Donor Engineering and Solid-State Confinement Enable Switchable RTP and TADF in Saccharin-Based Emitters

A thorough understanding of the conversion process of triplet excitons is crucial for achieving controllable thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) in donor-acceptor (D-A) emitters. Here, we combine a multiscale calculation method and thermal vibration correlation function theory to elucidate the excited-state dynamics of five saccharin-based D-A emitters (6-Cz-Sac, 5-Cz-Sac, DiCz-Sac, 5-Sac-Pxz, and 5-Sac-Ptz) and construct a unified mechanistic picture. In tetrahydrofuran solution, all emitters exhibit weak S₁ radiative decay accompanied by fast nonradiative loss, which intrinsically limits emission efficiency. 6-Cz-Sac and 5-Cz-Sac show inefficient triplet recycling due to the slow RISC process relative to the ISC process, resulting in dominant nonradiative loss from the T₁ state. In contrast, DiCz-Sac enables efficient triplet recycling through a compact triplet-state energy level with a dominant T₂-assisted RISC channel, while 5-Sac-Pxz and 5-Sac-Ptz operate via a conventional single-channel TADF mechanism driven by the very small S₁-T₁ gaps. In the solid state, crystal packing suppresses structural relaxation and extends the triplet state lifetimes. Hydrogen-bond short contacts around the saccharin acceptor further provide effective intermolecular locking and reduce nonradiative loss, thereby activating triplet branching as the decisive factor governing the emission. As a result, 6-Cz-Sac and 5-Cz-Sac become RTP-dominated, DiCz-Sac exhibits concurrent TADF and RTP due to competition between T₂-assisted up-conversion and phosphorescence, and 5-Sac-Pxz/5-Sac-Ptz maintains single-channel TADF emission. Overall, this work reveals how donor engineering and solid-state confinement together determine triplet fate, enabling selective access to RTP, mixed TADF/RTP, or conventional TADF within a unified saccharin-based platform.