Humidity Sensing through Reversible Isomerization of a Covalent Organic Framework

Area of Science:

• Materials Science
• Supramolecular Chemistry
• Chemical Sensing

Background:

• Covalent organic frameworks (COFs) are crystalline porous polymers with tunable properties.
• Developing sensitive and selective chemical sensors remains a key challenge in materials science.
• Understanding the relationship between molecular structure and optical properties is crucial for sensor design.

Purpose of the Study:

• To investigate the color-changing properties of a specific covalent organic framework (COF) containing diiminol moieties.
• To explore the mechanism behind the observed color changes and their reversibility.
• To evaluate the potential of this COF as a colorimetric sensor for solvents and humidity.

Main Methods:

• Synthesis of a covalent organic framework (COF) with 2,5-di(imine)-substituted 1,4-dihydroxybenzene (diiminol) units.
• Spectroscopic analysis (UV-Vis absorption) to monitor color changes in the presence of various solvents and solvent vapors.
• Density Functional Theory (DFT) and time-dependent DFT calculations to elucidate the electronic structure and tautomerization mechanism.
• Fabrication and testing of a colorimetric humidity sensing device using an oriented thin film of the COF.

Main Results:

• The COF exhibited rapid, passive, and reversible color changes detectable by the naked eye upon exposure to polar solvents, particularly water.
• A new visible absorption band emerged in the presence of polar solvents, indicating a reversible conversion.
• DFT calculations revealed that solvent-induced tautomerization between diiminol and iminol/cis-ketoenamine forms is responsible for the optical changes.
• The iminol/cis-ketoenamine tautomer exhibits longer wavelength absorption with significant charge-transfer character compared to the diiminol.
• A COF-based humidity sensor demonstrated quick response to water vapor and maintained stability for several months.

Conclusions:

• Tautomerization-induced electronic structure changes in COFs can be effectively utilized for sensing applications.
• The reported COF offers a promising platform for developing rapid, reversible, and long-term stable colorimetric sensors, especially for humidity detection.
• The facile and visible color change mechanism highlights the potential of exploiting dynamic covalent chemistry in functional materials.