Manipulating large luminescent shift from red to near-infrared by pressure via charge-transfer states in COFs

Abstract

Near-infrared piezochromic materials exhibiting luminescence responses are important in many fields, such as mechanical sensors and storage devices. Covalent organic frameworks (COFs), as an emerging subclass of crystalline porous materials, combine structural adaptability with tunable photophysical properties, making them highly promising for piezochromic applications. However, research in this specific area remains notably limited. Herein, a series of donor-acceptor structured two-dimensional covalent organic frameworks (2D COFs) exhibiting bright red emission was successfully synthesized, all of which demonstrate a pronounced red-shift spanning the red and near-infrared regions. Notably, Py-BO-COF shows the largest piezochromic shift of 187 nm with a high sensitivity of 44.52 nm GPa⁻¹, significantly exceeding that of Py-BT-COF, TPE-BO-COF, and most reported COF/MOF systems. Remarkably, Py-BO-COF exhibits fully reversible and repeatable emission switching over multiple cycles, consistently maintaining excellent linearity without degradation. This combination of high sensitivity and outstanding reversibility positions Py-BO-COF as a promising candidate for high-performance piezochromic materials. In situ spectroscopic analyses and theoretical simulations further reveal that the variation in piezochromic rates among the COFs arises from differences in charge-transfer (CT) processes, while the pronounced red-shifts 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 the field of smart piezochromic systems.