Achieving 1.0-s Thermally Activated Delayed Fluorescence via Synergistic Control of Reverse Intersystem Crossing and...

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.