The weak spin–orbit coupling (SOC) in metal-free organic molecules poses a challenge in achieving phosphorescence emission. To attain pure phosphorescence in RTP organic emitters, a promising molecular design concept has been proposed. This involves incorporating n → π* transitions and leveraging the heavy atomic effect within the spin–orbit charge transfer-induced intersystem crossing (SOCT-ISC) mechanism of bipolar molecules. Following this design concept, two bipolar metal-free organic molecules (PhSeB and PhSeDB) with donor–acceptor (D–A) and acceptor–donor–acceptor (A–D–A) configurations have been synthesized. When the molecular configuration changes from D–A to A–D–A, PhSeDB exhibits stronger electron coupling and n → π* transitions, which can further enhance the spin–orbit coupling (SOC) together with the heave atom effect from the selenium atom. By the advanced synergism among enhanced n → π* transitions, heavy atom effect and magnified electron coupling to efficiently promote phosphorescence emission, PhSeDB can achieve pure RTP emission in both the solution and doped solid film. Thanks to the higher spin–orbit coupling matrix elements (SOCMEs) for T₁ ↔ S₀, PhSeDB attains the highest phosphorescence quantum yield (ca. 0.78) among all the RTP organic emitters reported. Consequently, the purely organic phosphorescent light-emitting diodes (POPLEDs) based on PhSeDB achieve the highest external quantum efficiencies of 18.2% and luminance of 3000 cd m⁻². These encouraging results underscore the significant potential of this innovative molecular design concept for highly efficient POPLEDs.

Inspired by the intriguing optoelectronic characteristics of the 2-phenylpyridine-type (ppy-type) four-coordinate organoboron skeleton, we envisage a molecular design strategy by manipulating the MLCT transition character to develop high-performance long-wavelength Ir-based phosphors with a ppy-type four-coordinate organoboron skeleton for organic light-emitting diodes (OLEDs). Three ppy-type cyclometalated Ir(III) complexes are successfully prepared. IrOBN and IrPBN exhibit the expected long-wavelength phosphorescent emission at 620 and 604 nm, respectively, due to the electron-accepting ability of the pyridine coordinated with the boron atom (pyd(B)) in extending the π-conjugated length for the LUMO, thus leading to stabilization of the LUMO. Interestingly, IrMBN shows a green phosphorescence at 514 nm. The more electron-deficient pyd(B) in IrMBN leads to a reorganized and localized LUMO distribution pattern mainly on pyd(B) rather than the pyridine coordinated with the Ir atom (pyd(Ir)), shortening the π-conjugation length for the LUMO, hence resulting in an elevated LUMO. Benefiting from the high rigidity of the ppy-type four-coordinate organoboron skeleton, these three ppy-type cyclometalated Ir(III) complexes show high PLQY (ca. 0.6–1). Beneficially, we can achieve impressive electroluminescence (EL) performance based on IrPBN with the highest efficiencies of a maximum external quantum efficiency (η_ext) of 26.0%, a maximum current efficiency (η_L) of 42.0 cd A⁻¹, and a maximum power efficiency (η_P) of 38.5 lm W⁻¹, respectively. All these excellent results convincingly demonstrate the effectiveness of our molecular design strategy and the great potential of the ppy-type four-coordinate organoboron skeleton in developing high-performance Ir-based phosphors.

Through precise modulation of excited-state properties, four hot-exciton materials were designed and synthesized, enabling high-performance NUV-OLEDs.