Effect of Phenylsulfanyl Substitution on Thermally Activated Delayed Fluorescence: Mild Heavy‐Atom Sulfur Enhances Triplet Harvesting in Photo‐ and Radioluminescence

Ionizing radiation in organic scintillators generates a high fraction of triplet excitons that often act as loss channels, limiting light yield and temporal response. Here, we report a linker‐engineering strategy for all‐organic crystalline thermally activated delayed fluorescence (TADF) emitters and scintillators based on the introduction of moderate heavy atoms in the ortho position relative to the donor. Incorporation of phenylsulfanyl groups into the benchmark PXZ‐TRZ framework affords PXZ‐dSP‐TRZ, in which the singlet–triplet energy gap is reduced and spin–orbit coupling is enhanced, promoting selective reverse intersystem crossing (rISC). Single‐crystal analysis shows that sulfur substitution preserves the strongly twisted donor–acceptor geometry, flattens the donor fragment, and changes crystal packing from mixed donor···donor/acceptor···acceptor interactions to predominantly acceptor···acceptor stacking. Consequently, PXZ‐dSP‐TRZ exhibits markedly improved TADF performance, including a photoluminescence quantum yield of up to 90%, a delayed fluorescence lifetime of 0.9 µs, and a high rISC rate of 6 × 10 6 s −1 , enabled by an exceptionally low rISC activation energy of 6.2 meV. Radioluminescence and scintillation measurements under x‐ray and γ‐ray excitation confirm efficient triplet harvesting under ionizing radiation. These results identify sulfur linker modification as a promising route toward fast organic scintillators.