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 a highly functionalized, flexible, and yet infrequent hydrazone linkages, a new covalent organic framework, namely BTD, is synthesized by the 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···O=C), interlayer (N-H···O) H-bonding as well as π-π stacking, which provides in-plane rigidness in the staggered AB type stacking of layers. These strong interactions can be the possible reason for the prompt crystallization of BTD. The dual proton donor and acceptor sites offered by functionalized hydrazone linkage in BTD lead to the wide-range, colorimetric, luminescent detection of hydrogen ion concentration. Its luminescence is highest at pH 4 but quenched in strong basic medium (pH 13). Furthermore, to embark upon the characteristics of hydrazone connectivity and anchored functional groups (carbonyl and hydroxy), the trivalent lanthanides are incorporated onto the decorated pockets yielding Tb@BTD and Eu@BTD. Utilizing the remarkable thermal detection features of lanthanide-decorated COFs, the ratiometric temperature detection studies are performed in the 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 noting 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 to work for effective luminescent thermometric applications under physiological conditions.