A Dihydroxyterephthalic Acid-based Fluorescent Hydrogen-Bonded Organic Framework as a Multifunctional Platform for Acid Vapor Detection, pH Sensing and Proton Conductivity

Abstract

Hydrogen-bonded organic frameworks (HOFs) have gained significant research attention for their potential in addressing different environmental challenges such as pollution control and sustainable energy generation. In the present study, we have developed an ionic 3D HOF, viz. DHT-MEL, by combining 2,5-dihydroxyterephthalic acid (DHT) and melamine (MEL) as the molecular building blocks. The inherent photoluminescence of the HOF arising from fluorescent DHT moiety, coupled with the close proximity of the acid-responsive MEL unit, enables the material with highly efficient acid sensing behavior. The HOF undergoes a distinct reversible and visible change of fluorescence emission from yellow to blue upon exposure to acids. The acid sensing property works for several inorganic and organic acids in their liquid and vapor phases and was further extended for pH sensing of aqueous solutions. The HOF shows pH sensing capability at different acidic pHs with distinct emission colors: bright blue at pH <2, green in the pH range 2-3, greenish-yellow at pH~4 and yellow at pH >4, which is unprecedented. The sensing experiments can be performed by using HOF-coated paper strips that require only a few micrograms of the material and the testing strips can be reused. Furthermore, the HOF shows good proton conductivity owing to dense H-bonded network. A proton conductivity value of 4.74 × 10 -3 S cm -1 at 57 ℃ and 98% RH was achieved, which is comparable to many reported representative crystalline framework materials. This work establishes a basis for developing multifunctional stable HOFs that combine photoluminescent sensing and proton-conducting properties.