Visual recognition of ortho-xylene based on its host-guest crystalline self-assembly with α-cyclodextrin

Effect of the Molecular Weight of Polyelectrolyte and Surfactant Chain Length on the Solid-Phase Molecular Self-Assembly

Solid-phase molecular self-assembly (SPMSA) is emerging as an efficient approach, leading to scale-span self-assembled supramolecular films. With SPMSA, freestanding macroscopic supramolecular films can be formed upon mechanically pressing the precipitates formed with polyelectrolytes and oppositely charged surfactants. Herein, we report that the film formation ability and the mechanical strength of the resultant film depend highly on the surfactant chain lengths and the molecular weight of polyelectrolytes. A coarse-grained molecular dynamics study revealed that the longer surfactant chains are beneficial for the ordered assembly of surfactant bilayers in the film, whereas the larger molecular weight of PE favors the enhanced mechanical strength of the film by promoting the long-range order of the surfactant bilayers. The current results disclosed the physical insight of the surfactant chain length and the molecular weight of polyelectrolytes into the film structure and mechanical strength, which is of practical importance in guiding the creation of SPMSA materials.

New opportunities for cyclodextrins in supramolecular assembly: metal organic frameworks, crystalline self-assembly, and catalyzed assembly

Cyclodextrins (CDs) are widely used macrocycles in supramolecular assembly due to their easy availability, versatile functionality and excellent biocompatibility. Although they are well-known for forming host-guest complexes with a wide range of guests and this host-guest chemistry has long been utilized in industry and academia, new opportunities have arisen in recent years, particularly in supramolecular assembly. In the present review, we will first provide a basic introduction to CDs and then summarize their emerging roles in the fields of supramolecular chemistry and materials. This includes their involvement in hybrid frameworks with inorganic components such as metal ions and polyoxometalates, crystalline self-assembly with amphiphilic molecules, and their new possibility of "catassembly" and induced chiral supramolecular structures that have previously been overlooked. Finally, we will comment on the future perspectives of CDs to inspire more ideas and efforts, with the aim of promoting diverse applications of CDs in supramolecular materials.

Interfacial Rheological and Emulsion Properties of Self-Assembled Cyclodextrin-Oil Inclusion Complexes

To investigate the effect of the molecular size of alkanes and the cavity size of cyclodextrins (CDs) on the formation of interfacial host-guest inclusion complexes, the interfacial tension (IFT) of CD (α-CD, β-CD, γ-CD) solutions against oils (hexadecane, dodecylbenzene) was determined by interfacial dilational rheology measurements. The results show that the "space compatibility" between CDs and oil molecules is crucial for the formation of interface host-guest inclusion complexes. Hexadecane with a smaller molecular size can form host-guest inclusion complexes with small cavities of α-CD and β-CD, dodecylbenzene with a larger molecular size can form interfacial aggregates with the medium-sized cavity of β-CD easily, and the polycyclic aromatic hydrocarbon molecules in kerosene can form inclusion complexes with the large cavity of γ-CD. The formation of interfacial inclusion complexes leads to lower IFT values, higher interfacial dilational modulus, nonlinear IFT responses to the interface area oscillating, and skin-like films at the oil-water interface. What's more, the phase behavior of Pickering emulsions formed by CDs with different oils is explored, and the phenomena in alkane-CD emulsions are in line with the results in dilatation rheology. The interfacial active host-guest structure in the kerosene-γ-CD system improves the stability of the Pickering emulsion, which results in smaller emulsion droplets. This unique space compatibility characteristic is of great significance for the application of CDs in selective host-guest recognition, sensors, enhanced oil recovery, food industries, and local drug delivery.

Physical Insight into the Conditions Required in the Solid-Phase Molecular Self-Assembly of SDS Revealed by Coarse-Grained Molecular Dynamics Simulation

Molecular self-assembled materials have attracted considerable interest in recent years. As part of the efforts to overcome the shortcoming that the solution-based methods were hardly applicable in preparing bulk macroscopic molecular self-assemblies, Yan [ CCS Chem. 2020, 2, 98-106] developed a strategy of solid-phase molecular self-assembly (SPMSA) that allows scaling up the generation of massive supramolecular films. It is highly desired to understand the physical insight into the SPMSA at a molecular level. Here, in combination with the experimental study, we report molecular dynamics (MD) simulations on the SPMSA of the surfactant sodium dodecyl sulfate (SDS) using a coarse-grained method with the Martini force field model. The MD simulations clearly manifest that a small amount of water is required to endow the SDS molecules with sufficient mobility to self-assemble, and the smaller size of the preassembled SDS particles favors their further fusion into mesophases by reducing the total surface Gibbs free energy, while the smaller interparticle distance decreases the time for the particle fusion. The simulation results agree well with the conditions required in the experiment, confirming that SMPSA is a free-energy-favored process leading to bulk self-assembled materials.

2-O-Methylated β-Cyclodextrin as an Effective Building Block to Construct Supramolecular Assemblies with Various Morphologies and Molecular Arrangements

The preparation of supramolecular cyclodextrin (CD) assemblies and control of their assembly mode through guest inclusion in CD cavities have been actively studied. Contrarily, there are limited reports on the control of the assembly mode of guest-free CD molecules by external stimuli. Herein, we report the use of 2-O-methylated β-cyclodextrin (2-Me-β-CD) as an effective building block in fabricating supramolecular assemblies with diverse morphologies and molecular arrangements through assembly mode control by various stimuli, such as temperature and solvent. When methanol and diethyl carbonate were used as good and poor solvents, respectively, 2-Me-β-CD formed an amorphous assembly through solvent evaporation on a polyethylene terephthalate (PET) substrate. Increasing the drying temperature and using crystalline substrates, such as highly oriented pyrolytic graphite (HOPG) and sapphire, changed the assembly mode of 2-Me-β-CD to a head-to-tail channel assembly. However, when a 2-Me-β-CD/1-propanol solution was mixed with linear alkanes as a poor solvent, 2-Me-β-CD with head-to-head channel assembly was formed as a precipitate. Additionally, when the corresponding cyclic alkane was used as an alternative poor solvent, an organogel composed of 2-Me-β-CD with head-to-head channel assemblies was obtained. The organogel obtained became a precipitate composed of 2-Me-β-CD with cage-type assembly upon heating at 50 °C. Among the supramolecular assemblies fabricated in this study, the head-to-tail channel assembly is a rare molecular assembly of β-CD and its derivatives. It possesses a modified columnar cavity that has potential applications in molecular recognition and sensing.

Selectivity behaviour of two roof-shaped host compounds in the presence of xylene and ethylbenzene guest mixtures

α,α-Diphenyl-9,10-dihydro-9,10-ethanoanthracene-11-methanol possessed an enhanced selectivity for o-Xy when recrystallized from various xylene and EB mixed solvents (84.5–93.7%).