Hyperconjugation Engineering of π-Extended Azaphosphinines for Designing Tunable Thermally Activated Delayed Fluorescence Emitters

Implanting heteroatoms into organic π-conjugated molecules (OCMS) offered a great opportunity to fine-tune the chemical structures and optoelectronic properties. This work describes a new family of 1,4-azaphosphinines with extended σ–π hyperconjugations. The photophysical studies revealed that azaphosphinines exhibited narrow-band thermally activated delayed fluorescence (TADF) ( full width at half-maximum: 26–40 nm). According to the orbital localization analysis and natural bond orbital analysis, the effective σ*−π* hyperconjugation is believed to induce the multiple-resonance (MR) TADF, which is distinct from the p−π conjugation-induced MR-TADF in BN systems. Although having the large _ΔE_S1–T1s (>3.0 ev), the study suggested that σ*−π hyperconjugation endowed the system with the structural vibration favorable for the spin-vibronic-assisted RISC. Having the tunable p-centers (lp, O, S, Se, and Me⁺), azaphosphinines showed a fine-tuned TADF. Generally, azaphosphinines with strong σ*−π* hyperconjugations showed small _ΔE_S1–T1s, efficient RISCs, and high PLQYs. Leveraging on the efficient hyperconjugations, TADF emission of the system spanned from UV-blue to green. Particularly, extended azaphosphinines exhibited the high photoluminescence quantum yields (74% in toluene and 92% in the 10% doped PMMA). As a proof of concept, two azaphosphinines with a PO center were applied as the light-emitting materials in organic lighting-emitting diodes. The devices showed the narrow-band UV- and deep-blue emission with EQE as high as 10.3%. The current study offered us a new strategy, namely, σ–π hyperconjugation-induced MR-TADF, for designing OCMs with tunable light-emitting properties.