The adaptive properties of noncovalent materials allow easy processing, facile recycling, self‐healing, and stimuli responsiveness. However, the poor robustness of noncovalent systems has hampered ...their use in real‐life applications. In this Concept Article we discuss the possibility of creating robust noncovalent arrays by utilizing strong hydrophobic interactions. We describe examples from our work on aqueous assemblies based on aromatic amphiphiles with extended hydrophobic cores. These arrays exhibit fascinating properties, including robustness, multiple stimuli‐responsiveness, and pathway‐dependent self‐assembly. We have shown that this can lead to functional materials (filtration membranes) rivaling covalent systems. We anticipate that water‐based noncovalent materials have the potential to replace or complement conventional polymer materials in various fields, and to promote novel applications that require the combination of robustness and adaptivity.
Better than covalent? Noncovalent materials formed by reversible self‐assembly are more adaptive than covalent ones. This adaptivity facilitates easy processing, recycling, self‐healing, and stimuli‐responsiveness. While noncovalent interactions are generally considered weak, strong hydrophobic interactions in water can be exploited to create noncovalent materials that are unusually stable, thus allowing practical use in real‐life applications.
Hybrid organic/lead halide perovskites are promising materials for solar cell fabrication, resulting in efficiencies up to 18%. The most commonly studied perovskites are CH3NH3PbI3 and CH3NH3PbI3–x ...Cl x where x is small. Importantly, in the latter system, the presence of chloride ion source in the starting solutions used for the perovskite deposition results in a strong increase in the overall charge diffusion length. In this work we investigate the crystallization parameters relevant to fabrication of perovskite materials based on CH3NH3PbI3 and CH3NH3PbBr3. We find that the addition of PbCl2 to the solutions used in the perovskite synthesis has a remarkable effect on the end product, because PbCl2 nanocrystals are present during the fabrication process, acting as heterogeneous nucleation sites for the formation of perovskite crystals in solution. We base this conclusion on SEM studies, synthesis of perovskite single crystals, and on cryo-TEM imaging of the frozen mother liquid. Our studies also included the effect of different substrates and substrate temperatures on the perovskite nucleation efficiency. In view of our findings, we optimized the procedures for solar cells based on lead bromide perovskite, resulting in 5.4% efficiency and V oc of 1.24 V, improving the performance in this class of devices. Insights gained from understanding the hybrid perovskite crystallization process can aid in rational design of the polycrystalline absorber films, leading to their enhanced performance.
Protein crystallization is important in structural biology, disease research and pharmaceuticals. It has recently been recognized that nonclassical crystallization-involving initial formation of an ...amorphous precursor phase-occurs often in protein, organic and inorganic crystallization processes
. A two-step nucleation theory has thus been proposed, in which initial low-density, solvated amorphous aggregates subsequently densify, leading to nucleation
. This view differs from classical nucleation theory, which implies that crystalline nuclei forming in solution have the same density and structure as does the final crystalline state
. A protein crystallization mechanism involving this classical pathway has recently been observed directly
. However, a molecular mechanism of nonclassical protein crystallization
has not been established
. To determine the nature of the amorphous precursors and whether crystallization takes place within them (and if so, how order develops at the molecular level), three-dimensional (3D) molecular-level imaging of a crystallization process is required. Here we report cryogenic scanning transmission microscopy tomography of ferritin aggregates at various stages of crystallization, followed by 3D reconstruction using simultaneous iterative reconstruction techniques to provide a 3D picture of crystallization with molecular resolution. As crystalline order gradually increased in the studied aggregates, they exhibited an increase in both order and density from their surface towards their interior. We observed no highly ordered small structures typical of a classical nucleation process, and occasionally we observed several ordered domains emerging within one amorphous aggregate, a phenomenon not predicted by either classical or two-step nucleation theories. Our molecular-level analysis hints at desolvation as the driver of the continuous order-evolution mechanism, a view that goes beyond current nucleation models, yet is consistent with a broad spectrum of protein crystallization mechanisms.
Noncovalent systems are adaptive and allow facile processing and recycling. Can they be at the same time robust? How can one rationally design such systems? Can they compete with high-performance ...covalent materials? The recent literature reveals that noncovalent systems can be robust yet adaptive, self-healing, and recyclable, featuring complex nanoscale structures and unique functions. We review such systems, focusing on the rational design of strong noncovalent interactions, kinetically controlled pathway-dependent processes, complexity, and function. The overview of the recent examples points at the emergent field of noncovalent nanomaterials that can represent a versatile, multifunctional, and environmentally friendly alternative to conventional covalent systems.
Understanding inorganic nanocrystal (NC) growth dynamic pathways under their native fabrication environment remains a central goal of science, as it is crucial for rationalizing novel ...nanoformulations with desired architectures and functionalities. We here present an in-situ method for quantifying, in real time, NCs' size evolution at sub-nm resolution, their concentration, and reactants consumption rate for studying NC growth mechanisms. Analyzing sequential high-resolution liquid-state
F-NMR spectra obtained in-situ and validating by ex-situ cryoTEM, we explore the growth evolution of fluoride-based NCs (CaF
and SrF
) in water, without disturbing the synthesis conditions. We find that the same nanomaterial (CaF
) can grow by either a particle-coalescence or classical-growth mechanism, as regulated by the capping ligand, resulting in different crystallographic properties and functional features of the fabricated NC. The ability to reveal, in real time, mechanistic pathways at which NCs grow open unique opportunities for tunning the properties of functional materials.
The conclusions reached by a diverse group of scientists who attended an intense 2‐day workshop on hybrid organic–inorganic perovskites are presented, including their thoughts on the most burning ...fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.
Understanding the crystallization of organic molecules is a long‐standing challenge. Herein, a mechanistic study on the self‐assembly of crystalline arrays in aqueous solution is presented. The ...crystalline arrays are assembled from perylene diimide (PDI) amphiphiles bearing a chiral N‐acetyltyrosine side group connected to the PDI aromatic core. A kinetic study of the crystallization process was performed using circular dichroism spectroscopy combined with time‐resolved cryogenic transmission electron microscopy (cryo‐TEM) imaging of key points along the reaction coordinate, and molecular dynamics simulation of the initial stages of the assembly. The study reveals a complex self‐assembly process starting from the formation of amorphous aggregates that are transformed into crystalline material through a nucleation–growth process. Activation parameters indicate the key role of desolvation along the assembly pathway. The insights from the kinetic study correlate well with the structural data from cryo‐TEM imaging. Overall, the study reveals four stages of crystalline self‐assembly: 1) collapse into amorphous aggregates; 2) nucleation as partial ordering; 3) crystal growth; and 4) fusion of smaller crystalline aggregates into large crystals. These studies indicate that the assembly process proceeds according to a two‐step crystallization model, whereby initially formed amorphous material is reorganized into an ordered system. This process follows Ostwald’s rule of stages, evolving through a series of intermediate phases prior to forming the final structure, thus providing an insight into the crystalline self‐assembly process in aqueous medium.
Getting in order: A combined spectroscopic/structural study reveals four stages of crystalline self‐assembly of organic molecules: collapse into amorphous aggregates, nucleation as partial ordering, crystal growth, and fusion of smaller crystalline aggregates into large crystals. The process follows Ostwald's rule of stages, and evolves through a series of intermediate phases prior to forming the final structure (see figure).
Most molecular self‐assembly strategies involve equilibrium systems, leading to a single thermodynamic product as a result of weak, reversible non‐covalent interactions. Yet, strong non‐covalent ...interactions may result in non‐equilibrium self‐assembly, in which structural diversity is achieved by forming several kinetic products based on a single covalent building block. We demonstrate that well‐defined amphiphilic molecular systems based on perylene diimide/peptide conjugates exhibit kinetically controlled self‐assembly in aqueous medium, enabling pathway‐dependent assembly sequences, in which different organic nanostructures are evolved in a stepwise manner. The self‐assembly process was characterized using UV/Vis circular dichroism (CD) spectroscopy, and cryogenic transmission electron microscopy (cryo‐TEM). Our findings show that pathway‐controlled self‐assembly may significantly broaden the methodology of non‐covalent synthesis.
Choose your path: Well‐defined molecular systems exhibit kinetically controlled self‐assembly in aqueous medium, enabling pathway‐dependent assembly sequences, in which different organic nanostructures are evolved in a stepwise manner (see figure).
Investigation of supramolecular kinetics is essential for elucidating self-assembly mechanisms. Recently, we reported on a noncovalent system involving a bolaamphiphilic perylene diimide dimer that ...is kinetically trapped in water but can rearrange into a different, more ordered assembly in water/THF mixtures ( Angew. Chem. Int. Ed. 2014, 53, 4123 ). Here we present a kinetic mechanistic study of this process by employing UV–vis spectroscopy. The transformation exhibits a rapid decrease in the red-shifted absorption band, which is monitored in order to track the kinetics at different temperatures (15–50 °C) and concentrations. Fitting the data with the 1D KJMA (Kolmogorov–Johnson–Mehl–Avrami) model affords the activation parameters. The latter as well as seeding experiments indicates that the transformation occurs without the detachment of covalent units, and that hydration dynamics plays a significant role in nucleation, with entropic factors being dominant. Switching off the transformation, and the formation of off-pathway intermediates were observed upon heating to temperatures above 55 °C. These insights into kinetically controlled supramolecular polymer transformations provide mechanistic information that is needed for a fundamental understanding of noncovalent processes, and the rational design of noncovalent materials.
Designing supramolecular nanotubes (SNTs) with distinct dimensions and properties is highly desirable, yet challenging, since structural control strategies are lacking. Furthermore, relatively ...complex building blocks are often employed in SNT self-assembly. Here, we demonstrate that symmetric bolaamphiphiles having a hydrophobic core comprised of two perylene diimide moieties connected via a bipyridine linker and bearing polyethylene glycol (PEG) side chains can self-assemble into diverse molecular nanotubes. The structure of the nanotubes can be controlled by assembly conditions (solvent composition and temperature) and a PEG chain length. The resulting nanotubes differ both in diameter and cross section geometry, having widths of 3 nm (triangular-like cross-section), 4 nm (rectangular), and 5 nm (hexagonal). Molecular dynamics simulations provide insights into the stability of the tubular superstructures and their initial stages of self-assembly, revealing a key role of oligomerization via side-by-side aromatic interactions between bis-aromatic cores. Probing electronic and photonic properties of the nanotubes revealed extended electron delocalization and photoinduced charge separation that proceeds via symmetry breaking, a photofunction distinctly different from that of the fibers assembled from the same molecules. A high degree of structural control and insights into SNT self-assembly advance design approaches toward functional organic nanomaterials.
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