Orthogonal boron-nitrogen spiro compounds: donor-acceptor strength modulation for thermally activated delayed fluorescence emitters with aggregation induced emission enhancement

A series of boron-nitrogen spiro compounds were synthesized using the “intramolecular locking” strategy, and the effects on the properties of the compounds by changing the strength of the donor (D) and acceptor (A) were investigated. It showed that compounds PBA-s-FXylB and PBA-s-FXylBF, which have strong donors and acceptors, exhibit high photoluminescence quantum yields (PLQY ∼ 1) and obvious thermally activated delayed fluorescence (TADF) characteristics in degassed solvents and in the solid state. The aggregation-induced emission enhancement (AIEE) phenomenon can be observed in their solution in THF/H₂O mixtures with I/I₀ of up to 85.2 for PBA-s-FXylBF. Solid state photophysical property investigation revealed that their neat films exhibited stronger TADF character with prolonged lifetime and lower activation energy (E_a). DFT calculations confirmed the separation of the HOMO and LUMO in these molecules, which usually results in small ΔE_ST and facilitates the RISC process. Theoretical investigation of excited states revealed the participation of higher-lying triplet states (≥T₂) in the RISC process for PCZ-s-FXylB and PBA-s-Mes*B, and the vibronic coupling transitions for PBA-s-FXylB and PBA-s-FXylBF. This work systematically investigates the photophysical properties and structure-property relationships of boron-nitrogen spiro molecules, laying the foundation for the development of new optoelectronic materials.