrates that the LC-functionalization strategy effectively enables intense CP-EL, paving the way for high-performance CP-OLEDs.

Scheme 1. (a) Chemical structures of 3CzPh and liquid crystal-functionalized 3CzPCH; (b) chemical structure of chiral inducer R-D; schematic diagram of the chiral co-assembly process (c) before and (d) after thermal annealing; (e) the device configuration of CP-OLEDs.

Fig. 1. Optimized structures and HOMO/LUMO distributions of 3CzPCH and 3CzPh.

Fig. 2. POM images of 3CzPCH doped with different wt% amounts of R-D at 120 °C. (a) 0 wt%; (b) 1 wt%; (c) 3 wt%; (d) 5 wt%; (e) 7 wt%; (f) 10 wt%.

Fig. 3. (a and d) CD spectra, (b and e) CPL spectra, and (c and f) g_lum value curves of 3CzPCH doped with different wt% amounts of R/S-D (a–c: before thermal annealing; d–f: after thermal annealing at 120 °C).

Fig. 4. SEM images of 3CzPCH+3 wt% R-D (a–c) before and (d–f) after thermal annealing at 120 °C. (a and d) 10 mg mL⁻¹, (b and e) 10⁻¹ mg mL⁻¹ and (c and f) 10⁻³ mg mL⁻¹ in toluene.

Fig. 5. (a) The energy level diagram of the related materials; (b) chemical structures of functional layer molecules; (c) EL spectra of the R/S-Cz; (d) CIE 1931 chromaticity diagram; (e) J–V–L characteristics; (f) CE–L curves; (g) CP-EL spectra and (h) g_EL curves of the R/S-Cz.