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

Abstract

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.

A Copper Iodide Cluster-Based Coordination Polymer as an Unconventional Zero-Thermal-Quenching Phosphor

“…In an attempt to obtain high-efficiency PL materials, great efforts have been made to mitigate the detrimental TQ effect. − In principle, TQ could be largely minimized by suppressing the possible energy loss via nonradiative relaxation channels in the solid state by rigid molecular conformations or dense packing structures. − The understanding of the mechanisms behind TQ is rather beneficial for the predetermined design of highly photostable materials suitable for high-power LED lighting (Figure S1). The incorporation of discrete metallic clusters into robust and extended network structures has been proven to be an efficient strategy to improve both thermal- and photostabilities. − Apart from the rigidification strategy, it is also known that the metal–metal bonding interaction plays a crucial role in stabilizing the PL of metallic coinage compounds.…”

A Red-Emitting Cu(I)–Halide Cluster Phosphor with Near-Unity Photoluminescence Efficiency for High-Power wLED Applications

“…Hence, it is crucial to minimize structural distortion of local coordination spheres of metal centers for achieving highly emissive excited states [ 46 , 47 , 48 , 49 ]. We have previously demonstrated the synthesis and photophysical properties of cluster-based extended Cu(I) frameworks through monochelating imidazole derivatives, which showed negative/zero thermal quenching via electronic structural transition [ 15 , 16 ]. However, the monochelating metal complexes readily suffer from the heavy distortion of their excited states, which results in rather low PL efficiency, and the complexes are not attractive for LED phosphors.…”

“…[1][2][3][4][5][6]. The photophysical properties of metal complexes can be modified not only by the property of central metals but also by organic ligands and metal-ligand interactions [7][8][9][10][11][12][13][14][15][16][17]. Of particular interest is the potential use of Cu(I) complexes for light generation via electroluminescence [18][19][20][21][22][23][24][25], such as organic light-emitting diodes (oLEDs), because of their low cost, low toxicity, and earth-abundance of the copper element.…”

High-temperature negative thermal quenching phosphors from molecular-based materials

“…[26][27][28][29][30][31][32][33][34][35][36] By virtue of the delocalization-to-localization transition in imidazole-based ligands, we recently developed imidazole-copper(I) coordination networks to demonstrate the abnormal NTQ effect of their PL emission spectra below RT. 37,38 In this work, we continue to employ a prototype copper-iodide cubic cluster as a building block for the preparation of cluster-based coordination polymers (CPs) using bis-imidazole terminal ligands as the linker (ESI, † Fig. S1-S3), with the purpose of enhancing the NTQ temperature region (ESI, † Table S1).…”