A Flexible Purely Organic Molecule Exhibiting Strong Spin-Orbital Coupling: Toward Nondoped Room-Temperature Phosphorescence OLEDs

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-pyrimidinebased purely organic compound in an amorphous film state, which shows a fast intersystem crossing process and a high phosphorescence rate of 1.1 x 10(3) s(-1). The heavy atom sulfur and nitrogen atoms in the molecule can provide n-pi* transition character for efficient spinorbital 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%.

Automated summary

This study discovered a strong spin-orbital coupling effect in a purely organic compound, achieving a high phosphorescence rate and intersystem crossing process. The flexible molecule skeleton and heavy atom sulfur and nitrogen atoms providing n-pi* transition character contribute to efficient spin-orbital coupling, leading to the fabrication of a high-efficiency purely organic phosphorescence light-emitting diode.