Boosting Reverse Intersystem Crossing of TADF Emitter through Molecular Geometry Adaptation to Crystalline Host

Abstract Rapid reverse intersystem crossing (RISC) is one of the prime concerns for blue thermally activated delayed fluorescence (TADF) emitters, as it reduces triplet exciton population, the root cause of detrimental triplet‐mediated annihilation processes that accelerate device efficiency roll‐off and degradation. This work introduces a new concept to tailor the RISC of TADF emitters through their molecular geometry adaptation to crystalline hosts bearing a similar donor‐acceptor structure. A meticulously designed crystalline host comprising isophthalonitrile acceptor (A) and carbazole‐derived donor (D) units, characterized by nearly orthogonal D‐A arrangement, has been demonstrated to alleviate singlet‐triplet energy gap (ΔE_ST) of the TADF dopant by forcing it to adopt a more twisted D‐A configuration, as corroborated by X‐ray diffraction (XRD) measurements. The approach not only significantly reduces the RISC activation energy, resulting in a remarkable tenfold boost of the RISC rate (above 10⁷ s⁻¹) and fourfold shortening of delayed FL lifetime (down to 1.5 µs), but also offers the additional benefit of suppressing conformational disorder of TADF dopant, producing a narrower emission bandwidth. The presented concept, based on crystalline host‐driven RISC engineering, is anticipated to have a profound impact on the development of high‐performance, stable blue‐emitting TADF organic light‐emitting diodes (OLEDs).