Divergent Properties in Structural Isomers of Triphenylamine-Based Covalent Organic Frameworks

Covalent organic frameworks (COFs) are a class of crystalline and porous materials with tunable pore size, shape, and framework functionalities. More importantly, COFs can be designed to fulfill targeted applications, and therefore, the past decades have witnessed considerable efforts from scientists to formulate and functionalize COFs for desired properties. To archive that goal, a thorough understanding of COFs’ structures and their properties is essential. This work provides a novel perspective on the relationship between COFs’ structures and their inherent properties through the study of structural isomers in COFs. Structural isomers are commonly known in general and organic chemistry, yet a detailed study with direct comparison between COF structural isomers still hasn’t been accomplished. In this research, two imine-based COF isomers, TAPA-PDA COF and IISERP-COF2, were extensively studied and compared with respect to their synthetic conditions, framework properties, phase reversibility, optical properties, and surface energy. TAPA-PDA COF is an imine-based COF formed from the solvothermal reaction between tris(4-aminophenyl)amine (TAPA) and terephthalaldehyde, while its structural isomer IISERP-COF2 can be synthesized from tris(4-formylphenyl)amine and 1,4-diaminobenzene. Experimental data reveal that these COF structural isomers can have varied properties despite minor differences in their structures. Specifically, while TAPA-PDA COF can be made from mild solvothermal reaction at 70 °C, IISERP-COF2 requires a harsher synthesis condition with a higher reaction temperature and longer duration. Both TAPA-PDA COF and IISERP-COF2 exist in two different phases: either in solution or an activated, dry form. While the phase transition is reversible for TAPA-PDA COF, it is only partial for that of IISERP-COF2. Lastly, their framework properties, optical properties including UV-Vis absorption and fluorescence, solvatochromic behavior, and surface energy are also significantly different. In fact, this work provides another factor to consider when engineering COF materials for desired applications besides alternating pore size and geometry.