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Table of Contents

    Topic review

    Supramolecular Chemistry: Host–Guest Molecular Complexes

    Submitted by: Sadaf Bashir Khan

    Definition

    The host–guest (HG) interactions in two-dimensional (2D) permeable porous linkages are growing expeditiously due to their future applications in biocatalysis, separation technology, or nanoscale patterning. In host–guest (HG) interaction, distinctive structural complexes development occurs via non-covalent associations. There is a growing curiosity in executing supramolecular HG structures for assembling organic solvents and aqueous solutions on compact planes.

    1. Overview

    In recent times, researchers have emphasized practical approaches for capturing coordinated and selective guest entrap. The physisorbed nanoporous supramolecular complexes have been widely used to restrain various guest species on compact supporting surfaces. The host–guest (HG) interactions in two-dimensional (2D) permeable porous linkages are growing expeditiously due to their future applications in biocatalysis, separation technology, or nanoscale patterning. The different crystal-like nanoporous network has been acquired to enclose and trap guest molecules of various dimensions and contours. The host centers have been lumped together via noncovalent interactions (such as hydrogen bonds, van der Waals (vdW) interactions, or coordinate bonds). In this review article, we enlighten and elucidate recent progress in HG chemistry, explored via scanning tunneling microscopy (STM). We summarize the synthesis, design, and characterization of typical HG structural design examined on various substrates, under ambient surroundings at the liquid-solid (LS) interface, or during ultrahigh vacuum (UHV). We emphasize isoreticular complexes, vibrant HG coordination, or hosts functional cavities responsive to the applied stimulus. Finally, we critically discuss the significant challenges in advancing this developing electrochemical field. 

    2. Host–Guest (HG) Interaction

    In host–guest (HG) interaction, distinctive structural complexes development occurs via non-covalent associations [1]. There is a growing curiosity in executing supramolecular HG structures for assembling organic solvents and aqueous solutions on compact planes [2]. Previous studies reveal that compact, dense planes offer an adequate degree of crystallinity in host linkage, favoring efficient guest traps. It also delivers added firmness and constancy to the subsequent HG supramolecular assemblies using molecule substrate interactions [3][4]. Often, substrate accumulated host complexes show particularity in guest molecules displaying crystalline configurations analogous to enzymes or zeolites [4][5]. These nanostructured host planes can be easily assimilated into realistic purposeful supramolecular structures leading towards prospective applications in molecular sensors, drug delivery, flat panel display devices, cosmetics, catalysis, or separation equipment [6]. HG chemistry is investigated either in solution or on solid surfaces, each having its unique physiognomies and molecular recognition [7][8][9][10]. The conjointly specific interactions among the host system and the guest links take place over atomic dimensions [11][12]. At sub-molecular resolution, STM [11][13] helps to spot these HG links on the condition that it proceeds on the atomically plane conductive smooth substrate [14][15][16]. Molecular identification in traditional solution-phase host–guest interaction is more often comprehended using indirect methods, including calorimetry technique to measure the degree of heat alteration, UV-vis absorption, or chemical shift variation. The experimental evidence gathered from these measurements offers statistics on the selectivity and strength of intermolecular interactions, letting researchers evaluating step-by-step procedures keep the different influencing parameters consistent throughout the process. Even though these procedures are exceedingly progressing and scientifically demanding, they cannot yet capture the direct visual structure of the HG multiplexes. In this regard, STM offers fundamental facts about HG centers; additionally, it permits monitoring the dynamics and active features of this coordination, hence apprehending molecular proceedings. STM has advanced as a multipurpose method for investigating supramolecular HG networks operative in various environmental conditions [17][18]. On a flat conductive surface, the HG complex is every so often acquired through molecular self-accumulation.
    The carbon-based molecules accumulate in self-assemblies to generate a host network. The host complexes encompass cavities in the form of nanowells comprising of a single molecule. In these nanocavities, the guest molecules can adsorb or trap within these networks. Naturally, these HG linkage systems show persistence via vdW interactions, hydrogen bonding, metal-ligand association, or halogen bonding. At the LS interface, dynamic co-adsorption of solvent molecules takes place to stabilize the generated host complexes. Guests’ molecules become restrained and immobilized when the dimensions, including magnitude, feature, and contour of guest molecules, are suitable with the cavities offered by the host network. The constraint on guest molecules arises depending on solvent desorption. The guest molecules usually possess higher adsorption energy in comparison to solvent molecules. The attractive dispersion or diffusion plays a significant role in maintaining the equilibrium and stabilization among guest molecules and host networks, including the underlying surface. As a result, the HG surface interactions are often surface-dependent because both the host and guest molecules can be transported on the surface simultaneously. A complex and dynamic environment is often observed during liquid-solid (LS) solution interactions compared to UHV surroundings because of molecule-solvent and solvent-surface interactions. Besides this, added molecular and intermolecular surface interactions were present in the self-assemblies. Moreover, LS interactions deliver encouraging environments for molecular dynamics so that guest trapping occurs near equilibrium. However, the supporting substrate selection is narrow to the ones that possess stability in ambient conditions and do not experience oxidation. Commonly, the more frequently used substrates were highly oriented pyrolytic graphite (HOPG) or Au (111). However, MoS2 is also used to support STM experimentations in the ambient environment [19].
    Even though the LS interface offers an ‘apparent life’ sight of assembly development, ultra-high vacuum experimentations require special care, such as the ultra-clean environment. In UHV, the molecular assembly was carried out utilizing the organic molecular beam epitaxy (OMBE) system permitting precise mechanism over atomic ratios and stratum width. Earlier, under UHV conditions, various crystal surfaces comprising of metals such as gold, copper, platinum, or silver were used as supporting surfaces for the study of HG chemistry [20]. In UHV, the self-assembly formation takes place in a vacuum, so the surface temperature shows consistency. Moreover, it can be easily controlled, which allows for optimized annealing and even viewing at low temperatures. The surface is an essential feature in HG interfaces as it fundamentally directs the freedom of movement among molecules due to adsorption, self-healing, or self-alignment. During UHV settings, high-temperature annealing persuades the dynamics in supramolecular hybrids. At the LS interface, the annealing is sometimes avoided due to solvent evaporation. It is a significant issue in metals because metals intermingle strongly with aromatic molecular structures. As a result, to attain and monitor self-accumulated (long-range) complexes of physisorbed molecules is tricky on metallic substrates compared to HOPG because of the greater diffusion obstructions. That is the reason HOPG offers a wide range of multicomponent self-assemblies or supramolecular hybrids studies without influencing the molecular dynamics. Numerous correspondences subsist between HG approaches active on surfaces and those implemented in solution. STM plays an influential role in investigating and analyzing the intrinsically porous network encompassing stable covalent cavities and extrinsic permeable host complexes. The inherent pore in an isolated molecule exists due to molecule configuration. The intrinsically porous host system comprises macrocyclic compounds such as cyclodextrins [21], crown ethers [22], and calixarenes [23]. In comparison, the extrinsic porosity is an after-effect generated by constituent molecules due to covalent or non-covalent assemblage. Earlier different research groups focus on extrinsically porous structures that self-assemble upon adsorption supported on solid planes to generate a host system. Sometimes, the host establishing molecules act as guests, known as auto host-guest systems [24]. Usually, the HG network assembly comprises two components. The advanced order multi-component systems involve four different molecular components in which more than one kind of guest particle was accumulated in a paternal host system [25]. Thus, it is essential to know that the HG system comprises a multi-component system; still, the multicomponent network did not represent HG coordination.

    3. Conclusions

    At the beginning of HG chemistry in the 1980s, the essential principle of supramolecular chemistry briskly advanced due to the formation of diverse hosts, i.e., crown ethers, cryptands, pillararenes, or calixarenes. However, the investigation related to HG chemistry on compact substrates were initiated comparatively slow, perhaps due to the absence of appropriate modus operandi, which helps to access the hidden and out-of-the-way LS interface. STM specifically makes it possible to illustrate assemblies of intricate HG systems at submolecular resolution. These porous crystal complexes were exceedingly desired. Their elongated extended framework permits the elegant structural mechanisms required in molecular separation applications. As reviewed comprehensively above, HG interaction on compact substrates has progressed considerably in the preceding years. Initially, the host linkage structures were simple, in which the selection of guest atoms is usually limited to C60 or coronene. Various host’s structural design has been assembled on several solid planes to elaborate host void, strength, interactions, stability, or entrapping guest species. The development of efficient approaches in contemporary ages has revealed the immense potential to expand multifaceted and functional host arrangements. UHV–STM and LS interfaces have so far delivered illustrations of host–guest connecting systems, even though these are customarily qualitative consequences. The LS interface seems to be a methodology that can be scaled up for an upcoming device. The analyses conceded under UHV surroundings will prolong to have distinct prominence. The ultraclean environment with the nonexistence of solvent is a significant factor for straightening out important mechanical characteristics of guests associating progressions. Current development in the field specifies the mutual usage of the molecular scheme, supra-molecular amalgamation, and substrate interactions to comprehend HG arrangements designed for explicit purposes.
    In recent times, novel approaches have been used to advance host systems that display guest-binding performance showing response to external stimuli. The research work reviewed currently discloses supramolecular chemistry concepts, which can be cost-effectively utilized in the ‘real-life’ approaches. In the real world, objectives of the host–guest binding investigated on solid planes can be advanced in two distinctive routes: its applications in molecular separations or sensing. The HG interaction on a substrate will be significantly improved via different essential approaches that permit the adjustment of a chiral molecule in 2D voids. This modification facilitates a careful HG group based on the compound and chiral molecule. A remarkable option is to consume the restricted area inside the nanocavities to operate the modifications. The dimensions of porous membrane voids which as a host may be altered according to the requirement to persuade the controlled reaction field. A well-defined restrain on the porous or dense intricate structure is an advantageous alternative characteristic for upcoming host structures. This procedure permit target molecules storage in open pores until, via using external stimuli, to close the arrangement. The examples deliberated above already own these desirable characteristics. However, there is a possibility for advanced investigation. Corresponding investigative methods such as emission spectroscopy or optical absorption will possibly be applicable to trace modifications in solution dilution on the release of guest molecules on the condition that guest release-capture takes place on an assessable balance. Moreover, in HG systems, idiosyncratic guests resist for adsorption might significantly advantage in Raman spectroscopy, i.e., receptive to the guest elemental analysis.
    Besides these progressions, one cannot ignore the organic self-assembly kinetics and thermodynamics investigations at the LS interface, which is undergoing exciting and rapid succession. Through the last two epochs, the understanding of chemical reaction kinetics, i.e., rates of reaction or predicting the thermodynamic stability, is also debatable. In various chemical systems, the fundamental conflict takes place between thermodynamics and reaction kinetics in a system. It engenders significant arguments and comparisons between kinetics and thermodynamics. Consequently, at the LS interface, kinetic and thermodynamic mechanisms take place at LS to attain expecting surface assemblies associating with their chemical and nanoelectronic characteristics. Unluckily, quantitative knowledge of kinetics and thermodynamics is narrow compared to the qualitative explanation at the LS interface. The variables such as concentration, solvent, temperature, pressure, or time were considered external test center influencing parameters. However, at molecular dimensions, structure, polymorphism, the location of the process, i.e., boundaries, centers or defects, the interval at a specific position, or solvent co-adsorption, influence the supramolecular assembly. The desirable future aim is to investigate surface properties concerning different influencing variables and, by these means, govern entirely rate constants or thermodynamic variables linked together in progression. In the future, it is a prerequisite to consider the comprehensive information of solute solvation, solute–surface interaction, or occurrences of surface inhomogeneity due to defects, step edges, or reformations.

    The entry is from 10.3390/molecules26133995

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