A “Flexible” Purely Organic Molecule Exhibiting Strong Spin–Orbital Coupling: Toward Nondoped Room-Temperature Phosphorescence OLEDs
Weidong Qiu, Xinyi Cai, Zijian Chen, Xiaofan Wei, Mengke Li, Qing Gu, Xiaomei Peng, Wentao Xie, Yihang Jiao, Yiyang Gan, Weimin Liu, Shi‐Jian Su
Abstract
Purely organic materials usually exhibit weak spin–orbital coupling (SOC) effect because of the lack of noble heavy metals, and the generation and direct emission from the triplet state is spin-forbidden. This would lead to slow intersystem crossing, long triplet lifetime, and low phosphorescence quantum yield. Herein, strong spin–orbital coupling between singlet and triplet was observed in a “flexible” and twist thianthrene-pyrimidine-based purely organic compound in an amorphous film state, which shows a fast intersystem crossing process and a high phosphorescence rate of 1.1 × 103 s–1. The heavy atom sulfur and nitrogen atoms in the molecule can provide n−π* transition character for efficient spin–orbital coupling. Moreover, the flexible molecule skeleton enables conformational change and molecular vibration in excited states, which was proved to be vital for efficient vibrational spin–orbital coupling. Benefitting from the strong SOC effect, a nondoped purely organic phosphorescence light-emitting diode was fabricated, which achieves a maximum external quantum efficiency of 7.98%, corresponding to an exciton utilization ratio exceeding 87.6%.