Perylene-Based Covalent Organic Frameworks for Acid Vapor Sensing

Abstract

Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have not attracted much attention as functional components of the final material. We have developed a new series of dual-pore perylene-based COFs and demonstrate that their imine bonds can be protonated reversibly, causing significant protonation-induced colour shifts towards the nearinfrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapour sensors with a detection limit as low as 35 µg L-1 and a response range of at least four orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of non-aqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as functiondetermining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their possible applications.

Acid Exfoliation of Imine‐linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

“…The powder's color also changed from bright orange to red upon acid addition, which is consistent with protonation of the imine linkages. Analogous color changes were previously observed upon protonation of imine‐linked macrocycles and COFs . The exfoliated solids were isolated by centrifugation, washed, and dried under vacuum overnight prior to characterization.…”

“…The resulting film (400 nm thick) was imaged by AFM, which revealed an array of discrete circular particles piled into an extended structure, as expected for an assembly of 2D polymer sheets (Figure A). A portion of the COF film was submerged in reagent alcohol (90 % ethanol, 5 % methanol, 5 % isopropanol) immediately after drying, and its color rapidly changed from red to yellow, which we attribute to the deprotonation of its iminium ions back to neutral imines . Notably, the film also delaminated from the substrate, providing a freestanding COF film (Figure B) that could be transferred to any desired support.…”

“…Notably, when the exfoliated TAPB‐PDA and Methyl COF solids were isolated, purified, and activated by supercritical CO 2 drying, high surface areas (Figures S15, S28) and crystallinity (Figures S13, S26) were still observed. In these cases, the iminium ions were deprotonated by large volumes of methanol during the workup, as indicated by a color change from red to bright yellow . HRTEM images of exfoliated TAPB‐PDA and Methyl COF particles both contained lattice fringes with spacings of approximately 32 Å, which corresponds to d 100 of both structures (Figures S21, S34).…”

Acid Exfoliation of Imine-linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films

“…Aportion of the COF film was submerged in reagent alcohol (90 %e thanol, 5% methanol, 5% isopropanol) immediately after drying, and its color rapidly changed from red to yellow,w hich we attribute to the deprotonation of its iminium ions back to neutral imines. [41,42] Notably,t he film also delaminated from…”

Ultrahigh Responsivity Photodetectors of 2D Covalent Organic Frameworks Integrated on Graphene

“…In addition to optimizing the combination of π‐electron monomers, COF‐graphene photodetectors can be further regulated by the external environment . COFs have been proven to be ideal materials for gas adsorption due to their highly porous structure with high surface areas.…”