Variation in the Singlet-Triplet Energy Gap (ΔE_ST) Governs Multistep Thermal Responsive Photoluminescence in a Coordination Polymer Exhibiting Thermally Activated Delayed Fluorescence

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 (ΔE_ST). However, the mechanistic relationship between specific ΔE_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 ΔE_ST, while simultaneously elucidating how ΔE_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.