Design and prompt synthesis of a hydrazone-linked covalent organic framework with binding pockets for lanthanides: luminescent pH and ratiometric temperature sensing

Abstract

Focusing on the exceptional traits of the highly functionalized, flexible, and yet infrequent hydrazone linkages, a new covalent organic framework, namely BTD, is synthesized by a fast crystallization technique within 1.5 h. Extensive characterization by various spectroscopic and microscopic techniques, powder X-ray diffraction, and computational simulation confirms its purity, structural features, and porous nature. The framework is locked due to the presence of secondary interactions, such as intralayer (N⋯H–O and C–H⋯OC) and interlayer (N–H⋯O) H-bonding as well as π–π stacking, which provide in-plane rigidness in the staggered AB-type stacking of layers. These strong interactions can be a possible reason for the prompt crystallization of BTD. The dual proton donor and acceptor sites offered by the functionalized hydrazone linkage in BTD lead to the wide-range, colorimetric, luminescent detection of hydrogen ion concentrations. Its luminescence is highest at pH 4 but quenched in a strong basic medium (pH 13). Furthermore, utilizing the characteristics of hydrazone connectivity and anchored functional groups (carbonyl and hydroxy), trivalent lanthanides are incorporated onto the decorated pockets, yielding Tb@BTD and Eu@BTD. Utilizing the remarkable thermal detection features of lanthanide-decorated COFs, ratiometric temperature detection studies were performed in aqueous and methanolic slurries. The computed relative sensitivity values for Tb@BTD and Eu@BTD are 1.404% K⁻¹ and 1.118% K⁻¹ at 333 K in water and 2.73% K⁻¹ and 2.29% K⁻¹ at 313 K in methanol, respectively. For demonstrating the utility of both COFs in real environmental conditions, the change in luminescence is studied in the temperature range of 308–320 K. With a quenching of 22%, Eu@BTD proves to be a better candidate for effective luminescent thermometric applications under physiological conditions.