Hyperconjugation Engineering of π-Extended Azaphosphinines for Designing Tunable Thermally Activated Delayed Fluorescence Emitters
Zhaoxin Liu, Lingqiang Meng, Yanrong Jiang, Chao Li, Huanchao Gu, Kexuan Zhao, Ji Zhang, Hong Meng, Yi Ren
Abstract
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 Δ E S1–T1 s (>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 Δ E S1–T1 s, 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.