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

Abstract

Developing highly efficient blue thermally activated delayed fluorescence (TADF) emitters with a narrowband emission is still a challenge. Here, novel ultrapure blue TADF emitters of TSBA-Cz and TSBA-PhCz were designed and synthesized for organic light-emitting diodes (OLEDs). Photophysical and time-dependent density functional theory calculation results simultaneously show the similar intramolecular charge-transfer character of MR-type TADF emitters. Benefiting from the symmetrical and rigid molecular configuration, compounds TSBA-Cz and TSBA-PhCz emit a pure blue emission peak at 463 and 470 nm, a narrow full width at half-maximum (FWHM) of 30 and 36 nm, and a small singlet-triplet energy gap (ΔE ST ) of 0.21 and 0.18 eV, respectively, facilitating their excellent TADF behavior in doped films. Furthermore, highly efficient TADF-OLED devices using the TSBA-Cz and TSBA-PhCz with external quantum efficiencies of 23.4 and 21.3% emit ultrapure blue electroluminescence (EL) at 464 and 472 nm with a narrow FWHM of about 35 nm and CIE color coordinates of (0.14, 0.11) and (0.12, 0.18). This work provides novel TADF emitters for blue OLEDs with narrowband EL.

Organoboron-based multiple-resonance emitters: synthesis, structure–property correlations, and prospects

“…130 For TSBA-Cz and TSBA-PhCz , both S 1 and T 1 states were LE-dominant, resulting in substantially similar photophysical properties based on the MR effect. 131 In BSS-Cz , BSS-PA , BSS-SpAc , and BSS-Ac , the HOMO levels of donor units become shallower in the order of Cz < PA < SpAc < Ac. 133 Similar to TSBA-Cz , BSS-Cz showed LE-dominated emission with a narrow FWHM of 28 nm because of the small HOMO offset.…”

“…8 and Table 6). 116,120,122–138 The reaction temperature of the first lithium–halogen exchange was kept relatively low (−78 °C to rt), probably to suppress the undesired exchange at the remaining bromine atom. Although the yield of Br-TDBA was relatively low (15%) in Kwon's work, 122 the same product was synthesised later with up to 49% yield, 129 which suggests that lithium–halogen exchange proceeds with high selectivity.…”

Development of novel acceptors with deep LUMO energy levels

“…Recently, more and more researchers have committed to exploring efficient exciplex-based OLEDs and made great progress. For the blue and green emissions, the external quantum efficiencies (EQEs) of some pureorganic exciplex-based devices have approached and exceeded 20%, gradually catching up with the pace of single-molecule TADF OLEDs [5][6][7]. However, the development of pure-organic red exciplex emitters with emission peaks over 600 nm has been almost stagnant.…”

Synthesis and Properties of Short-Lifetime Thermally Activated Delayed Fluorescence Materials

“…The delayed lifetime of TADF materials is in the range of hundreds of microseconds as a result of their endothermic RISC process . Most of TADF materials reveal a long delayed fluorescent lifetime, , which leads to enhancement of the triplet-related nonradiative process, − such as triplet–triplet annihilate (TTA), triplet-polaron annihilate (TPA), and severe efficiency roll-off at a high current density.…”