Unveiling Multichannel Triplet Excitons Up-Conversion Mechanisms in Blue Thermally Activated Delayed Fluorescence...

Understanding how molecular structure and environment cooperatively govern reverse intersystem crossing (RISC) is central to advancing blue thermally activated delayed fluorescence (TADF) emitters. Here, we combine the multiscale calculation method, thermal vibration correlation function (TVCF) theory, and explicit minimum-energy crossing point (MECP) and conical intersection (CI) analysis to elucidate the excited-state dynamics of 2CzIPN, i-2CzdOXDMe, and i-2CzdOXDPh. We find that, despite larger spin-orbit couplings involving the lowest triplet excited state (T₁), triplet up-conversion in all systems is dominated by the second triplet excited state (T₂) to the lowest singlet excited state (S₁) manifold. The efficiency of this pathway is dictated by the interplay among S₁-T₂ energy gaps, reorganization energies, and T₁ ↔ T₂ population exchange. In solution, large structural changes and inefficient population of T₂ suppress the RISC process, yielding weak (2CzIPN) or absent (i-2CzdOXDMe and i-2CzdOXDPh) TADF features. While in the solid state, packing-induced suppression of low-frequency torsions reduces reorganization energies, accelerates T₁ ↔ T₂ equilibration, and activates both adiabatic and MECP-mediated S₁-T₂ RISC channels, leading to pronounced aggregation-induced enhancement emission. Thus, the excited-state decay rates were calculated to investigate the dynamics of triplet excitons, revealing how molecular structure and surrounding environments influence the TADF behavior. Our findings reveal that achieving efficient TADF emission requires not only small S₁-T₂ gaps but also structurally accessible triplet exchange pathways, offering clear design principles for high-efficiency deep-blue TADF materials.