Metal-driven folding and assembly of a minimal ß-sheet into a 3D-porous honeycomb framework.

In contrast to short helical peptides, constrained peptides, and foldamers, the design and fabrication of crystalline 3D frameworks from the ß-sheet peptides are rare because of their high self-aggregation propensity to form 1D architectures. Herein, we demonstrate the formation of a 3D porous honeycomb framework through the silver coordination of a minimal ß-sheet forming a peptide having terminal metal coordinated 4- and 3-pyridyl ligands.

Helitwistacenes-Combining Lateral and Longitudinal Helicity Results in Solvent-Induced Inversion of Circularly Polarized Light.

Helicity is expressed differently in ortho- and para-fused acenes-helicenes and twistacenes, respectively. While the extent of helicity is constant in helicenes, it can be tuned in twistacenes, and the handedness of flexible twistacenes is often determined by more rigid helicenes. Here, we combine helicenes with rigid twistacenes consisting of a tunable degree of twisting, forming helitwistacenes. While the X-ray structures reveal that the connection does not affect the helicity of each moiety, their electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra are strongly affected by the helicity of the twistacene unit, resulting in solvent-induced sign inversion. ROESY NMR and TD-DFT calculations support this observation, which is explained by differences in the relative orientation of the helicene and twistacene moieties.

Molecular Engineering of Ordered Piezoelectric Sulfonic Acid-Containing Assemblies.

Sulfonic acid-containing bioorganic monomers with wide molecular designability and abundant hydrogen bonding sites hold great potential to design diverse functional biocrystals but have so far not been explored for piezoelectric energy harvesting applications due to the lack of strategies to break the centrosymmetry of their assemblies. Here, a significant molecular packing transformation from centrosymmetric into non-centrosymmetric conformation by the addition of an amide terminus in the sulfonic acid-containing bioorganic molecule is demonstrated, allowing a high electromechanical response. The amide-functionalized molecule self-assembles into a polar supramolecular parallel ß-sheet-like structure with a high longitudinal piezoelectric coefficient d11 = 15.9 pm V-1 that produces the maximal open-circuit voltage of >1 V and the maximal power of 18 nW in nanogenerator devices pioneered. By contrast, molecules containing an amino or a cyclohexyl terminus assemble into highly symmetric 3D hydrogen bonding diamondoid-like networks or 2D double layer structures that show tunable morphologies, thermostability, and mechanical properties but non-piezoelectricity. This work not only presents a facile approach to achieving symmetry transformation of bioorganic assemblies but also demonstrates the terminal group and the property correlation for tailor-made design of high-performance piezoelectric biomaterials.

Factors Controlling Complex Morphologies of Isomorphous Metal-Organic Frameworks.

We demonstrate here how nitrate salts of bivalent copper, nickel, cobalt, and manganese, along with an achiral organic ligand, assemble into various structures such as symmetrical double-decker flowers, smooth elongated hexagonal bipyramids, and hexagonal prisms. Large morphological changes occur in these structures because of different metal cations, although they maintain isomorphous hexagonal crystallographic structures. Metal cations with stronger coordination to ligands (Cu and Ni) tend to form uniform crystals with unusual shapes, whereas weaker coordinating metal cations (Mn and Co) produce crystals with more regular hexagonal morphologies. The unusual flower-like crystals formed with copper nitrate have two pairs of six symmetrical petals with hexagonal convex centers. The texture of the petals indicates dendritic growth. Two different types of morphologies were formed by using different copper nitrate-to-ligand ratios. An excess of the metal salt results in uniform and hexagonal crystals having a narrow size distribution, whereas the use of an excess of ligand results in double-decker morphologies. Mechanistically, an intermediate structure was observed with slightly concave facets and a domed center. Such structures most likely play a key role in the formation of double-decker crystals that can be formed by fusion processes. The coordination chemistry results in isostructural chiral frameworks consisting of two types of continuous helical channels. Four pyridine units from four separate ligands are coordinated to the metal center in a plane having a chiral (propeller-type) arrangement. The individual double-decker flower crystals are homochiral and a batch consists of crystals having both handedness.

Entropically-Driven Co-assembly of l-Histidine and l-Phenylalanine to Form Supramolecular Materials.

Molecular self- and co-assembly allow the formation of diverse and well-defined supramolecular structures with notable physical properties. Among the associating molecules, amino acids are especially attractive due to their inherent biocompatibility and simplicity. The biologically active enantiomer of l-histidine (l-His) plays structural and functional roles in proteins but does not self-assemble to form discrete nanostructures. In order to expand the structural space to include l-His-containing materials, we explored the co-assembly of l-His with all aromatic amino acids, including phenylalanine (Phe), tyrosine (Tyr), and tryptophan (Trp), all in both enantiomeric forms. In contrast to pristine l-His, the combination of this building block with all aromatic amino acids resulted in distinct morphologies including fibers, rods, and flake-like structures. Electrospray ionization mass spectrometry (ESI-MS) indicated the formation of supramolecular co-assemblies in all six combinations, but time-of-flight secondary-ion mass spectrometry (ToF-SIMS) indicated the best seamless co-assembly occurs between l-His and l-Phe while in the other cases, different degrees of phase separation could be observed. Indeed, isothermal titration calorimetry (ITC) suggested the highest affinity between l-His and l-Phe where the formation of co-assembled structures was driven by entropy. In accordance, among all the combinations, the co-assembly of l-His and l-Phe produced single crystals. The structure revealed the formation of a 3D network with nanocavities stabilized by hydrogen bonding between -N (l-His) and -NH (l-Phe). Taken together, using the co-assembly approach we expanded the field of amino acid nanomaterials and showed the ability to obtain discrete supramolecular nanostructures containing l-His based on its specific interactions with l-Phe.

Energy Transport in Dichroic Metallo-organic Crystals: Selective Inclusion of Spatially Resolved Arrays of Donor and Acceptor Dyes in Different Nanochannels.

In this study, the precise positioning and alignment of arrays of two different guest molecules in a crystalline host matrix has been engineered and resulted in new optically active materials. Sub-nm differences in the diameters of two types of 1D channels are sufficient for size-selective inclusion of dyes. Energy transport occurs between the arrays of different dyes that are included in parallel-positioned nanochannels by Förster resonance energy transfer (FRET). The color of individual micro-sized crystals are dependent on their relative position under polarized light. This angular-dependent behavior is a result of the geometrically constrained orientation of the dyes by the crystallographic packing of the host matrix and is concentration dependent.

Crystallographic-Morphological Connections in Star Shaped Metal-Organic Frameworks.

The symmetry of a crystal's morphology usually reflects the symmetry of the crystallographic packing. For single crystals, the space and point groups allow only a limited number of mathematical descriptions of the morphology (forms), all of which are convex polyhedrons. In contrast, concave polyhedrons are a hallmark of twinning and polycrystallinity and are typically inconsistent with single crystallinity. Here we report a new type of structure: a concave polyhedron shape single crystal having a multidomain appearance and a rare space group (P622). Despite these unusual structural features, the hexagonal symmetry is revealed at the morphological levels.

Ternary host-guest complexes with rapid exchange kinetics and photoswitchable fluorescence.

Confinement within molecular cages can dramatically modify the physicochemical properties of the encapsulated guest molecules, but such host-guest complexes have mainly been studied in a static context. Combining confinement effects with fast guest exchange kinetics could pave the way toward stimuli-responsive supramolecular systems-and ultimately materials-whose desired properties could be tailored "on demand" rapidly and reversibly. Here, we demonstrate rapid guest exchange between inclusion complexes of an open-window coordination cage that can simultaneously accommodate two guest molecules. Working with two types of guests, anthracene derivatives and BODIPY dyes, we show that the former can substantially modify the optical properties of the latter upon noncovalent heterodimer formation. We also studied the light-induced covalent dimerization of encapsulated anthracenes and found large effects of confinement on reaction rates. By coupling the photodimerization with the rapid guest exchange, we developed a new way to modulate fluorescence using external irradiation.

Modulating the Electromechanical Response of Bio-Inspired Amino Acid-Based Architectures through Supramolecular Co-Assembly.

Supramolecular packing dictates the physical properties of bio-inspired molecular assemblies in the solid state. Yet, modulating the stacking modes of bio-inspired supramolecular assemblies remains a challenge and the structure-property relationship is still not fully understood, which hampers the rational design of molecular structures to fabricate materials with desired properties. Herein, we present a co-assembly strategy to modulate the supramolecular packing of N-terminally capped alanine-based assemblies (Ac-Ala) by changing the amino acid chirality and mixing with a nonchiral bipyridine derivative (BPA). The co-assembly induced distinct solid-state stacking modes determined by X-ray crystallography, resulting in significantly enhanced electromechanical properties of the assembly architectures. The highest rigidity was observed after the co-assembly of racemic Ac-Ala with a bipyridine coformer (BPA/Ac-DL-Ala), which exhibited a measured Young's modulus of 38.8 GPa. Notably, BPA crystallizes in a centrosymmetric space group, a condition that is broken when co-crystallized with Ac-L-Ala and Ac-D-Ala to induce a piezoelectric response. Enantiopure co-assemblies of BPA/Ac-D-Ala and BPA/Ac-L-Ala showed density functional theory-predicted piezoelectric responses that are remarkably higher than the other assemblies due to the increased polarization of their supramolecular packing. This is the first report of a centrosymmetric-crystallizing coformer which increases the single-crystal piezoelectric response of an electrically active bio-inspired molecular assembly. The design rules that emerge from this investigation of chemically complex co-assemblies can facilitate the molecular design of high-performance functional materials comprised of bio-inspired building blocks.

Exploiting Minimalistic Backbone Engineered γ-Phenylalanine for the Formation of Supramolecular Co-Polymer.

Ordered supramolecular hydrogels assembled by modified aromatic amino acids often exhibit low mechanical rigidity. Aiming to stabilize the hydrogel and understand the impact of conformational freedom and hydrophobicity on the self-assembly process, two building blocks based on 9-fluorenyl-methoxycarbonyl-phenylalanine (Fmoc-Phe) gelator which contain two extra methylene units in the backbone, generating Fmoc-γPhe and Fmoc-(3-hydroxy)-γPhe are designed. Fmoc-γPhe spontaneously assembled in aqueous media forming a hydrogel with exceptional mechanical and thermal stability. Moreover, Fmoc-(3-hydroxy)-γPhe, with an extra backbone hydroxyl group decreasing its hydrophobicity while maintaining some molecular flexibility, self-assembled into a transient fibrillar hydrogel, that later formed microcrystalline aggregates through a phase transition. Molecular dynamics simulations and single crystal X-ray analyses reveal the mechanism underlying the two residues' distinct self-assembly behaviors. Finally, Fmoc-γPhe and Fmoc-(3-OH)-γPhe co-assembly to form a supramolecular hydrogel with notable mechanical properties are demonstrated. It has been believed that the understanding of the structure-assembly relationship will enable the design of new functional amino acid-based hydrogels.