Geomimetic Hydrothermal Synthesis of Polyimide‐Based Covalent Organic Frameworks

Abstract

Despite its abundance, water is not widely used as a medium for organic reactions. However, under geothermal conditions, water exhibits unique physicochemical properties, such as viscosity and a dielectric constant, and the ionic product become similar to those of common organic solvents. We have synthesized highly crystalline polyimide‐based covalent organic frameworks (PICs) under geomimetic hydrothermal conditions. By exploiting triphenylene‐2,3,6,7,10,11‐hexacarboxylic acid in combination with various aromatic diamines, PICs with various pore dimensions and crystallinities were synthesized. XRD, FT‐IR, and DFT calculations revealed that the solubility of the oligomeric intermediates under hydrothermal conditions affected the stacking structures of the crystalline PICs. Furthermore, the synthesized PICs demonstrate promising potential as an anode material in lithium‐ion batteries owing to its unique redox‐active properties and high surface area.

Direct and Linker-Exchange Alcohol-Assisted Hydrothermal Synthesis of Imide-Linked Covalent Organic Frameworks

“…(2) While in the supernatant of the alcohol containing reaction mixture imide and amic acid species could be detected, the supernatant of the water reaction mixture contained predominantly deprotonated pyromellitic acid and protonated amine linker molecules (Figures S7–S10). This indicates the formation of a monomer-salt intermediate in the COF formation reaction prior to polymerization, by protonation of the amine linker by the hydrolyzed PMDA as described previously. , We observed that this monomer-salt intermediate (Figure S11) dissolves in water and also in the water/n-alcohol mixtures during the heating process, and amorphous PI begins to precipitate thereafter. Thus, while water could serve as a good solvent for PMDA and the intermediate salt, enabling the primary diffuse polymerization, and enhances the reversibility of the equilibrium reaction (Le Chatelier’s principle), the alcohol could contribute to enhancing the solubility of the amine precursor, enabling exchange of the amine linker molecule from the precipitated amorphous network during the defect healing processes.…”

“…Interestingly, the imide polymerization itself seemed to occur in the presence of water as evident from the typical imide vibrational bands in the fourier transform infrared (FT-IR) spectra of the precipitate (Figure S1). We thus reasoned that crystallization may either be prevented by a lack of solubility of the amine precursor molecules and imide oligomers in water, hindering the growth of well oriented COF sheets, or the lack of polarity differences in the reaction mixture. Indeed, most COFs have been reported to form crystalline structures in two-solvent mixtures of varying polarity, e.g., mesitylene (ε = 2.4) and NMP (ε = 32) for imide-linked COFs .…”

“…In addition, the less polar imide dimers and oligomers, detected in the supernatant of the alcohol-containing reaction, are possibly kept longer in solution due to a surfactant-like behavior of the longer chain n-alcohols which can interact with the less polar benzylic groups of the oligomers in a manner similar to that observed in surfactant-based MOF formation . This could explain the requirement of progressively smaller alcohol concentrations as one moves from n-propanol to n-hexanol and provides a great advantage over the pure hydrothermal synthesis approach, circumventing the need for water-soluble linker molecules and oligomers to achieve high quality COFs …”

“…Unterlass and co-workers demonstrated that polycondensation under hydrothermal conditions can lead to crystalline 1D polyimides (PI). − Although this method works well for 1D PIs, the adaptation of this hydrothermal synthetic strategy for 2D COFs is rather challenging. Following the work of Unterlass and co-workers, Kim and co-workers recently reported the synthesis of PIC-Ph COF under hydrothermal conditions . However, its applicability in synthesizing crystalline, high surface area COFs is restricted to the p -phenylenediamine linker, a water-soluble linker molecule.…”

“…Following the work of Unterlass and co-workers, Kim and co-workers recently reported the synthesis of PIC-Ph COF under hydrothermal conditions. 23 However, its applicability in synthesizing crystalline, high surface area COFs is restricted to the pphenylenediamine linker, a water-soluble linker molecule. Using less water-soluble linker molecules such as 4,4′diaminobiphenyl or 4,4′-diamino-p-terphenyl results in a limited long-range growth of the 2D structure and therefore in COFs with very low surface areas.…”