Spiro Units Embedded in the B/N Center for Constructing Highly Efficient Multiple Resonance TADF Emitters.

Departing from conventional molecular design strategies that rely on spiro units merely as peripheral components (side chains, terminal groups, or linkage units), we fully or partially incorporate the rigid 9,9'-spirobi[fluorene] (SF) unit into the boron/nitrogen multiple resonances (B/N-MR) emitting core, thereby successfully developing a series of proof-of-concept isomerized multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters, namely SF-BN1, SF-BN2, SF-BN3, and SF-BN4. Remarkably, these novel emitters exhibit exceptionally narrow full-width at half-maximum (FWHM) values of 15-21 nm in dilute toluene solutions and high photoluminescence quantum yields (PLQYs) of up to 90% in doped films. The corresponding organic light-emitting diode (OLED) based on SF-BN1 achieved high external quantum efficiency (EQE) of up to 29.0%, with CIE coordinates of (0.13, 0.08), closely aligning with the BT.2020 blue emission standard. Sky-blue OLEDs based on SF-BN3 can achieve a high EQE of 29.8%, with a narrow FWHM value of 18 nm; the hyperfluorescent (HF) OLEDs based on SF-BN3 improved the EQE of 35.5%. Moreover, we elucidated subtle variations in the connectivity of chemical functional groups within emitters and the polar environment and doping concentrations of OLEDs, which can significantly impact these isomers' optical and electroluminescent (EL) properties.