Tailored Energy\nLandscape to Foster Reverse Intersystem\nCrossing and Radiative Rates in TADF Emitters

Effective triplet exciton utilization is essential for\nhigh electroluminescence\nefficiency and device durability in the development of OLEDs. Thermally\nactivated delayed fluorescence (TADF) has proven to be a powerful\nstrategy for this purpose. However, balancing a low singlet–triplet\nenergy offset (Δ<i>E</i><sub>ST</sub>) with a high\noscillator strength (<i>f</i>) remains a significant challenge,\nas these two factors often conflict due to their different dependencies\non orbital overlap integrals. In this study, we developed a TADF emitter\nwith a minimal Δ<i>E</i><sub>ST</sub>, where both\nshort-range and long-range charge transfer transitions contributed\nto a high <i>f</i> and an enhanced <i>k</i><sub>rISC</sub>. A reliable strategy for fostering electronic coupling\nbetween the <sup>1</sup>CT and <sup>3</sup>LE or hybridized triplet\nstates in newly synthesized TADF emitters featuring a strongly twisted\nD–A structure is reported here. Furthermore, a suitable dielectric\nsolvation technique is employed to effectively modulate the <sup>3</sup>CT energy, revealing a clear correlation between the photoluminescence\nquantum yield and the dielectric constant of the matrix.