Chiral Perturbation Strategies for Circularly Polarized Thermally Activated Delayed-Fluorescence Small Molecules: Progress in the Application of Organic Light-Emitting Diodes

Abstract

The application of organic light-emitting diodes (OLEDs) has become widespread, with polarizers commonly employed to mitigate the influence of external light sources on OLED displays. However, when the light signal generated by the OLED emissive layer passes through the polarizer, approximately 50% of the light energy is inevitably lost. Circularly polarized luminescent (CPL) molecules, capable of emitting specific left- or right-handed circularly polarized light, theoretically enable 100% light energy utilization in corresponding OLED devices (CP-OLEDs). With this breakthrough, CPL mechanisms exhibit significant potential for applications in data storage, bioimaging, and 3D displays. In this review, we focus on molecules constructed via a chiral perturbation strategy, analyzing their CPL generation mechanisms and molecular engineering principles. The relationship between these molecular structures and OLED performance is systematically analyzed and summarized. Finally, we critically address current challenges in developing both CPL active materials and devices based on the chiral perturbation strategies, while providing perspectives on future developments and potential challenges in this field.

Keywords: chirality perturbation; circularly polarized luminescence; light-emitting diode; molecular structure; thermally activated delayed fluorescence.

Figure 1

Three generations of OLED emitting material luminescence mechanism. (F = fluorescence; P = phosphorescence; PF = prompt fluorescence; DF = delayed fluorescence; ISC = intersystem crossing; RISC = reverse intersystem crossing; ΔE_ST = the energy difference between the first excited singlet and triplet states; nr = nonradiative).

Figure 2

Schematic diagram of an OLED and CP-OLED with anti-glare filters.

Figure 3

(R/S) – 1 molecular design schematic diagram. Reproduced with permission. Copyright © 2016 American Chemical Society [26].

Figure 4

Binaphthol based chiral CP-TADF molecules (The red part in the picture represents the chiral unit).

Figure 5

Performance of CP-doped OLED based on (R/S)-OBN-DPA: (a) EQE–luminance curves with insertion of EL spectrum at 6 V and lighting of NJU logo by device D-D(R). (b) Current–efficiency/power–efficiency–luminance curves and insertion of current–density/luminance–voltage curves. (c) CPEL spectra. (d) g_EL versus wavelength curves. Copyright 2019, Wiley-VCH [41].

Figure 6

Octahydrobinaphthol-based chiral CP-TADF molecules. (The red part in the picture represents the chiral unit).

Figure 7

Performance of OLEDs based on (+)-(S,S)-CAI-Cz and (−)-(R,R)-CAI-Cz as emitters. (a) EQE–luminance characteristics. Inset: EL spectra of the devices at 9 V. (b) The g_EL value of the enantiomer-based OLEDs as a function of emission wavelength. Copyright 2018, Wiley-VCH [50].

Figure 8

1.2-Diaminocyclohexane-based chiral CP-TADF molecules. (The red part in the picture represents the chiral unit).

Figure 9

(a) Energy diagram of the solution-processed non-doped CP-OLEDs based on (R/S)-TpAc-TRZ. (b) EQE–current density characteristics of the device G(S). Inset: EL spectra of the device at 6 V. (c) Current density–voltage–luminance (J–V–L) characteristics of the device G(S). (d). g_EL values of CP-OLEDs based on (S)-(+)-TpAc-TRZ (G(S)) and (R)-(−)-TpAc-TRZ (G(R)) as a function of emission wavelength. Copyright 2021, Wiley-VCH [53].

Figure 10

Chiral CP-TADF molecules constructed based on other chiral units. (The red part in the picture represents the chiral unit).