Evolution of π Electronic Structure and Luminescence Function by Introducing Imidazole Functional Groups in Covalent Organic Frameworks

The integration of functional groups with enhanced π-electron conjugation into covalent organic frameworks (COFs) has been reported to significantly influence their fluorescence characteristics. In this study, we synthesized covalent organic framework (COF) powders incorporating imidazole functional groups, which feature regular pore structures and imidazole side groups. We systematically investigated the incorporation of imidazole functional groups─known for their significant conjugation effects─into the COF framework and explored their luminescence behaviors in various solvents. Our research revealed that the fluorescence behavior of these COFs exhibits a pronounced dependence on solvent polarity. Specifically, exposure to solvents of different polarities induces systematic changes in the luminescence properties of the material. Theoretical simulations, in conjunction with experimental observations, indicated that the enhanced conjugation effect resulting from the incorporation of imidazole groups leads to a redshift in the fluorescence emission wavelength. This redshift is primarily attributed to the expansion of π-π orbitals within the molecular structure and the consequent reduction in the energy gap between excited states. These findings provide valuable theoretical guidance for the rational design of efficient optoelectronic devices and sensors based on COF materials, highlighting the potential for tuning luminescence properties of COF materials through molecular engineering.