Achieving 1.0‐s Thermally Activated Delayed Fluorescence via Synergistic Control of Reverse Intersystem Crossing and Exciton Cycling

Persistent thermally activated delayed fluorescence (p‐TADF) is fundamentally constrained by the kinetic trade‐off between reverse intersystem crossing (rISC) and triplet exciton decay, including phosphorescence and non‐radiative processes, which intrinsically limits its lifetime ( τ DF ). Here we present a synergistic strategy that overcomes this limitation by concurrently slowing the rISC rate ( k rISC ) while preserving the condition k rISC ≫ k Ph + k nr,T and deliberately promoting multiple intersystem crossing (ISC)/rISC exciton cycles. The efficacy of this approach is validated by o ‐TFBCz, which achieves an unprecedented τ DF of 1.00 s even in unannealed poly(methyl methacrylate), despite originating from a phosphorescence core with a lifetime ( τ Ph ) of only 1.92 s. This system exhibits bluish‐green afterglow under blue‐light excitation and outstanding thermal stability. Quantitative photophysical analysis reveals an average of 2.1 ISC/rISC cycles per exciton in this material, enabled by an ISC rate ( k ISC ) that dominates over fluorescence ( k Fl ) and internal conversion ( k IC ) rates ( k ISC > k Fl + k IC ). These results establish a clear, generalizable blueprint for breaking the lifetime ceiling of pure organic p‐TADF materials.