Mark Olson

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H‐bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG‐DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non‐emissive building blocks, the H‐bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid‐state optical hydrochromism in response to relative humidity and temperature.