Manipulating Large Luminescent Shift from Red to Near-Infrared by Pressure via Charge-Transfer States in Covalent Organic Frameworks
Near-infrared piezochromic materials exhibiting luminescence responses are critical for mechanical sensors and storage devices. Covalent organic frameworks (COFs), as crystalline porous materials, combine structural adaptability with tunable photophysical properties, yet their piezochromic applications remain underexplored. Here, we report a series of donor-acceptor structured two-dimensional COFs (2D COFs) with bright red emission, all showing pronounced red-shifts spanning red to near-infrared regions. Notably, Py-BO-COF exhibits the largest piezochromic shift of 187 nm with a high sensitivity of 44.52 nm GPa−1, significantly surpassing Py-BT-COF, TPE-BO-COF, and most reported COF/MOF systems. Py-BO-COF also demonstrates fully reversible and repeatable emission switching over multiple cycles, maintaining excellent linearity without degradation. In situ spectroscopic analyses and theoretical simulations reveal that variations in piezochromic rates arise from differences in charge-transfer (CT) processes, while the pronounced red-shift in Py-BO-COF is associated with reduced interlayer distance and enhanced coplanarity. This study systematically establishes the structure-property relationship in piezochromic 2D COFs, offering strategic guidance for designing highly sensitive and reversible pressure-responsive materials, thereby advancing smart piezochromic systems.