Enzymatic reactions in nature efficiently trap reactants and release products within well-defined spaces. Inspired by this, researchers have developed artificial catalysts with specific apertures to enhance proximity effects and increase local substrate concentration. Control over catalytic activity is often achieved by modifying the environment around active centers using external triggers. However, continuously exchanging products for reagents within confined spaces remains a significant challenge. Most porous structures rely on rigid pores where products occupy the cavity, hindering further reactions. The need exists for catalysts that dynamically exchange reagents for products using external triggers. Self-assembly of small molecules via noncovalent interactions offers a route to creating stimuli-responsive dynamic devices. Expansion of transient states enhances exchange due to increased entropy, while contraction accelerates reagent collisions. However, transient pores based on noncovalent interactions are often too delicate for reliable exchange. Aromatic rod amphiphiles, composed of conjugated carbon and hydrophilic dendritic segments, can self-assemble into porous structures with hydrophobic characteristics, ideal for organic catalysis. Compared to rigid porous structures, these hydrophobic pores exhibit remarkable recognition of organic molecules. For instance, hollow spherical adsorbents from folded aromatic rod assemblies show excellent organic contaminant removal efficiency from wastewater, and well-defined helical polymers can selectively extract enantiomers of fullerenes based on size and chirality. Incorporating active atoms like nitrogen or metals into conjugated carbon substrates creates active sites within aromatic apertures. Unlike traditional rigid porous structures, supramolecular catalysts can spontaneously release products through disassembly and regain catalytic activity upon reassembly. Building on the recognition and regeneration capabilities of supramolecular apertures, this research presents a tubular catalyst with reversible contraction-expansion for efficient reagent and product exchange. This 'pulsating' transition is achieved using thermo-responsive alkyl blocks in pyridine-based aromatic amphiphiles.

The authors extensively review existing literature on artificial catalysts, focusing on those utilizing porous structures and external stimuli for control. They highlight the limitations of existing rigid porous materials in continuously exchanging reactants and products, emphasizing the need for dynamic systems. The review covers supramolecular catalysis and self-assembling systems, particularly those employing aromatic amphiphiles and their ability to form structures with specific recognition properties for organic molecules. The use of thermo-responsive elements for controlling the structure and function of catalysts is discussed, citing examples of 'breathing' vesicles and other stimuli-responsive materials. The review concludes by emphasizing the potential of self-assembling systems with tunable porosity for creating dynamic catalysts with efficient product turnover.

The study employed bent-shaped aromatic amphiphiles with thermo-responsive alkyl chains at both ends and hydrophilic oligoether dendrons at the apex for the creation of the tubular catalysts. Two variants, 1 and 2, were synthesized, differing in the length of the alkyl chains (decyl for 1 and butyl for 2). Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were used to characterize the morphology and dimensions of the self-assembled tubules formed in aqueous solution. Two-dimensional X-ray diffraction (2D XRD) was employed to analyze the arrangement of the tubules and determine the number of molecules in the unit cell. UV-Vis and fluorescence spectroscopy were used to investigate the aggregation behavior of the amphiphiles in different solvents. The catalytic activity of the tubules was evaluated using a nucleophilic aromatic substitution (SNAr) reaction between 1,3-dinitro-4-chlorobenzene (R1) and 1-octanethiol (R2). Fourier transform infrared (FTIR) spectroscopy was used to investigate the interactions between the reagents and the catalyst's pyridine groups. Adsorption experiments were conducted to determine the encapsulation efficiency of reagents and products within the tubules. Temperature-dependent FTIR and TEM were used to investigate the thermo-responsive behavior of the tubules, and HPLC was used to analyze the reaction products and monitor reagent and product exchange. The SNAr reaction was performed at room temperature using tubule 1 (decyl chains) and tubule 2 (butyl chains). The conversion was confirmed by HPLC analysis. FTIR was used to show the interactions between the catalyst and the reactants. The adsorption studies showed distinct behavior between tubule 1 (contracted) and tubule 2 (expanded) regarding the adsorption of reactant and product. Temperature-dependent FTIR, TEM, and XRD were used to demonstrate the reversible transition between the contracted and expanded states of tubule 1. The recyclability and reusability of tubule 1 was studied by repeated heating and cooling cycles, monitoring the reaction conversion with HPLC.

The study successfully synthesized and characterized two types of self-assembling tubular catalysts (1 and 2) with different alkyl chain lengths. TEM and STEM images revealed the formation of well-defined tubules with varying inner and outer diameters depending on the alkyl chain length. 2D XRD patterns confirmed the hexagonal packing arrangement of the tubules, and the number of molecules per unit cell was determined. The catalytic activity of the tubules was tested using a SNAr reaction. Tubule 1, with shorter alkyl chains, exhibited high catalytic activity (77%), while tubule 2 showed no activity. FTIR spectroscopy revealed that the active pyridine groups in tubule 1 interacted with the thiol reagent, leading to catalytic activity, whereas the pyridine groups in tubule 2 were blocked by the flexible butyl chains, resulting in inactivity. Adsorption studies showed that tubule 2 adsorbed both reactants and products more efficiently due to its expanded pore structure. The thermo-responsive behavior of tubule 1 was investigated, showing that it undergoes a reversible transition between a contracted trimeric and an expanded hexameric state upon heating above 60 °C. The expanded state allows for increased reagent adsorption, while the contracted state facilitates the SNAr reaction. The dynamic pulsating motion between the contracted and expanded states was demonstrated, showing that the catalyst could facilitate the continuous exchange of products and reagents while maintaining high catalytic activity. This system enabled the efficient and recyclable catalysis of SNAr reactions with high conversion rates, showing that the material could be recycled over 5 cycles.

The findings demonstrate the successful design and fabrication of a dynamic, pulsating tubular catalyst with highly efficient product turnover. The results support the hypothesis that controlling the pore size and the accessibility of active sites through stimuli-responsive conformational changes can significantly enhance the catalytic performance of porous materials. The distinct catalytic behaviors of tubules 1 and 2 highlight the crucial role of the alkyl chain length in determining the pore size and accessibility of active sites. The reversible transition between contracted and expanded states, driven by thermal stimuli, provides a mechanism for efficient exchange of products and reagents, addressing a significant challenge in heterogeneous catalysis. The high recyclability and reusability of the catalyst demonstrate the potential for practical applications in various chemical processes. The approach of using self-assembling structures with incorporated active sites and thermo-responsive elements opens new avenues for designing advanced heterogeneous catalysts with tailored properties and functionalities.

This study successfully demonstrated a novel strategy for creating a dynamic catalyst by combining self-assembly of aromatic amphiphiles with thermo-responsive behavior. The resulting pulsating tubules exhibit high catalytic activity, recyclability, and reusability due to the efficient exchange of products and reagents facilitated by their reversible conformational changes. This work offers a new perspective on the design and development of highly efficient and sustainable heterogeneous catalysts. Future research could explore the application of this approach to other catalytic reactions and investigate different stimuli-responsive elements to expand the tunability and control of the catalytic process.

The study primarily focused on a specific SNAr reaction. Further research is needed to investigate the applicability of this approach to a wider range of chemical reactions. The current system relies on thermal stimuli for the reversible transition, and exploration of alternative stimuli (e.g., light, pH) could enhance its versatility. The study primarily used 1-octanethiol as a reactant. Further investigation into the applicability of this system to various substrates is required. Finally, scaling up the synthesis of the tubular catalyst for industrial applications requires further investigation.