Ultrapure Blue Thermally Activated Delayed Fluorescence (TADF) Emitters Based on Rigid Sulfur/Oxygen-Bridged Triarylboron Acceptor: MR TADF and D–A TADF

Abstract

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.

Rational Molecular Design Strategy for High-Efficiency Ultrapure Blue TADF Emitters: Symmetrical and Rigid Sulfur-Bridged Boron-Based Acceptors

“…The corresponding rate constants for fluorescence radiative decay ( k r ), intersystem crossing ( k ISC ), and reverse intersystem crossing ( k RISC ) were calculated based on the aforementioned quantum yields and lifetimes of the prompt and delayed fluorescence, and corresponding data are shown in Table S1. Remarkably, owing to the presence of the extremely rigid MR framework, the k r values of TSBA-Cz and TSBA-PhCz are higher (10.35 × 10 7 and 6.98 × 10 7 s –1 ), comparing with the most of the common MR-type TADF materials. − ,− The k nr values of TSBA-Cz and TSBA-PhCz are 2.52 × 10 7 and 1.78 × 10 7 s –1 , respectively, which was responsible for PLQY and OLED device performances of these two emitters. The k ISC and k RISC are 4.49 × 10 8 and 4.17 × 10 8 and 5.29 × 10 4 and 7.62 × 10 4 s –1 , respectively, indicating that the RISC process plays a key role in the photophysical processes for these two reported MR-type TADF emitters.…”

“…The k ISC and k RISC are 4.49 × 10 8 and 4.17 × 10 8 and 5.29 × 10 4 and 7.62 × 10 4 s –1 , respectively, indicating that the RISC process plays a key role in the photophysical processes for these two reported MR-type TADF emitters. The k RISC value suggests that the introduction of sulfur atom into the rigid molecular structure can accelerate the RISC process in comparison with the oxygen/sulfur-bridged cyclized boron-based emitters …”

“…The OLED devices using TSBA-Cz and TSBA-PhCz as doped emitters were prepared, as described previously (ref ), and the EL performance was carried out using the apparatus described in ref .…”

“…In our previous work, we found that oxygen/sulfur-bridged cyclized boron-based emitters exhibit a narrow intramolecular charge-transfer (ICT) blue emission and fast reverse intersystem crossing (RISC) process . The sulfur atom can lock the structure to increase molecular rigidity and sever as a heavy atom to promote the RISC process in TADF molecules. − With these considerations in mind, sulfur atoms are introduced into the TAB acceptor structure to make this new structure appropriate as an acceptor for blue TADF emitters with narrowband EL. On the one hand, the rigid and symmetrical molecular structure can inhibit vibrational relaxation, which raises the Franck–Condon factor and results in the high photoluminescence quantum yield (PLQY) with a narrowband emission.…”

“…The k RISC value suggests that the introduction of sulfur atom into the rigid molecular structure can accelerate the RISC process in comparison with the oxygen/sulfur-bridged cyclized boron-based emitters. 27 We fabricated multilayered TADF-OLED devices to evaluate the difference in the EL performance of TSBA-Cz and TSBA-PhCz as emitters. The energy level diagram of fabricated blue TADF-OLED devices is presented in Figure 6a.…”