Piezochromism in Dynamic Three-Dimensional Covalent Organic Frameworks

Piezochromic materials with pressure-dependent photoluminescence-tuning properties are important in many fields, such as mechanical sensors, security papers, and storage devices. Covalent organic frameworks (COFs), as an emerging class of crystalline porous materials (CPMs), feature structural dynamics and tunable photophysical properties, which are suitable for designing piezochromic materials, but related research is scarce. Herein, we report two dynamic three-dimensional COFs based on aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ) chromophores, termed JUC-635 and JUC-636, and for the first time, study their piezochromic behavior by diamond anvil cell technique. Due to the various luminescent groups, JUC-635 has completely different solvatochromism and molecular aggregation behavior in the solvents. More importantly, JUC-635 with AIE effect exhibited a maintained fluorescence with increasing pressure (~3 GPa), and a reversible sensitivity with high-contrast emission differences (Δλem = 187 nm) up to 12 GPa, which is superior to other CPMs reported so far. Therefore, this research will open a new gate for expanding the potential applications of COFs as exceptional piezochromic materials in pressure sensing, barcoding, and signal switching.