Dielectric Solvation-Regulated Excited-State Energetics Dictate Reverse Intersystem Crossing Dynamics

The efficiency of thermally activated delayed fluorescence (TADF) is fundamentally governed by the rate of the reverse intersystem crossing process (krISC), which depends exponentially on the singlet–triplet activation energy barrier (ΔEST) according to the Arrhenius relationship. This work demonstrates that dielectric solvation provides an effective means of modulating the energetic landscape governing rISC. By systematically probing the dielectric response of the 1CT, 3CT, and 3LE states, we reveal how the local environmental polarity alters both ΔEST and the 1CT–3LE energy separation. Moreover, we explored the crucial role of the medium’s rigidity in maximizing the radiative properties of TADF emitters by suppressing excessive molecular motions. The photophysical properties from solution to solid-state films further establish that the dielectric characteristics of the host matrix exert a decisive influence on excited-state energetics and delayed fluorescence performance. These findings identify solid-state solvation as a previously underappreciated strategy for controlling excited-state energetics and establish host dielectric engineering as a powerful strategy for maximizing TADF efficiency while simultaneously mitigating dipole–dipole interaction-induced photoluminescence (PL) quenching.