Dynamic covalent chemistry concept toward preparing crystalline covalent organic frameworks for dual-mode acidochromic responses

Covalent organic frameworks (COFs), as an emerging class of porous materials, have recently received increasing interest because of their diverse and designable structure. Developing structural stable COFs is still a long-term goal for practical application. Herein, a chemical agent-induced dynamic covalent chemistry (DCC) regrowth strategy was proposed to synthesize urchin-like imine-linked COF with high crystallinity and chemical stability, evidenced by X-ray diffraction spectroscopy, high-resolution transmission electron microscopy, selected area electron diffraction, and thermal gravimetric analyzer. Additionally, the reversible protonation of the imine nitrogen in the framework endowed the COF with excellent water dispersibility, which can readily form a stable solution system. The corresponded changes of the conjugated π electronic structure led to characteristic colorimetric and fluorescent acidochromic responses over a wide pH range without COF decomposition. The drastically increased structural stability provides useful insights into the design and synthesis of functional COFs for practical application.