Water-stable boroxine structure with dynamic covalent bonds

Abstract

Boroxines are significant structures in the synthesis and construction of biological-active compounds, covalent organic frameworks, anion receptors, self-healing materials, and others. However, their utilization in aqueous media is a formidable task due to hydrolytic instability. Here we report a water-stable boroxine structure discovered from 2‑hydroxyphenylboronic acid (HO‑PBA). We find that HO‑PBA exists as a dimer with dynamic covalent bonds (DCBs) and aggregation-induced enhanced emission activity, instead of a monomer. Intriguingly, upon exposure to water, HO‑PBA dimer rapidly transforms into a boroxine structure with excellent pH stability and water-compatible DCBs. Building upon these discoveries, we report the ultra-strong binding capacity of boroxines toward fluoride ions in aqueous media, and develop a boroxine-based hydrogel with high acid–base stability and reversible gel–sol transition. This discovery of the water-stable boroxine structure breaks the constraint of boroxines not being applicable in aqueous environments, opening a new era of researches in boroxine chemistry.