Covalent Organic Frameworks: Linkage Chemistry and Its Critical Role in The Evolution of Electronic Structures and Functions

Covalent organic frameworks (COFs) provide a molecular platform for designing a novel class of functional materials with well-defined structures. A crucial structural parameter is the linkage, which dictates how knot and linker units are connected to form two-dimensional polymers and layer frameworks, shaping ordered pi-array and porous architectures. However, the roles of linkage in the development of ordered pi electronic structures and functions remain fundamental yet unresolved issues. Here we report the designed synthesis of COFs featuring four representative linkages: hydrazone, imine, azine, and C=C bonds, to elucidate their impacts on the evolution of pi electronic structures and functions. Our observations revealed that the hydrazone linkage provides a non-conjugated connection, while imine and azine allow partial pi conjugation, and the C=C bond permits full pi-conjugation. Importantly, the linkage profoundly influences the control of pi electronic structures and functions, unraveling its pivotal role in determining key electronic properties such as band gap, frontier energy levels, light absorption, luminescence, carrier density and mobility, and magnetic permeability. These findings highlight the significance of linkage chemistry in COFs and offer a general and transformative guidance for designing framework materials to achieve electronic functions.