Negative/Zero Thermal Quenching of Luminescence via Electronic Structural Transition in Copper-Iodide Cluster-Based Coordination Networks.

Photoluminescence (PL) intensity in organic or metal-organic emitters usually suffers from thermal quenching (TQ), which severely hinders their industrial applications. The development of negative thermal quenching (NTQ) and/or zero thermal quenching (ZTQ) materials depends on a better understanding of the mechanisms underpinning TQ in luminescent solids. In this work, we investigated the temperature dependence of thermally activated delayed fluorescence (TADF) in copper(I)-organic coordination polymers (CP) ligated with an imidazole or triazole derivative over a broad temperature range. The efficient PL emission of CP1 is heavily quenched as the crystalline samples are cooled to 77 K; the PL intensity shows the NTQ effect in the region of 77-238 K followed by a ZTQ effect in the temperature range of 238-318 K. No NTQ or ZTQ effect is observed for reference coordination polymer CP2, where the 1,2,4-triazole group was used instead of the imidazole one. Our work highlights the important role of the ligand's electronic structure in optimizing photophysical properties of coordination polymer emitters and may stimulate new efforts to design luminescent materials exhibiting NTQ and ZTQ effect at higher temperature.