Organic covalent frameworks with reactive site side chains: synthesis, fluorescence effects and solvent-based colour development

In this paper, two covalent organic polymers (COPs) materials with different frameworks, COP-3N and COP-CN, with high heat resistance and obvious fluorescence effects, were synthesised using 3-(methylamino )-[1,1‘-biphenyl]-4,4’-dicarbaldehyde (DAH) and different phenyl monomers. The two COPs have a unique solvent-based chromogenic effect, and when dispersed in solvents with different polarities, the fluorescence spectra of the two COPs are significantly blue-shifted from the yellow fluorescence of the solid state of about 560 nm to the blue fluorescence of 460 nm or even the ultraviolet light of about 356 nm. Under the excitation of the most common 365 nm UV lamp, COP-3N shows prolonged yellow, cyan and blue luminescence in different solvents, which can be used as a simple raw material for encryption of handwritten messages.The amine active hydrogen side group brought by the introduction of DAH to COP-CN and COP-3N makes it a unique reactive site, which provides a pathway for COP to link to other functional groups or small molecules in the future.