Compared to Ir­(III) complexes with octahedral geometries, Pt­(II) complexes with square planar geometries show superior optical properties because their flat shapes lead to an orientation that enhances the outcoupling of organic light-emitting diodes (OLEDs). However, the flat shapes of Pt­(II) complexes typically induce a bathochromic shift, limiting their application in high-performance deep-blue phosphorescent OLEDs with high color purity. In this study, bulky trimethylsilyl (TMS)-substituted blue phosphorescent Pt­(II) complex (PtON7-TMS) is successfully synthesized to improve color purity. The TMS substituent containing Si atom effectively suppresses intermolecular interaction and aggregation even when the complex concentration in the film state is higher than 30 wt %. As a result, the PtON7-TMS-based OLEDs exhibit a maximum external quantum efficiency of 21.4%, along with a pure-blue color of CIE (0.14, 0.09) at 20 wt % doping concentration and a full-width at half maximum of 30 nm. The pure blue color is maintained at a higher doping concentration (>30 wt %).

Three donor−acceptor-type thermally activated delayed fluorescence (TADF) emitters (PXZBAO (1), PXZBTO (2), and PXZBPO (3)) comprising a phenoxazine (PXZ) donor and differently π-expanded boroncarbonyl (BCO) hybrid acceptor units are proposed. The emitters exhibit red (1) to orange (3) emissions with an increase in the π-expansion in the BCO acceptors. The control of the strength of local aromaticity for the BCO unit and the corresponding LUMO level is attributed to inducing the unusual emission color shifts. The photoluminescence quantum yield and delayed fluorescence lifetime of the emitters are also adjusted by the π-expansion. Notably, although 1 possesses a 3 nπ* state in the acceptor unit as a local triplet excited state ( 3 LE, T 2 ), the T 2 states of 2 and 3 mainly comprise a 3 ππ* state in the acceptor. Consequently, all of the emitters exhibit strong spin−orbit coupling between their T 2 and excited singlet (S 1 ) states, leading to a fast reverse intersystem crossing with rate constants of ∼10 6 s −1 . By employing the emitters as dopants, we realize efficient red-to-orange TADF-OLEDs. Maximum external quantum efficiencies of 17.7% for the yellowish-orange (3), 15.5% for the orange (2), and 13.9% for the red (1) devices are achieved, and the values are very close to the theoretical limit predicted from the optical simulation.

Formamidinium lead bromide (FAPbBr3) nanocrystals (NCs) that exhibit ultra‐pure green emission are the most promising candidates for future displays. Despite the rapid development of light‐emitting diodes (LEDs) based on perovskite NCs (PeNCs), there is limited research detailing their intrinsic light outcoupling. Herein, the use of a short‐chain fluoroaromatic ligand, 4‐fluoro‐phenethylammonium bromide (FPEABr), via a facile spin‐casting method is proposed to fine‐tune the refractive index (n) and horizontal dipole ratio values (ΘH) of the perovskite emitter layer and simultaneously suppress the defects formed during film deposition. After FPEABr ligand exchange, the FAPbBr3 NC films exhibit a refractive index n that is significantly lower (by about 0.4) than bulk (2D/quasi‐2D or 3D) perovskite films and show an enhanced ΘH value of 77%. Therefore, ultra‐pure green perovskite LEDs are successfully produced with a maximum current efficiency of ≈50 cd A−1, a maximum luminance of 21 304 cd m−2, and a peak external quantum efficiency of 11.34% at a high luminance of 2804 cd m−2, approaching the theoretical value of 11.90% given the structure, photoluminescence quantum yield, and ΘH.