Modulating circularly polarized luminescent and thermally activated delayed fluorescence properties by introducing chiral unit and extending acceptor unit strategies

Abstract

Circularly polarized thermally activated delayed fluorescence (CP-TADF) organic light-emitting diodes (OLEDs) have emerged as promising optoelectronic devices due to their high exciton utilization efficiency and simplified device architecture, demonstrating broad application prospects in 3D displays, quantum computing, and information storage. However, the inherent trade-off between luminescent efficiency and luminescent dissymmetry factor (g_lum) has significantly limited the diversity of high-performance CP-TADF materials. In this study, we systematically investigated the luminescent mechanism of a reported thermally activated delayed fluorescence molecule QAO using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods coupled with the thermal vibration correlation function (TVCF) approach. Through calculations of intersystem crossing rates, reverse intersystem crossing rates, radiative decay rates, and non-radiative decay rates, we confirmed the TADF characteristics of QAO. Moreover, the evaluation of electronic circular dichroism (ECD) spectrum and g_lum values (1.92 × 10^-3) confirmed the circularly polarized luminescent (CPL) feature. Furthermore, based on these findings, two strategies (introducing chiral unit and extending the acceptor unit) were proposed to enhanced the CPL and TADF properties, and two new efficient CP-TADF molecules (QAO-Cz and QAO-CzNCF) were constructed. Results indicate that the g_lum values of QAO-Cz (4.06 × 10^-3) and QAO-CzNCF (4.46 × 10^-3) were largely improved compared with that of initial QAO with decreased electric transition dipole moment (|μ|) and magnetic transition dipole moment (|m|) of the molecules. Compared to |m| (0.723 × 10^-20 erg·G^-1, 0.474 × 10^-20 erg·G^-1, 0.349 × 10^-20 erg·G^-1), |μ| (3.885 × 10^-18 esu·cm, 1.242 × 10^-18 esu·cm, 0.786 × 10^-18 esu·cm) exhibits a greater degree of reduction, leading to an enhancement in |m|/|μ| and g_lum values and further improving the CPL performance. Meanwhile, the energy gap of the molecules QAO-Cz (0.03 eV) and QAO-CzNCF (0.05 eV) are significantly reduced compared to QAO (0.14 eV), while the spin-orbit coupling values are remarkably increased, ensuring the excellent TADF properties of these molecules. Thus, the fundamental structure-property relationship is revealed and inner luminescent mechanism is illustrated. Wise strategy for designing CP-TADF molecules is proposed, and both designed CP-TADF molecules successfully achieve synergistic optimization of CPL and TADF properties, this work provides new insights for developing high-performance CP-TADF emitters.

Keywords: Circularly polarized luminescent; Luminescent asymmetry factor; TVCF method; Thermally activated delayed fluorescence.