Facile access to high-performance reverse intersystem crossing OLED materials through an unsymmetrical D-A-D’ molecular scaffold

Abstract

Organic light-emitting diode (OLED) materials based on a reverse intersystem crossing (RISC) triplet harvesting mechanism have attracted tremendous attention. Currently, many research efforts have been devoted to the construction of D-A or D-A-D RISC-OLED materials whose charge-transfer-featured singlet excited states (1CTD-A) are very close to the local triplet excited states of their D or/and A moieties (3LED or/and 3LEA), so that a small singlet–triplet energy splitting and a large spin–orbit coupling matrix element can be acquired concurrently. However, the chief technical difficulty lies in the accurate prediction and delicate tuning of the 1CTD–A energy levels of these compounds. Herein, by using PCz-TXO2 as an example, we demonstrated that high-performance RISC-OLED materials can be readily achieved by integrating an appropriate D’ subunit into a traditional D-A fluorophore that lacks RISC property, i.e., constructing a D–A–D’ triad whose 1CTD-A is close to its 3LED’. In comparison with its D-A or D-A-D counterpart of P-TXO2 or DP-TXO2 showing satisfactory photoluminescence efficiency, pure blue gamut but poor RISC, the presence of a carbazole-based D’ subunit has negligible influence on the transition feature of its lowest singlet excited state (S1), but triggers significantly enhanced RISC property. Consequently, PCz-TXO2 shows pure blue electroluminescence (EL) inherited from DP-TXO2 [CIE1931: (0.160, 0.089) vs. (0.154, 0.102)], but significantly enhanced external quantum efficiency (EQEmax: 9.5% vs. 6.1%) and exciton utilization efficiency (EUEmax: 93% vs. 42%). Our results present a new method to facilely access RISC materials and can greatly extend the design rationales for high-performance OLED materials.