Inverse Gas Chromatography Demonstrates the Crystallinity-Dependent Physicochemical Properties of Two-Dimensional Covalent Organic Framework Stationary Phases

Abstract

Two-dimensional polymers (2DPs), in the form of layered 2D covalent organic frameworks (COFs), are promising candidates for adsorbent-based separations because their pore sizes, shapes, functionalities, and interlayer stacking arrangements can be tuned by modifying their building blocks. Recently, high-quality single crystals of two 2D COFs exhibited distinct and improved separation characteristics in the gas chromatography (GC) separation of benzene and cyclohexane relative to polycrystalline samples of the same materials. These surprising findings motivate the present study, in which inverse pulse gas chromatography (IGC) was used to characterize the dispersive and specific adsorption properties of the surfaces of single-crystalline and polycrystalline TAPPy-PDA COFs for the separation of linear n-alkanes as well as a series of standard polar probes. Major differences in separation behavior were again observed that provide insight into how analytes interact with the single-crystalline and polycrystalline 2D COFs. A polarity study based on McReynolds constants revealed the nonpolar nature of the single-crystalline TAPPy-PDA COF, whereas the polycrystalline TAPPy-PDA COF surface was found to have a slightly polar character. Three common approaches to calculating the specific interaction parameter, I sp, were tested to examine their validity in the context of 2D COFs, revealing that the single-crystalline TAPPy-PDA COF possessed an electron donor character that we attribute to the imine nitrogen atoms inside the well-defined pore channels. In contrast, the polycrystalline TAPPy-PDA COF showed a relative electron acceptor character, which may be more heavily influenced by interactions between the analytes and dangling bonds or functionalities at grain boundaries. These findings provide a quantitative comparison of 2D COF materials quality by determining the acid–base interactions (represented by the electron donor–acceptor properties), polarity, and other physiochemical parameters. Furthermore, these results indicate the importance of establishing high materials quality for 2D COF samples prior to establishing rigorous structure–property relationships for separation performance.

Some Irregularities in the Evaluation of Surface Parameters of Solid Materials by Inverse Gas Chromatography

“…The results obtained by these authors − ,,,,− cannot be considered as confident results, because of the following reasons:…”

“…Many authors − ,,,,− recently studied the London dispersive energy and the Lewis’s acid–base properties of some solid surfaces by using IGC technique and the classical methods of Dorris–Gray and Schultz et al that were proved to give wrong results. Furthermore, in their study they used constant values of the surface areas of organic solvents and also supposed as constant the dispersive component of the surface tension of the various organic molecules. − ,− …”

Wavelength-Dependent Excitonic Properties of Covalent Organic Frameworks Explored by Theory and Experiments

“…For instance, the VB 1 −CB transition of N2N3 has a significant local character at the bipyridine bridge (row 2, column 2). However, there is also a significant electron correlation between the imine ES with the phenylene GS (1,3) and the bipyridine ES with the phenylene GS (2, 3), indicative of CT-like behavior. Note that the lower triangular correlations are larger than their transposed correlations between the phenylene ES with the imine GS (3, 1) and the phenylene ES with the bipyridine GS (3,2), which indicates that the CT is asymmetric ands support where neesfer (ET) toward bipyridine.…”

“…However, there is also a significant electron correlation between the imine ES with the phenylene GS (1,3) and the bipyridine ES with the phenylene GS (2, 3), indicative of CT-like behavior. Note that the lower triangular correlations are larger than their transposed correlations between the phenylene ES with the imine GS (3, 1) and the phenylene ES with the bipyridine GS (3,2), which indicates that the CT is asymmetric ands support where neesfer (ET) toward bipyridine. 47 Compared to VB 1 −CB, the VB 2 −CB transition exhibits more LE density at imine (1, 1) and less phenylene/ imine and phenylene/bipyridine correlations, explaining its lower CTC and smaller exciton size.…”

“…Covalent organic frameworks (COFs) are a promising class of polymeric materials with future uses across a wide range of applications such as gas storage, chromatography, drug delivery, and catalysis . Their versatility is the result of ordered structures that form into predictable, porous topologies defined by the shape and size of their monomeric units.…”