Introductory Chapter: Self-Assembly of Molecules into Supramolecular Structures

1. Overview

The bottom-up approach for the self-assembly of molecules, macromolecules, and particles into well-defined superstructures provides superior structural control of materials compared to top-down methods. Nature largely utilizes macromolecules to construct supramolecular materials, ultimately contributing to a wide range of applications. Thus, the self-assembly of materials and the formation of superstructures have been of great interest in the fields of materials science, nanoscience, and nanoengineering.

This book provides the self-assembly of materials and supramolecular chemistry design principles for a broad spectrum of materials, including bio-inspired amphiphiles, metal oxides, metal nanoparticles, and organic-inorganic hybrid materials. It describes the fundamental concepts of self-assembly design approaches and supramolecular chemistry principles for research ideas in nanotechnology-enabled applications. The book focuses on three main themes, which include: the self-assembly and supramolecular chemistry of amphiphiles, the supramolecular structures and devices of inorganic materials, and the assembly-disassembly of organic-inorganic hybrid materials.

2. Self-assembly of bioinspired amphiphiles

Assembly of amphiphilic molecular and polymeric materials into precise functional structures is being explored actively for patterning and fabricating periodic array structures from mesoscale to nanoscale. Leading examples can be found in biosystems where assemblies of different amphiphilic components yield unusual microstructures that enable the performance of highly specific cellular functions [1, 2, 3]. For example, cell membrane’s lipid bilayer, which is constructed from self-assembled phospholipids (PLs), is one of nature’s well-organized nanoscale machines and is vital to cell’s signal transduction. The self-assembly of biologically based lipids into unusual microstructures provides superior structural control due to the amphiphilic nature of lipids that are composed of a polar head group and a nonpolar hydrocarbon tail [4]. The dimensionality of the hydrated hydrophilic head group (0.7 to 1.0 nm) and the hydrophobic core of the bilayer (2.5 to 3.5 nm) results in varied tail lengths, which provide unprecedented control over the structural complexity and function at the nanoscale. For instance, most long-chain PLs self-assemble into spherical bilayer aggregates, known as liposomes [5]. However, by exploiting the subtle interplay between appropriately modified head groups and fatty acid chains, certain synthetic phospholipids self-assemble into novel microstructures [6]. One such example consists of synthetic PLs with photopolymerizable diacetylenic moieties in the acyl chains that self-assemble into hollow, cylindrical structures, known as tubules [7, 8], which have also been observed in other synthetic surfactants [9] and in bile [10].

Owing to these remarkable supramolecular assemblies, bio-conjugated nanomaterials, consisting of lipid bilayers and inorganic nanomaterials, such as metal nanoparticles [11, 12, 13] and nanocarbon [14, 15], have been demonstrated as new functional materials for electronic devices. In these devices, lipid bilayers are used as electrically insulating substrates that confine the nanomaterials. Particularly, the recent research advancements in this field set forth the supramolecular design principles, enabling access to many of the current examples of nanomaterials conjugated lipid bilayer architectures [11, 16, 17, 18, 19, 20, 21]. These efforts toward biological mimicry of supramolecular assemblies suggest that metal-nanoparticle arrays can be self-assembled in the lipid bilayer vesicles, and one can separate an array of nanoparticles with a uniform gap distance through supramolecular principles. By controlling the separation distance between nanoparticles or arrays with nanoscale precision using selective molecular interactions of lipids, many variations in the size and shape of the assembled nanostructures are possible [22, 23]. Additionally, it has been demonstrated that lipids can alter the dielectric properties of metallic nanoparticles, providing a way to modulate the optical properties of an integrated architecture and serve as optical sensors [24]. It has also been proven that the interaction between the lipid and nanoparticle can be adjusted by altering the nature of the polar head groups [25]. Because the study of PLs assembly is used to model biological membranes [26], current studies of the hybrid structures of metallic nanoparticles and PLs have shown the potential for the development of new biosensing devices and drug delivery methods across cellular membranes.

Utilizing bioinspired molecular and polymeric systems, stimuli-responsive nanomaterials with a variety of functionalities that respond to light [27], pH [28], temperature [29], and chemical [30] stimuli have been also developed. Although a variety of synthetic strategies enables the development of responsive materials, their structures exhibit limitations in morphological deformation in response to external stimuli. Transition metal ions are a unique tool for engineering responsive character based on their various binding stoichiometries and geometries, allowing a single material to have a diverse set of responses to different metal ions.⁷ Utilizing the dynamic nature of transition metal coordination bonds, structural control in small molecules [31, 32, 33, 34] and proteins, [35, 36, 37] to develop responsive films, [38, 39] self-healing soft materials, [39, 40, 41, 42] subcomponent self-assembly of polymeric materials, [43, 44, 45, 46] and hierarchical assemblies of nanoparticles has been demonstrated [47, 48].

3. Supramolecular nanoassemblies of π-conjugated molecular systems

Conjugated organic molecules have been the subject of continuous interest in organic electronics. Synthetic efforts aimed at π-conjugated systems having well-defined architectures are indeed driven by the desire to impart specific optical and electrical properties to materials by controlling their molecular structure. Crafting the structure and function of organic materials using the strategies of self-assembly and supramolecular chemistry has progressed over the past two decades in materials chemistry. A variety of systems have been engineered where function is directly linked to non-covalent interactions, such as ionic, hydrophobic, van der Waals, hydrogen, and coordination bonds. They also influence solid-state self-organization to produce an organized structure at any scale, ranging from the nano and micrometer scales to microscopic dimensions.

Great efforts have been directed to the solution-processable self-assembly of π- conjugated small molecules, oligomers, or polymers into shape-defined nanostructures. Bridging the gap between natural and artificial systems, well-organized nanomaterials can be prepared using self-assembly approaches [49, 50, 51, 52, 53, 54]. The morphologies of these self-assembled nanostructures cooperatively control by non-covalent interactions, such as H- bonding, dipole-dipole attraction, π-π stacking, van der Waals force, hydrophobic effect, electrostatic interaction, and metal-ligand coordination. Most predominantly, intermolecular π-π interactions and H-bonding have been the major driving forces, which often cooperatively drive other weak non-covalent interactions [50, 55]. Moreover, non-covalent interactions are highly dependent on the molecular structure where external environmental parameters, such as solvent, temperature, concentration, and fabrication process are responsible for the morphology of the supramolecular structure [50, 52, 56].

1D nanostructures of organic semiconductors with morphologies, including nanowires, nanobelts, nanorods, nanotubes, and helices, have merited intensive study over the past two decades. These 1D functional nanostructures hold great potential for enabling next-generation electronic and optoelectronic nanodevices [57, 58]. Consequently, the self-assembly of π-conjugated organic molecules into 1D nanostructures has been an active and rapidly developing field [52, 59, 60, 61, 62, 63, 64].

In recent years, the application of Watson-Crick pairing of nucleic acids, that is, the pairing of nucleic acids, using specific hydrogen bonding to pattern self-assembling and supramolecular organic materials, has transformed how we design, engineer, and synthesize structures at all scales [65, 66, 67, 68]. In the context of organic materials, the ability to generate ordered one-dimensional structures (1D) is fundamentally useful and functional. Thus, there remains considerable untapped potential in 1D organic materials. Successful fabrication of 1D nanostructures demands a tight correlation between the self-assembling kinetics and the molecular design and engineering. This usually requires a strong interplay between chemical synthesis, materials fabrication, and physical characterization that relate to a broad range of applications in electronic devices. Thus, understanding how to design molecules that can form 1D structures through non-covalent interactions is a key objective in organic material chemistry. So far, there is no universal principle that can predict the formation of any sort of 1D molecular self-assembly. Therefore, in order to design self-assembled 1D nanostructures, the supramolecular chemistry approaches should not only focus on the design principles of the molecular and supramolecular structure but also control the dynamic assembly-disassembly of the architecture. For example, they can be cylindrical assemblies, flat or twisted ribbons, tubes, and many other shapes but retain their morphologies in one dimension during the assembly and disassembly process.

Planar and rigid aromatic molecules are known to form one-dimensional (1D) and three-dimensional (3D) nanostructures through strong π-π interactions, evidencing that such interactions could be an effective approach for the formation of 1D nanoassemblies [69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80]. However, it still remains a difficult task to fabricate 1D nanostructures with well-defined morphology and molecular arrangement of these systems. Although impressive research have been performed on covalent and hydrogen-bonded dyads, triads, and higher order polycyclic aromatic systems in solution and solid phase [69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80]. research on further self-assembly of these complexes utilizing other strong non-covalent interactions, in particular, a combination of σ-holes and π−π interactions, is relatively scarce. This is quite unfortunate since self-assembly may have enormous potential for organic electronics, by providing a pathway to arrange electron donating (donors) and electron-accepting (acceptors) semiconducting fused-arene moieties into higher architectures through a wide variety of functional groups. In the solution phase, these systems can utilize for studying energy transfer phenomena as well as for understanding the feasibility of making dynamic self-assembled supramolecular structures by tailoring supramolecular interactions. For example, synergistic effect of non-covalent interactions, such as π-π stacking and hydrogen bonding on the chromophore stacking can reveal from their solution phase assembly behavior. These studies eventually can lead to the transfer of shape-persistent structures from solution to the thin film of the active layer of organic devices. Regardless, it is necessary to be programmed the molecular building blocks for the successful implementation of supramolecular electronics in practice, where the design enables the organization of functional chromophores into ordered, nano-sized aggregates.

Conflict of interest

The authors declare no conflict of interest.