Since their initial report, research efforts into covalent organic frameworks (COFs) have resulted in a vast library of structures and linkages that can form crystalline polymeric materials. These materials have in turn demonstrated utility in applications such as gas storage and separation, electrochromics, flexible supercapacitors and solid electrolytes. While there have been a number of prominent electronic and sensor devices utilizing COFs, very little of these reports discuss the selection of monomer pairings. Not only are monomer pairings important for sensor and electronic properties but they play a significant role in material properties and synthesis conditions. To gain a better understanding of the relationship between pairings we focused on switching the chemical moieties of monomers in an imine based COF from aldehyde to amine and documented the divergent properties. The subsequent porosity, phase reversibility, optical properties and surface energy of resulting imine-based COFs were then compared. Our findings show that the optical properties of these donor-acceptor COFs are quite different. This is exemplified by the fact that one of the monomer pair COFs, TAPA-PDA COF, displays a maximum emission at 625 nm, while its monomer pair analog exhibits a maximum emission at 532 nm. These differences clearly point to significant differences in electron density and is further illustrated by their solvachromatic response to solvents of increasing. Further characterization of these materials documents the differences in synthesis conditions, phase reversibility and hydrophobicity/hydrophilicity. These findings provide evidence that more than monomer geometry should be considered when constructing novel organic materials.

Exploring divergent properties of structural isomer covalent organic frameworks

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