Triptycene-derived heterocalixarene: A new type of macrocycle-based stationary phases for gas chromatography

Triptycene-like naphthopleiadene as a readily accessible scaffold for supramolecular and materials chemistry

Triptycene derivatives are used extensively in supramolecular and materials chemistry, however, most are prepared using a multi-step synthesis involving the generation of a benzyne intermediate, which hinders production on a large scale. Inspired by the ease of the synthesis of resorcinarenes, we report the rapid and efficient preparation of triptycene-like 1,6,2',7'-tetrahydroxynaphthopleiadene directly from 2,7-dihydroxynaphthalene and phthalaldehyde. Structural characterisation confirms the novel bridged bicyclic framework, within which the planes of the single benzene ring and two naphthalene units are fixed at an angle of ∼120° relative to each other. Other combinations of aromatic 1,2-dialdehydes and 2,7-disubstituted naphthalenes also provided similar triptycene-like products. The low cost of the precursors and undemanding reaction conditions allow for rapid multigram synthesis of 1,6,2',7'-tetrahydroxynaphthopleiadene, which is shown to be a useful precursor for making the parent naphthopleiadene hydrocarbon. The great potential for the use of the naphthopleiadene scaffold in supramolecular and polymer chemistry is demonstrated by the preparation of a rigid novel cavitand, a microporous network polymer, and a solution-processable polymer of intrinsic microporosity.

Increasing Mass Transfer Resistance of MOFs as a Reverse Tuning Strategy to Achieve High-Resolution Gas Chromatographic Separation

In separation science, precise control and regulation of the MOF stationary phase are crucial for achieving a high separation performance. We supposed that increasing the mass transfer resistance of MOFs with excessive porosity to achieve a moderate mass transfer resistance of the analytes is the key to conducting the MOF stationary phase with a high resolution. Three-dimensional UiO-67 (UiO-67-3D) and two-dimensional UiO-67 (UiO-67-2D) were chosen to validate this strategy. Compared with UiO-67-3D with overfast mass transfer and low retention, the reduced porosity of UiO-67-2D increased the mass transfer resistance of analytes in reverse, resulting in improved separation performance. Kinetic diffusion experiments were conducted to verify the difference in mass transfer resistance of the analytes between UiO-67-3D and UiO-67-2D. In addition, the optimization of the UiO-67-2D thickness for separation revealed that a moderate diffusion length of the analytes is more advantageous in achieving the equilibrium of absorption and desorption.

Investigation of the chiral recognition role of cyclodextrin hydroxyl moieties via high performance liquid chromatography

Cyclodextrin (CD) is known to afford excellent enantioselectivities due to its hydrophobic cavity and external H-bonding sites from hydroxyl moieties. However, there is still a lack of direct and comprehensive evidence clearly illustrating the origin of the important H-bonding effect. Regarding this issue, herein, four allylimidazole CD derivatives by selective substitution of the primary (6-position) and/or secondary (2,3-position) CD were synthesized and clicked onto silica surfaces to afford the corresponding chiral stationary phases (CSPs). The chiral chromatographic performances were systematically evaluated by separating 35 racemic analytes including isoxazolines, dansyl-amino acids, flavonoids and other racemates under reversed-phase HPLC. The chiral selection factors (α) and retention times (k) of the analytes on the as-prepared CSPs were comprehensively compared and it reveals that the enantioseparation ability was significantly altered due to the selective substituents of CD hydroxyl groups. The natural allylimidazole CD CSP (AICDCSP) was superior to the 6-O-tert-butyldimethylsilyl AICDCSP (6-TBDMAICDCSP) for most analytes. Dansyl amino acids and Ar-Pys were well separated on AICDCSP and 6-TBDMAICDCSP, where dansyl amino leucine gained the highest resolution up to 4.72 on AICDCSP, and flavonoids and Ar-Oprs were only separated on AICDCSP. These interesting separation results demonstrate that the secondary hydroxyl groups play a pivotal role in the separation of chiral compounds. In addition, the size of the CD cavity and the choice of solute also have an effect on the separation of substances. The mechanism involved in enantioselective discrimination of the selectively substituted CDs was further investigated by the molecular docking simulation.

Recent development of chiral ionic liquids for enantioseparation in liquid chromatography and capillary electrophoresis: A review

Ionic liquids (ILs), which are salts in a molten state below 100 °C, have become a hot topic of research in various fields because of their negligible vapour pressure, high thermal stability, and tunable viscosity. Chiral ionic liquids (CILs) can be applied in chromatography and capillary electrophoresis fields to improve the performance of enantiomeric separation, such as chiral stationary phases (CSPs) and mobile phase additives in high-performance liquid chromatography (HPLC); CSPs in gas chromatography (GC); and background electrolyte additives (BGE), chiral ligands and chiral selectors (CSs) in capillary electrophoresis (CE). This review focuses on the applications of CILs in HPLC and CE for the separation of enantiomers in the past five years. The mechanism for separating enantiomers was explained, and the prospect of the application of CILs in chiral liquid chromatography (LC) and CE analysis was also discussed.

High-Resolution Performance of Polycaprolactone Functionalized with Guanidinium Ionic Liquid for Gas Chromatography

This work firstly reported a new polycaprolactone based material functionalized with guanidinium ionic liquid (PCL-GIL) as the stationary phase with high resolution performance for capillary gas chromatography (GC). It is composed of polycaprolactone (PCL) and guanidinium ionic liquid (GIL) with amphiphilic conformation. The PCL-GIL capillary column coated by static method exhibited high column efficiency of 3942 plates/m and moderate polarity. As a result, the PCL-GIL column exhibited high-resolution capability. For a mixture of 27 analytes with a wide ranging polarity and outperformed the PCL-2OH and HP-35 columns, showing its advantageous separation capability for analytes of diverse types. Moreover, the PCL-GIL column showed high resolving capability for various positional isomers and cis-/trans-isomers, including alkylbenzenes, chlorobenzenes, naphthalenes, bromonitrobenzenes, chloronitrobenzenes, benzaldehydes, phenols, alcohols, respectively. In a word, PCL derivatized by GIL units as a new type of stationary phase has a promising future in GC separations.

Preparation of chiral stationary phase based on a [3+3] chiral polyimine macrocycle by thiol-ene click chemistry for enantioseparation in normal-phase and reversed-phase high performance liquid chromatography

Polyimine macrocycles are a new class of organic macrocycles with cyclic structures, well-defined molecular cavities, and multiple cooperative binding sites, which have recently aroused considerable research interest in molecular recognition and separation. Herein, we report the bonding of a [3+3] chiral polyimine macrocycle (H₃L, C₇₈H₇₈N₆O₃) on thiol-functionalized silica gel using thiol-ene click chemistry to prepare a chiral stationary phase (CSP) for high performance liquid chromatography (HPLC). The fabricated column exhibited excellent chiral separation capability under both normal-phase and reversed-phase conditions. Fourteen and 10 racemates were well resolved on the column in normal-phase mode (using n-hexane/isopropanol as the mobile phase) and reversed-phase mode (using methanol/water as the mobile phase), respectively, including alcohols, esters, ethers, ketones, aldehydes, epoxides and organic acids. Moreover, the column also shows good selectivity toward positional isomers. Six positional isomers (dinitrobenzene, chloroaniline, bromoaniline, iodoaniline, nitrobrobenzene and nitrochlorobenzene) were well separated on the column. In addition, the effects of the injection mass and mobile phase composition on the separation were investigated. The column shows good reproducibility and stability after multiple injections with the relative standard deviation (RSD) (n = 5) of the retention time and resolution being < 0.96 % and 0.65 %, respectively. This study indicates that this type of chiral polyimine macrocycles is a promising chiral selector for HPLC enantioseparation and will push forward the applications of more novel chiral macrocycles for chiral chromatographic separation.

High selectivity of a novel D–A structured copolymer as a gas chromatographic stationary phase toward aromatic isomers

This work reports a new π-conjugated copolymer with D–A structure (PBDB-T) as the stationary phase for gas chromatography (GC) with high resolving performance towards aromatic isomers of high similarity in structure and properties.

A selective and inert stationary phase combining triptycene with tocopheryl polyethylene glycol succinate for capillary gas chromatography

This work reports a selective and inert triptycene-based stationary phase (TPT) combining the triptycene framework with tocopheryl polyethylene glycol succinate (TPGS) units for capillary gas chromatography (GC). The TPT stationary phase was physically coated onto a capillary column by static coating method with the column efficiency of 4200 plates/m and moderate polarity. As demonstrated, the TPT column exhibited high inertness towards organic bases, including basic heterocycles, aliphatic and aromatic amines, showing distinct advantages over the TPGS and commercial columns. Also, the TPT column displayed high-resolution performance towards the isomers of methylpyridines, toluidines, xylidines and alkanes (C6-C8). Moreover, it showed excellent separation repeatability and reproducibility with RSD values in the range of 0.03%-0.07% for run-to-run, 0.12%-0.18% for day-to-day and 2.3%-3.6% for column-to- column (n = 4). Its applications to purity test of chemical products and to GC-MS analysis of the essential oil of Artemisia annua L. demonstrated its good potential for practical analyses. The present work has novelty in constructing highly selective and inert stationary phases and providing a feasible strategy for concurrently addressing the related problems in GC analyses. Its methodology and findings is of important value in terms of fundamental researches and practical applications.

The selectivity of a polydimethylsiloxane-based triblock copolymer as the stationary phase for capillary gas chromatography

A triblock copolymer (PCL-PDMS-PCL) constructed from polydimethylsiloxane (PDMS) and poly(ε-caprolactone) (PCL) chains was synthesized and used as the stationary phase for capillary gas chromatography (GC).