Thermal Enhancement of Luminescence for Negative Thermal Expansion in Molecular Materials

Abstract

Overcoming thermal quenching is an essential issue in the practical application of luminescent materials. Herein, we found that negative thermal expansion (NTE) can achieve the thermal enhancement of luminescence in molecular materials based on three metal–organic frameworks CuX-bpy (X = Cl, Br, I; bpy = 4,4′-bipyridine). All complexes exhibit NTE on the c-axis, and the strongest NTE leads to a contraction of the Cu...Cu distance in CuCl-bpy, which further intensifies the luminescence emission. This phenomenon indicates the existence of thermally enhanced charge transfer. Moreover, the origin of the distinction in charge transfer attributed to the different valence states of the copper is investigated through the combined studies of X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and density functional theory calculations. This research provides a new approach to modulating the luminescence thermal enhancement by NTE.

Anomalous thermal expansion of strontium squarate trihydrate induced by hydrogen-bond weakening

“…14,15 Some uniaxial NTE compounds can be processed as preferred fibers or membranes, which can also meet practical applications. 16 Moreover, NTE can also be utilized to modify materials’ physical and chemical properties, such as luminescence, 17,18 ferroelectric, 19 nonlinear optics, 20 gas adsorption, etc . 21,22…”

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

“…S13). Despite the characteristic frequencies of the imidazole group in the variable-temperature Raman spectra showing no convincing thermally induced changes, the in situ solid-state 13 C NMR spectra provide useful information on the electronic structural transition. At 293 K, the strong chemical shift peaks (at 135.5, 133.2, and 128.2 ppm in Fig.…”

“…[10][11][12] Molecular-based emitters typically exhibit a TQ effect or NTQ in a low temperature region, i.e., far below room temperature (RT), whereas high-temperature NTQ is most desirable for practical applications because the temperature generated from the Joule effect in gallium nitride LEDs can exceed 100 1C, at which the PL will be almost quenched for traditional phosphors via the non-radiative relaxation of excited electrons to the ground state. 13 By contrast, the emission from inorganic solid phosphors benefits from high NTQ temperatures; however, their emission energies are difficult to regulate via molecular modifications. 14,15 Copper(I) complexes have attracted much attention in the past decade, mainly because of the interest in their thermally activated delayed fluorescence (TADF).…”

“…To investigate the TD electronic structural transition at high temperatures, the Raman spectra and solid-state 13 C NMR spectra were obtained from RT to 500 K (Fig. 4 and ESI, † Fig.…”

“…, far below room temperature (RT), whereas high-temperature NTQ is most desirable for practical applications because the temperature generated from the Joule effect in gallium nitride LEDs can exceed 100 °C, at which the PL will be almost quenched for traditional phosphors via the non-radiative relaxation of excited electrons to the ground state. 13 By contrast, the emission from inorganic solid phosphors benefits from high NTQ temperatures; however, their emission energies are difficult to regulate via molecular modifications. 14,15…”