Development of Ester-Based Covalent Organic Framework Film Optical Waveguides for Their Competitive SO2 Gas Sensing Application

Ester-based covalent organic frameworks (COFs) demonstrate improved sensing stability and enhanced dipole-dipole (μ-μ) interactions for competitive SO2 sensing upon various acidic gases. An ester-based COF, referred to as [H4bptc-TOPT]n-COF, was synthesized using biphenyl-3,3',5,5'-tetracarboxylic acid (H4bptc) monomer and 4,4',4″-(1,3,5-triazine-2,4,6-triyl) triphenol (TOPT) linker through a one-pot solvothermal method. The resulting COF films were optically transparent and exhibited an irregular hexagonal mesoporous architecture and a high refractive index (from 1.7247 to1.9073), which supports strong single-mode (TE0) guided light, meeting the essential criteria for an optical waveguide sensor (OWGS). As an OWGS, the [H4bptc-TOPT]n-COF film demonstrated high efficiency in SO2 sensing upon various acidic gases. The spectral analysis and density-functional theory computations indicated that the ester group and benzene ring in [H4bptc-TOPT]n-COF contribute to SO2 sensing through μ-μ interactions, which are associated with increased adsorption energy, elongated ester bond length, and reduced bond angles, leading to an increase in the refractive index, adsorption coefficient, and almost reversible morphological change of the [H4bptc-TOPT]n-COF. The presented OWGS features a fast response (tres = 2 s), a wide detection range (100 ppm to 10 ppb), a low detection limit (2.5 ppb), and excellent resistance to interference at 293 K despite its weak immunity to humidity. Furthermore, SO2 adsorption weight of 17.18 ng·cm-2 was determined on per unit area of the [H4bptc-TOPT]n-COF film surface upon μ-μ interactions.