Variation in the Singlet‐Triplet Energy Gap (Δ E ST ) Governs Multistep Thermal Responsive Photoluminescence in a Coordination Polymer Exhibiting Thermally Activated Delayed Fluorescence

ABSTRACT Negative thermal quenching (NTQ) materials remain scarce yet show promise for advanced applications, particularly in high‐power LEDs. Thermally activated delayed fluorescence (TADF) materials may exhibit thermal quenching (TQ) or NTQ simultaneously due to different Singlet‐Triplet Energy Gap (Δ E ST ). However, the mechanistic relationship between specific Δ E ST values and temperature‐responsive luminescence remains unresolved. This work investigates the photophysical properties of a TADF coordination polymer (CuIP‐OPY). CuIP‐OPY exhibits a distinctive multistep photoluminescence response upon heating: TQ → zero thermal quenching (ZTQ) → NTQ (+1.67% K ‒1 ) → ZTQ. Computational simulations and variable‐temperature single‐crystal x‐ray diffraction reveal that this phenomenon originates from phase‐transition‐induced modulation of Δ E ST , while simultaneously elucidating how Δ E ST variations govern thermal responsive behavior. Leveraging these unique properties, we fabricated an LED operating at high temperature (421 K). Finally, we introduced a halogen‐doping strategy to precisely control the NTQ operational window (370–400 K), expanding material options for different operation temperatures.