Organic light-emitting diodes (OLEDs) still face a significant challenge in finding blue thermally activated delayed fluorescence (TADF) emitters that can achieve narrowband emission and high efficiency. In this work, we successfully design and synthesize a novel kind of TADF emitters based on rigid sulfur/oxygen-bridged triarylboron acceptor for ultrapure blue with narrowband electroluminescence. Time-dependent density functional theory (TD-DFT) calculations and photophysical results indicate the different intramolecular charge-transfer (ICT) character of two emitters. Benefiting from the rigid aromatic framework, both emitters exhibited deep-blue emission at 444 and 447 nm with a small full-width at half-maximum (fwhm) of about 33 nm, and a small singlet (S1)–triplet (T1) energy gap (ΔE ST) of 0.23 and 0.36 eV. Consequently, OLEDs based on PhCz-TOSBA and TPA-TOSBA exhibit deep blue electroluminescence at 456 nm with fwhm of about 55 nm, affording high external quantum efficiencies (EQEs) of 16.69% with CIE coordinates of (0.14, 0.15) and 16.65% with CIE coordinates of (0.14, 0.12), respectively. These findings show that PhCz-TOSBA and TPA-TOSBA are superior emitters in ultrapure blue TADF devices.

In this study, we intend to solve the problem of the low glass transition temperature (T g ) and the high coefficient of thermal expansion (CTE) that colorless polyimide (PI) films always exhibit. First, we copolymerized 2,2′-bis(trifluoromethyl)-[1,1′-biphenyl]-4,4′-diamine (TFDB) with 4,4′-(hexafluoroisopropene) diphthalic anhydride (6FDA) and p-phthaloyl chloride (TPC) and obtained a series of transparent and colorless films (PAI-10). These PAI films showed a high T g of 346−362 °C and a low CTE of 25.2−62.9 ppm/K, which were better than those of PI 6FDA-TFDB . Subsequently, three rigid diamines, p-phenylenediamine (p-PDA), 5-amino-2-(4-aminophenyl)benzoxazole (DAPBO) and 5-amino-2-(4-aminophenyl)benzimidazole (DAPBI), were used to replace 10% of the TFDB (PAI-9P-5, PAI-9O-5, and PAI-9I-5, respectively) to investigate the effect of the rigid structure on the thermal and optical properties. The results showed that rigid structures can greatly improve the thermal properties but impair transparency to some extent. Among these, PAI-9O-5 possessed the best performance with T 430 of 81.9%, T g of 364 °C, and CTE of 19.8 ppm/K. Furthermore, we fabricated an organic light-emitting diode (OLED) with PAI-9O-5 as the substrate. The flexible and bendable OLED@PAI-9O-5 showed typical green emission at 520 nm, a turn-on voltage of 2.9 V, a luminance of 1000 cd/m 2 at 5 V, a current efficiency (CE) of 2.53 cd/A, and an external quantum efficiency (EQE) of 0.81%, which were comparable to those of devices fabricated on conventional glass substrates.