Four emitters based on the naphthyridine acceptor moiety and various donor units exhibiting thermally activated delayed fluorescence (TADF) were designed and synthesized. The emitters exhibited excellent TADF properties with a small ΔE ST and a high photoluminescence quantum yield. A green TADF organic light-emitting diode based on 10-(4-(1,8-naphthyridin-2-yl)phenyl)-10H-phenothiazine exhibited a maximum external quantum efficiency of 16.4% with Commission Internationale de L'ećlairage coordinates of (0.368, 0.569) as well as a high current and power efficiency of 58.6 cd/A and 57.1 lm/W, respectively. The supreme power efficiency is a recordhigh value among the reported values of devices with naphthyridine-based emitters. This results from its high photoluminescence quantum yield, efficient TADF, and horizontal molecular orientation. The molecular orientations of the films of the host and the host doped with the naphthyridine emitter were explored by angle-dependent photoluminescence and grazing-incidence small-angle X-ray scattering (GIWAXS). The orientation order parameters (Θ ADPL ) were found to be 0.37, 0.45, 0.62, and 0.74 for the naphthyridine dopants with dimethylacridan, carbazole, phenoxazine, and phenothiazine donor moieties, respectively. These results were also proven by GIWAXS measurement. The derivative of naphthyridine and phenothiazine was shown to be more flexible to align with the host and to show the favorable horizontal molecular orientation and crystalline domain size, benefiting the outcoupling efficiency and contributing to the device efficiency.

Based on the classic 1,1′:4′,1″‐terphenyl fluorophore, a novel near‐ultraviolet (NUV) fluorescent BB4Ph (4,4″′‐bis(2,3,4,5‐tetraphenylphen‐1‐yl)(1,1′:4′,1″‐terphenyl) is designed, synthesized, and characterized. BB4Ph exhibits a NUV fluorescence with a λmax of 373 and 380 nm in solution and as a thin film, respectively. The grazing incidence wide‐angle X‐ray scattering (GIWAXS) reveals BB4Ph having a high value of order parameter (SGIWAXS) of 0.59 for a molecular layer array normal to the substrate; the angle‐dependent photoluminescence (ADPL) measurement shows Θ (horizontal–dipole ratios) as high as 95% with the neat film. The non‐doped BB4Ph organic light‐emitting diodes (OLEDs) show external quantum efficiency (EQE) up to 5.24% with 1931 Commission Internationale de l’ Eclairage coordinates (CIEx,y) of (0.16, 0.04), a NUV electroluminescence. With 4P‐Cz (9,9′‐(1,1′:4′,1″:4″,1′″‐quaterphenyl‐4,4′″‐diyl)dicarbazole) as a dopant emitter, doped BB4Ph OLEDs show the highest EQE of 6.99% and a NUV 1931 CIEx,y (0.16, 0.04). The photoluminescence quantum yield is measured for the thin film of BB4Ph and 4P‐Cz:BB4Ph as 68% and 96%, respectively. Hence, the low limits of light outcoupling efficiency (ηout) of the two individual OLEDs are estimated as 31% and 29%, respectively.