Facile Access to High-Performance Reverse Intersystem Crossing Oled Materials Through an  Unsymmetrical D-a-D' Molecular Scaffold

Abstract

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 ( 1 CT D-A ) are very close to the local triplet excited states of their D or/and A moieties ( 3 LE D or/and 3 LE A ), 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 1 CT D – A energy levels of these compounds. Herein,  PCz-TXO 2 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 1 CT D-A is close to its 3 LE D’ . In comparison with its D-A or D-A-D counterpart of P-TXO 2 or DP-TXO 2 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 , but triggers significantly enhanced RISC property. Consequently, PCz-TXO 2 shows pure blue electroluminescence (EL) inherited from DP-TXO 2 [CIE1931: (0.160, 0.089) vs . (0.154, 0.102)], but significantly enhanced external quantum efficiency (EQE max : 9.5% vs . 6.1%) and exciton utilization efficiency (EUE max : 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.

Keywords: reverse intersystem crossing material, unsymmetrical D-A-D' triad, electroluminescence