Simultaneously achieving smaller ΔEST and narrower FWHM of long- and short-range charge-transfer MR emitters via precisely regulating peripheral modification

The long-/short-range charge-transfer (LR/SR-CT) type thermally activated delayed fluorescence (TADF) emitter is designed to solve the problem of slow reverse intersystem crossing rate (k_RISC) of the traditional MR-TADF emitter and obtain the full-color emission. However, the introduction of LR-CT transition inevitably leads to the broadening of the emission spectrum. In the design of LR/SR-CT type TADF emitters, it is still quite difficult to balance the smaller singlet-triplet energy gap (ΔE_ST) and the narrower full-width at half-maximum (FWHM). Here, we investigated the LR-CT and SR-CT proportion of LR/SR-CT TADF emitters by adjusting the strength and position of the peripheral electron-donating units on the MR skeleton (TSBA), and prepared three MR-TADF emitters. The investigation of photophysical properties reveals that the increase in the strength and number of donors significantly reduces the ΔE_ST and accelerates the RISC process. The double substitution strategy of the TSBA skeleton effectively reduces the dihedral angle between the MR skeleton and the electron donor unit, and the highest occupied molecular orbital (HOMO)/hole is more distributed on the MR skeleton, which realizes the increase of the excited state SR-CT component. Consequently, the dual-substituted DTPA-TSBA emitter overcomes the contradiction between the small ΔE_ST and the narrow FWHM, and achieves the reduction of both. The corresponding device based on DTPA-TSBA achieved a green light emission with a maximum external quantum efficiency (EQE_max) of 26.2 % and a FWHM of only 55 nm. This work provides profound guidance for the development of high-performance narrowband TADF emitters.