Multicomponent Supramolecular Systems: Self‐Organization in Coordination‐Driven Self‐Assembly

The self‐organization of multicomponent supramolecular systems involving a variety of two‐dimensional (2 D) polygons and three‐dimensional (3 D) cages is presented. Nine self‐organizing systems, SS₁ – SS₉ , have been studied. Each involves the simultaneous mixing of organoplatinum acceptors and pyridyl donors of varying geometry and their selective self‐assembly into three to four specific 2 D (rectangular, triangular, and rhomboid) and/or 3 D (triangular prism and distorted and nondistorted trigonal bipyramidal) supramolecules. The formation of these discrete structures is characterized using NMR spectroscopy and electrospray ionization mass spectrometry (ESI‐MS). In all cases, the self‐organization process is directed by: 1) the geometric information encoded within the molecular subunits and 2) a thermodynamically driven dynamic self‐correction process. The result is the selective self‐assembly of multiple discrete products from a randomly formed complex. The influence of key experimental variables ‐ temperature and solvent ‐ on the self‐correction process and the fidelity of the resulting self‐organization systems is also described.     

Biomimetic Self‐Healing

Self‐healing is a natural process common to all living organisms which provides increased longevity and the ability to adapt to changes in the environment. Inspired by this fitness‐enhancing functionality, which was tuned by billions of years of evolution, scientists and engineers have been incorporating self‐healing capabilities into synthetic materials. By mimicking mechanically triggered chemistry as well as the storage and delivery of liquid reagents, new materials have been developed with extended longevity that are capable of restoring mechanical integrity and additional functions after being damaged. This Review describes the fundamental steps in this new field of science, which combines chemistry, physics, materials science, and mechanical engineering.     

Self‐Assembly of Non‐Biological Polymeric Strands Undergoing Enforced Helical Self‐Organization

The polycondensation of a dihydrazino‐pyrimidine ( 5 and 6 ) with a pyrimidine‐dicarbaldehyde ( 7 and 8b ) provides an efficent access to helical polymeric strands based on the formation of hydrazone connections between the pyrimidine groups. The folding into a helical structure is enforced by the helicity codon defined by the (hydrazone? pyrimidine) sequence. The polymers obtained have been characterized by mass spectrometry, indicating molecular weights up to ca. 12000 Da. Electronic spectra display specific absorption and emission features. These helical polymers present a core diameter of ca. 20 Å, a pitch of 3.5 Å, and, for a molecular weight around 9000 Da, a height of ca. 42 Å with 12 turns. The self‐assembled helical polymers obtained represent stable frameworks for the lateral attachment of functional residues in a helical disposition. Such entities may possess a range of novel chemical as well as biological properties.     

pH‐Switchable Self‐Assembled Materials

Self‐assembled materials, which are able to respond to external stimuli, have been extensively studied over the last decades. A particularly exciting stimulus for a wide range of biomedical applications is the pH value of aqueous solutions, since deprotonation‐protonation events are crucial for structural and functional properties of biopolymers. In living cells and tissues, intra‐ and extracellular pH values are stringently regulated, but can deviate from pH neutral as observed for example in tumorous, inflammatory sites, in endocytic pathways, and specific cellular compartments. By using a pH‐switch as a stimulus, it is thereby possible to address specific targets in order to cause a programmed response of the supramolecular material. This strategy has not only been successfully applied in fundamental research but also in clinical studies. In this feature article, current strategies that have been used in order to design materials with pH‐responsive properties are illustrated. This discussion only addresses selected examples from the last four years, the self‐assembly of polymer‐based building blocks, assemblies emerging from small molecules including surfactants or derived from biological macromolecules, and finally the controlled self‐assembly of oligopeptides.

     

RAFT‐Mediated Polymerization‐Induced Self‐Assembly

After a brief history that positions polymerization‐induced self‐assembly (PISA) in the field of polymer chemistry, this Review will cover the fundamentals of the PISA mechanism. Furthermore, this Review will also give an overview of some of the features and limitations of RAFT‐mediated PISA in terms of the choice of the components involved, the nature of the nanoobjects that can be obtained and how the syntheses can be controlled, as well as some potential applications.     

Self‐Healing Colloidal Crystals

Healing hands : A complex interplay between colloidal and polymeric energetics in microgel self‐assembly behavior results in soft colloidal assemblies with self‐healing properties. Repulsive soft spheres can adopt highly compressed conformations in colloidal crystalline lattices without directly contacting the nearest neighbors (see picture). This distant action is directly responsible for the self‐healing of the assemblies.

     

Biocatalytic Self‐Assembly Cascades

The properties of supramolecular materials are dictated by both kinetic and thermodynamic aspects, providing opportunities to dynamically regulate morphology and function. Herein, we demonstrate time‐dependent regulation of supramolecular self‐assembly by connected, kinetically competing enzymatic reactions. Starting from Fmoc‐tyrosine phosphate and phenylalanine amide in the presence of an amidase and phosphatase, four distinct self‐assembling molecules may be formed which each give rise to distinct morphologies (spheres, fibers, tubes/tapes and sheets). By varying the sequence or ratio in which the enzymes are added to mixtures of precursors, these structures can be (transiently) accessed and interconverted. The approach provides insights into dynamic self‐assembly using competing pathways that may aid the design of soft nanostructures with tunable dynamic properties and life times.     

Self‐Healing Molecular Crystals

One of the most inevitable limitations of any material that is exposed to mechanical impact is that they are inexorably prone to mechanical damage, such as cracking, denting, gouging, or wearing. To confront this challenge, the field of polymers has developed materials that are capable of autonomous self‐healing and recover their macroscopic integrity similar to biological organisms. However, the study of this phenomenon has mostly remained within the soft materials community and has not been explored by solid‐state organic chemists. The first evidence of self‐healing in a molecular crystal is now presented using crystals of dipyrazolethiuram disulfide. The crystals were mildly compressed and the degree of healing was found to be 6.7 %. These findings show that the self‐healing properties can be extended beyond mesophasic materials and applied towards the realm of ordered solid‐state compounds.     

Self‐Assembled Cage‐Like Protein Structures

Proteins and protein‐based assemblies represent the most structurally and functionally diverse molecules found in nature. Protein cages, viruses and bacterial microcompartments are highly organized structures that are composed primarily of protein building blocks and play important roles in molecular ion storage, nucleic acid packaging and catalysis. The outer and inner surface of protein cages can be modified, either chemically or genetically, and the internal cavity can be used to template, store and arrange molecular cargo within a defined space. Owing to their structural, morphological, chemical and thermal diversity, protein cages have been investigated extensively for applications in nanotechnology, nanomedicine and materials science. Here we provide a concise overview of the most common icosahedral viral and nonviral assemblies, their role in nature, and why they are highly attractive scaffolds for the encapsulation of functional materials.     

Biomimetic Strain‐Stiffening Self‐Assembled Hydrogels

Supramolecular structures with strain‐stiffening properties are ubiquitous in nature but remain rare in the lab. Herein, we report on strain‐stiffening supramolecular hydrogels that are entirely produced through the self‐assembly of synthetic molecular gelators. The involved gelators self‐assemble into semi‐flexible fibers, which thereby crosslink into hydrogels. Interestingly, these hydrogels are capable of stiffening in response to applied stress, resembling biological intermediate filaments system. Furthermore, strain‐stiffening hydrogel networks embedded with liposomes are constructed through orthogonal self‐assembly of gelators and phospholipids, mimicking biological tissues in both architecture and mechanical properties. This work furthers the development of biomimetic soft materials with mechanical responsiveness and presents potentially enticing applications in diverse fields, such as tissue engineering, artificial life, and strain sensors.     

Self‐Propelled Polymer Nanomotors

Polymer conformations: The picture shows instantaneous conformations of product molecules produced in a catalytic reaction at the head of a polymer chain. The extended (left) and collapsed (right) polymers correspond to chains in good and poor solvent conditions, respectively. The product molecule concentration gradient leads to a directed force on the polymer, so that it functions as a self‐propelled nanomotor.

     

Macroscopic Self‐Propelled Objects

Self‐propelled systems are currently in the spotlight of the research community. We review the progress of the construction of both millimeter‐ and centimeter‐sized self‐propelled macroscopic objects. We will also discuss the various sources of energy used by these systems, such as the electromagnetic field, electric field, thermal gradient, and chemical energy, and present how these millimeter‐ and centimeter‐sized devices can move at velocities of tens cm s^?1 and distances of several tens of meters.