Structural and Optical Modulations in Covalent Organic Frameworks via Pressure-treated Engineering

Piezochromic materials, exhibiting outstanding pressure-responsive performance including pressure-induced emission en-hancement (PIEE), blue-shifted emission, white-light emitting and so on, are crucial in various fields such as in advanced sensing and smart photonics. Covalent organic frameworks (COFs), an emerging class of crystalline porous materials (CPMs), possess dynamic structures and adjustable photophysical properties, making them promising candidates for the development of piezochromic materials. Nevertheless, this area remains relatively understudied. In this study, a successful synthesis of a series of bicarbazole-based COFs with varying topologies, dimensions, and linkages was conducted, followed by an investigation of their structural and optical properties under hydrostatic pressure generated by a diamond anvil cell. Consequently, these COFs exhibited distinct piezochromic behaviors, particularly an astonishing PIEE phenomenon with a 16-fold increase in fluorescence intensity from three-dimensional COFs, surpassing the performance of CPMs and nearly all known organic small molecules with PIEE behavior. In contrast, two-dimensional COFs, with rigid and conjugated structures, typically showed red-shifted and reduced emission, while the flexible variants exhibited rare blue-shifted emission. Mecha-nism research further revealed that these different piezochromic behaviors were primarily determined by interlayer distance and interaction. This study represents the first systematic exploration of the structures and optical properties of COFs through pressure-treated engineering and provides a new perspective on the design of piezochromic materials.