Modulating excited state <i>via</i> diversified electron-donating units in MR-TADF emitters: a theoretical exploration of structure–property relationships

Multi-resonant thermally activated delayed fluorescence (MR-TADF) molecules have emerged as promising candidates for high-resolution organic light-emitting diode (OLED) displays. However, their performance is often limited by intrinsically large singlet-triplet energy gaps (Δ<i>E</i>_ST), leading to an unsatisfactory reverse intersystem crossing rate (<i>k</i>_RISC). Herein, we systematically investigate how to modulate excited-state characteristics by strategically integrating diversified electron-donating units with an MR skeleton, enabling rational control over short-range charge transfer (SRCT) and long-range charge transfer (LRCT) components. The excited-state characters of S₁, including SRCT, SRCT + LRCT, and LRCT, are achieved by fine-tuning the donor and MR interactions. Compared with unsubstituted analogues, the increase of the LRCT component significantly reduces Δ<i>E</i>_ST, thereby elevating <i>k</i>_RISC values. However, the LRCT-dominated S₁ states show broad and structureless emission spectra due to substantial relaxation energy. For molecules with mixed SRCT and LRCT characters, triplet up-conversion occurs efficiently owing to the small Δ<i>E</i>_ST mediated by the mixed characters. Furthermore, the SRCT character with relatively small relaxation energy relevant to the S₁ → S₀ process could help achieve narrowband emission. This work establishes a molecular design framework for high-efficiency and narrowband MR-TADF materials, highlighting the critical role of donor unit engineering in exciton utilization and color purity.