A Supramolecular Sorting Hat: Stereocontrol in Metal–Ligand Self‐Assembly by Complementary Hydrogen Bonding

A combination of self‐complementary hydrogen bonding and metal–ligand interactions allows stereocontrol in the self‐assembly of prochiral ligand scaffolds. A unique, non‐tetrahedral M₄L₆ structure is observed upon multicomponent self‐assembly of 2,7‐diaminofluorenol with 2‐formylpyridine and Fe(ClO₄)₂. The stereochemical outcome of the assembly is controlled by self‐complementary hydrogen bonding between both individual ligands and a suitably sized counterion as template. This hydrogen‐bonding‐mediated stereoselective metal–ligand assembly allows the controlled formation of nonsymmetric discrete cage structures from previously unexploited ligand scaffolds.     

Hierarchical Organization of Ferrocene–Peptides

Hierarchical self‐assembly of disubstituted ferrocene (Fc)–peptide conjugates that possess Gly‐Val‐Phe and Gly‐Val‐Phe‐Phe peptide substituents leads to the formation of nano‐ and micro‐sized assemblies. Hydrogen‐bonding and hydrophobic interactions provide directionality to the assembly patterns. The self‐assembling behavior of these compounds was studied in solution by using ¹H NMR and circular dichroism (CD) spectroscopies. In the solid state, attenuated total reflectance (ATR) FTIR spectroscopy, single‐crystal X‐ray diffraction (XRD), powder X‐ray diffraction (PXRD), and scanning electron microscopy (SEM) methods were used. Spontaneous self‐assembly of Fc–peptides through intra‐ and intermolecular hydrogen‐bonding interactions induces supramolecular assemblies, which further associate and give rise to fibers, large fibrous crystals, and twisted ropes. In the case of Fc[CO‐Gly‐Val‐Phe‐OMe]₂ ( 1 ), molecules initially interact to form pleated sheets that undergo association into long fibers that form bundles and rectangular crystalline cuboids. Molecular offsets and defects, such as screw dislocations and solvent effects that occur during crystal growth, induce the formation of helical arrangements, ultimately leading to large twisted ropes. By contrast, the Fc–tetrapeptide conjugate Fc[CO‐Gly‐Val‐Phe‐Phe‐OMe]₂ ( 2 ) forms a network of nanofibers at the supramolecular level, presumably due to the additional hydrogen‐bonding and hydrophobic interactions that stem from the additional Phe residues.     

Water‐Regulated Self‐Assembly Structure Transformation and Gelation Behavior Prediction Based on a Hydrazide Derivative

Herein, we report the water‐regulated supramolecular self‐assembly structure transformation and the predictability of the gelation ability based on an azobenzene derivative bearing a hydrazide group, namely, N‐(3,4,5‐tributoxyphenyl)‐N′‐4‐[(4‐hydroxyphenyl)azophenyl] benzohydrazide (BNB‐t4). The regulation effects are demonstrated in the morphological transformation from spherical to lamellar particles then back to spherical in different solvent ratios of n‐propanol/water. The self‐assembly behavior of BNB‐t4 was characterized by minimum gelation concentration, microstructure, thermal, and mechanical stabilities. From the spectroscopy studies, it is suggested that gel formation of BNB‐t4 is mainly driven by intermolecular hydrogen bonding, accompanied with the contribution from π–π stacking as well as hydrophobic interactions. The successfully established correlation between the self‐assembly behavior and solubility parameters yields a facile way to predict the gelation performance of other molecules in other single or mixed solvents.     

Hierarchical Self‐Assembly in Liquid‐Crystalline Block Copolymers Enabled by Chirality Transfer

Helical topological structures are often found in chiral biological systems, but seldom in synthesized polymers. Now, controllable microphase separation of amphiphilic liquid‐crystalline block copolymers (LCBCs) consisting of hydrophilic poly(ethylene oxide) and hydrophobic azobenzene‐containing poly(methylacrylate) is combined with chirality transfer to fabricate helical nanostructures by doping with chiral additives (enantiopure tartaric acid). Through hydrogen‐bonding interactions, chirality is transferred from the dopant to the aggregation, which directs the hierarchical self‐assembly in the composite system. Upon optimized annealing condition, helical structures in film are fabricated by the induced aggregation chirality. The photoresponsive azobenzene mesogens in the LCBC assist photoregulation of the self‐assembled helical morphologies. This allows the construction and non‐contact manipulation of complicated nanostructures.     

Solvent‐Induced Helical Assembly and Reversible Chiroptical Switching of Chiral Cyclic‐Dipeptide‐Functionalized Naphthalenediimides

Understanding the roles of various parameters in orchestrating the preferential chiral molecular organization in supramolecular self‐assembly processes is of great significance in designing novel molecular functional systems. Cyclic dipeptide (CDP) chiral auxiliary‐functionalized naphthalenediimides (NCDPs 1 – 6 ) have been prepared and their chiral self‐assembly properties have been investigated. Detailed photophysical and circular dichroism (CD) studies have unveiled the crucial role of the solvent in the chiral aggregation of these NCDPs. NCDPs 1 – 3 form supramolecular helical assemblies and exhibit remarkable chiroptical switching behaviour (M‐ to P‐type) depending on the solvent composition of HFIP and DMSO. The strong influence of solvent composition on the supramolecular chirality of NCDPs has been further corroborated by concentration and solid‐state thin‐film CD studies. The chiroptical switching between supramolecular aggregates of opposite helicity (M and P) has been found to be reversible, and can be achieved through cycles of solvent removal and redissolution in solvent mixtures of specific composition. The control molecular systems (NCDPs 4 – 6 ), with an achiral or D ‐isomer second amino acid in the CDP auxiliary, did not show chiral aggregation properties. The substantial roles of hydrogen bonding and π–π interactions in the assembly of the NCDPs have been validated through nuclear magnetic resonance (NMR), photophysical, and computational studies. Quantum chemical calculations at the ab initio, semiempirical, and density functional theory levels have been performed on model systems to understand the stabilities of the right (P‐) and left (M‐) handed helical supramolecular assemblies and the nature of the intermolecular interactions. This study emphasizes the role of CDP chiral auxiliaries on the solvent‐induced helical assembly and reversible chiroptical switching of naphthalenediimides.     

Water‐Binding‐Mediated Gelation/Crystallization and Thermosensitive Superchirality

Determination of molecular structural parameters of hydrophobic cholesterol–naphthalimide conjugates for water binding capabilities as well as their moisture‐sensitive supramolecular self‐assembly were revealed. Water binding was a key factor in leading trace water‐induced crystallization against gelation in apolar solvent. Ordered water molecules entrapped in self‐assembly arrays revealed by crystal structures behave as hydrogen‐bonding linkers to facilitate three‐dimensional growth into crystals rather than one‐dimensional gel nanofibers. Water binding was also reflected on the supramolecular chirality inversion of vesicle self‐assembly in aqueous media via heating‐induced dehydration. Structural parameters that favor water binding were evaluated in detail, which could help rationally design organic building units for advancing soft materials, crystal engineering, and chiral recognition.     

The Helix to Super‐Helix Transition in the Self‐Assembly of π‐Systems: Superseding of Molecular Chirality at Hierarchical Level

Higher‐order super‐helical structures derived from biological molecules are known to evolve through opposite coiling of the initial helical fibers, as seen in collagen protein. A similar phenomenon is observed in a π‐system self‐assembly of chiral oligo(phenyleneethylene) derivatives (S )‐ 1 and (R )‐ 1 that explains the unequal formation of both left‐ and right‐handed helices from molecule having a specific chiral center. Concentration‐ and temperature‐dependent circular dichroism (CD) and UV/Vis spectroscopic studies revealed that the initial formation of helical aggregates is in accordance with the molecular chirality. At the next level of hierarchical self‐assembly, coiling of the fibers occurs with opposite handedness, thereby superseding the command of the molecular chirality. This was confirmed by solvent‐dependent decoiling of super‐helical structures and concentration‐dependent morphological analysis.     

Evidence for a Bulky Unit of a Pillar[5]arene Flipping in the Solid State

It is well known that pillar[5]arenes have two most stable conformations (pS and pR) in their crystal structures. Because of the intramolecular H‐bonding interactions, substituents, temperature, solvent and so on, the rotational behaviors of the phenolic units on pillararenes are also common. This paper showed some other kinds of conformations in the functionalized pillar[5]arenes and gave evidence for a bulky unit (1,4‐methoxycarbonylmethoxybenzene unit) flipping in the solid state. The presence of hydrogen bonding facilitated the intermolecular self‐assembly in terms of energy‐minimized packing in the crystals. Thus, the main driving force for the flipping of this bulky unit might be both the intramolecular hydrogen bonding between the phenolic units on pillararenes and quadrupolar hydrogen bonding between the host and water. This paper helps us to have a better understanding on the conformations of pillar[5]arenes.     

Entropically Favoured Assembly of Pyrazine‐Based Helical Fibers into Superstructures: Achiral/ Chiral Guest‐Induced Chirality Transformation

The development of synthetic helical structures undergoing stimuli‐responsive chirality transformations is important for an understanding of the role of chirality in natural systems. However, controlling supramolecular chirality in entropically driven assemblies in aqueous media is challenging. To develop stimuli‐responsive assemblies, we designed and synthesized pyrazine derivatives with l ‐alanine groups as chiral building blocks. These systems undergo self‐assembly in aqueous media to generate helical fibers and the embedded alanine groups transfer their chirality to the assembled structures. Furthermore, these helical fibers undergo a Ni²⁺‐induced chirality transformation. The study demonstrates the role of intermolecular hydrogen bonding, π–π stacking, and the hydrophobic effect in the Ni²⁺‐mediated transition of helical fibers to supercoiled helical ensembles which mimic the formation of superstructures in biopolymers.     

From Nongelating to Gelating: Synthesis and Structural Self‐Assembling Property Relationships of a Homologous Series of Oligo(amide–triazole)s

A homologous series of oligo(amide–triazole)s (OAT) [ OAT‐CO₂H‐2 n and OAT‐COPrg‐(2 n +1) ] with an increasing number of primary amide (CONH) and triazole hydrogen‐bonding functionalities was prepared by an iterative synthetic procedure. It was found that their self‐assembly and thermoreversible gelation strength had a strong correlation to the number of hydrogen‐bonding moieties in the oligomers. There also existed a threshold value of the number of CONH units, above which all the oligomers became organogelators. Hence, oligomers with ≤4 CONH units are devoid of intermolecular hydrogen bonding and also non‐organogelating, whereas those that contain >4 CONH units show intermolecular association and organogelating properties. For the organogelators, the T_gel value increases monotonically with increasing number of CONH units. On the basis of FTIR measurements, both the CONH and triazole C? H groups were involved in the hydrogen‐bonding process. A mixed xerogel that consisted of a 1:1 weight ratio of two oligomers of different lengths ( OAT‐CO₂H‐6 and OAT‐CO₂H‐12 ) was found to show microphase segregation according to differential scanning calorimetry, thus indicating that oligomers that bear a different number of hydrogen‐bonding units exhibited self‐sorting to maximize the extent of intermolecular hydrogen bonding in the xerogel state.     

Desymmetrization of 3,3′‐Bis(acylamino)‐2,2′‐bipyridine‐Based Discotics: The High Fidelity of Their Self‐Assembly Behavior in the Liquid‐Crystalline State and in Solution

Two novel nonsymmetrical disc‐shaped molecules 1 and 2 based on 3,3′‐bis(acylamino)‐2,2′‐bipyridine units were synthesized by means of a statistical approach. Discotic 1 possesses six chiral dihydrocitronellyl tails and one peripheral phenyl group, whereas discotic 2 possesses six linear dodecyloxy tails and one peripheral pyridyl group. Preorganization by strong intramolecular hydrogen bonding and subsequent aromatic interactions induce self‐assembly of the discotics. Liquid crystallinity of 1 and 2 was determined with the aid of polarized optical microscopy, differential scanning calorimetry, and X‐ray diffraction. Two columnar rectangular mesophases (Col_r) have been identified, whereas for C₃‐symmetrical derivatives only one Col_r mesophase has been found. 1 In solution, the molecularly dissolved state in chloroform was studied with ¹H NMR spectroscopy, whereas the self‐assembled state in apolar solution was examined with optical spectroscopy. Remarkably, these desymmetrized discotics, which lack one aliphatic wedge, behave similar to the symmetric parent compound. To prove that the stacking behavior of discotics 1 and 2 is similar to that of reported C₃‐symmetrical derivatives, a mixing experiment of chiral 1 with C₃‐symmetrical 13 has been undertaken; it has shown that they indeed belong to one type of self‐assembly. This helical J‐type self‐assembly was further confirmed with UV/Vis and photoluminescence (PL) spectroscopy. Eventually, disc 2 , functionalized with a hydrogen‐bonding acceptor moiety, might perform secondary interactions with molecules such as acids.     

Control of H‐ and J‐Type π Stacking by Peripheral Alkyl Chains and Self‐Sorting Phenomena in Perylene Bisimide Homo‐ and Heteroaggregates

The synthesis, self‐assembly, and gelation ability of a series of organogelators based on perylene bisimide (PBI) dyes containing amide groups at imide positions are reported. The synergetic effect of intermolecular hydrogen bonding among the amide functionalities and π–π stacking between the PBI units directs the formation of the self‐assembled structure in solution, which beyond a certain concentration results in gelation. Effects of different peripheral alkyl substituents on the self‐assembly were studied by solvent‐ and temperature‐dependent UV‐visible and circular dichroism (CD) spectroscopy. PBI derivatives containing linear alkyl side chains in the periphery formed H‐type π stacks and red gels, whereas by introducing branched alkyl chains the formation of J‐type π stacks and green gels could be achieved. Sterically demanding substituents, in particular, the 2‐ethylhexyl group completely suppressed the π stacking. Coaggregation studies with H‐ and J‐aggregating chromophores revealed the formation of solely H‐type π stacks containing both precursor molecules at a lower mole fraction of J‐aggregating chromophore. Beyond a critical composition of the two chromophores, mixed H‐aggregate and J‐aggregate were formed simultaneously, which points to a self‐sorting process. The versatility of the gelators is strongly dependent on the length and nature of the peripheral alkyl substituents. CD spectroscopic studies revealed a preferential helicity of the aggregates of PBI building blocks bearing chiral side chains. Even for achiral PBI derivatives, the utilization of chiral solvents such as (R)‐ or (S)‐limonene was effective in preferential population of one‐handed helical fibers. AFM studies revealed the formation of helical fibers from all the present PBI gelators, irrespective of the presence of chiral or achiral side chains. Furthermore, vortex flow was found to be effective in macroscopic orientation of the aggregates as evidenced from the origin of CD signals from aggregates of achiral PBI molecules.     

Orthogonal Halogen‐Bonding‐Driven 3D Supramolecular Assembly of Right‐Handed Synthetic Helical Peptides

Peptide‐mediated self‐assembly is a prevalent method for creating highly ordered supramolecular architectures. Herein, we report the first example of orthogonal C?X???X?C/C?X???π halogen bonding and hydrogen bonding driven crystalline architectures based on synthetic helical peptides bearing hybrids of l ‐sulfono‐γ‐AApeptides and natural amino acids. The combination of halogen bonding, intra‐/intermolecular hydrogen bonding, and intermolecular hydrophobic interactions enabled novel 3D supramolecular assembly. The orthogonal halogen bonding in the supramolecular architecture exerts a novel mechanism for the self‐assembly of synthetic peptide foldamers and gives new insights into molecular recognition, supramolecular design, and rational design of biomimetic structures.     

Self‐assembly and secondary structures of linear polypeptides tethered to polyhedral oligomeric silsesquioxane nanoparticles through click chemistry

In this study, we used click chemistry to synthesize a new macromolecular self‐assembling building blocks, linear polypeptide‐b‐polyhedral oligomeric silsesquioxane (POSS) copolymers, from a mono‐azido–functionalized POSS (N₃‐POSS) and several alkyne‐poly(γ‐benzyl‐L ‐glutamate) (alkyne‐PBLG) systems. The incorporation of the POSS unit at the chain end of the PBLG moiety allowed intramolecular hydrogen bonding to occur between the POSS and PBLG units, thereby enhancing the α‐helical conformation in the solid state, as determined through Fourier transform infrared spectroscopy and wide‐angle X‐ray diffraction analyses. POSS‐b‐PBLG underwent hierarchical self‐assembly, characterized using small‐angle X‐ray scattering, to form a bilayer‐like nanostructure featuring α‐helical or β‐sheet conformations and POSS aggregates. Thermogravimetric analysis indicated that the thermal degradation temperature increased significantly after incorporation of the POSS moiety, which presumably formed an inorganic protection layer on the nanocomposite's surface. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Hydrogen‐Bonding‐Mediated Vesicular Assembly of Functionalized Naphthalene–Diimide‐Based Bolaamphiphile and Guest‐Induced Gelation in Water

This paper describes the spontaneous vesicular assembly of a naphthalene–diimide (NDI)‐based non‐ionic bolaamphiphile in aqueous medium by using the synergistic effects of π‐stacking and hydrogen bonding. Site isolation of the hydrogen‐bonding functionality (hydrazide), a strategy that has been adopted so elegantly in nature, has been executed in this system to protect these moieties from the bulk water so that the distinct role of hydrogen bonding in the self‐assembly of hydrazide‐functionalized NDI building blocks could be realized, even in aqueous solution. Furthermore, the electron‐deficient NDI‐based bolaamphiphile could engage in donor–acceptor (D–A) charge‐transfer (CT) interactions with a water‐insoluble electron‐rich pyrene donor by virtue of intercalation of the latter chromophore in between two NDI building blocks. Remarkably, even when pyrene was located between two NDI blocks, intermolecular hydrogen‐bonding networks between the NDI‐linked hydrazide groups could be retained. However, time‐dependent AFM studies revealed that the radius of curvature of the alternately stacked D–A assembly increased significantly, thereby leading to intervesicular fusion, which eventually resulted in rupturing of the membrane to form 1D fibers. Such 2D‐to‐1D morphological transition produced CT‐mediated hydrogels at relatively higher concentrations. Instead of pyrene, when a water‐soluble carboxylate‐functionalized pyrene derivative was used as the intercalator, non‐covalent tunable in‐situ surface‐functionalization could be achieved, as evidenced by the zeta‐potential measurements.     

Orthogonally Self‐Assembled Multifunctional Block Copolymers

We report the synthesis of telechelic poly(norbornene) and poly(cyclooctene) homopolymers by ring‐opening metathesis polymerization (ROMP) and their subsequent functionalization and block copolymer formation based on noncovalent interactions. Whereas all the poly(norbornene)s contain either a metal complex or a hydrogen‐bonding moiety along the polymer side‐chains, together with a single hydrogen‐bonding‐based molecular recognition moiety at one terminal end of the polymer chain. These homopolymers allow for the formation of side‐chain‐functionalized AB and ABA block copolymers through self‐assembly. The orthogonal natures of all side‐ and main‐chain self‐assembly events were demonstrated by ¹H NMR spectroscopy and isothermal titration calorimetry. The resulting fully functionalized block copolymers are the first copolymers combining both side‐ and main‐chain self‐assembly, thereby providing a high degree of control over copolymer functionalization and architecture and bringing synthetic materials one step closer to the dynamic self‐assembly structures found in nature.     

A Carbon Dioxide Bubble‐Induced Vortex Triggers Co‐Assembly of Nanotubes with Controlled Chirality

It is challenging to prepare co‐organized nanotube systems with controlled nanoscale chirality in an aqueous liquid flow field. Such systems are responsive to a bubbled external gas. A liquid vortex induced by bubbling carbon dioxide (CO₂) gas was used to stimulate the formation of nanotubes with controlled chirality; two kinds of achiral cationic building blocks were co‐assembled in aqueous solution. CO₂‐triggered nanotube formation occurs by formation of metastable intermediate structures (short helical ribbons and short tubules) and by transition from short tubules to long tubules in response to chirality matching self‐assembly. Interestingly, the chirality sign of these assemblies can be selected for by the circulation direction of the CO₂ bubble‐induced vortex during the co‐assembly process.     

An Enantiopure Hydrogen‐Bonded Octameric Tube: Self‐Sorting and Guest‐Induced Rearrangement

The assembly of a discrete hydrogen‐bonded molecular tube from eight small identical monomers is reported. Tube assembly was accomplished by means of selective heterodimerization between isocytosine and ureidopyrimidinone hydrogen‐bonding motifs embedded in an enantiopure bicyclic building block, leading to the selective formation of an octameric supramolecular tube. Upon introduction of a fullerene guest molecule, the octameric tube rearranges into a tetrameric inclusion complex and the hydrogen‐bonding mode is switched. The dynamic behavior of the system is further explored in solvent‐ and guest‐responsive self‐sorting experiments.     

Thioamides: Versatile Bonds To Induce Directional and Cooperative Hydrogen Bonding in Supramolecular Polymers

The amide bond is a versatile functional group and its directional hydrogen‐bonding capabilities are widely applied in, for example, supramolecular chemistry. The potential of the thioamide bond, in contrast, is virtually unexplored as a structuring moiety in hydrogen‐bonding‐based self‐assembling systems. We report herein the synthesis and characterisation of a new self‐assembling motif comprising thioamides to induce directional hydrogen bonding. N,N′,N′′‐Trialkylbenzene‐1,3,5‐tris(carbothioamide)s (thioBTAs) with either achiral or chiral side‐chains have been readily obtained by treating their amide‐based precursors with P₂S₅. The thioBTAs showed thermotropic liquid crystalline behaviour and a columnar mesophase was assigned. IR spectroscopy revealed that strong, three‐fold, intermolecular hydrogen‐bonding interactions stabilise the columnar structures. In apolar alkane solutions, thioBTAs self‐assemble into one‐dimensional, helical supramolecular polymers stabilised by three‐fold hydrogen bonding. Concentration‐ and temperature‐dependent self‐assembly studies performed by using a combination of UV and CD spectroscopy demonstrated a cooperative supramolecular polymerisation mechanism and a strong amplification of supramolecular chirality. The high dipole moment of the thioamide bond in combination with the anisotropic shape of the resulting cylindrical aggregate gives rise to sufficiently strong depolarised light scattering to enable depolarised dynamic light scattering (DDLS) experiments in dilute alkane solution. The rotational and translational diffusion coefficients, D_trans and D_rot, were obtained from the DDLS measurements, and the average length, L, and diameter, d, of the thioBTA aggregates were derived (L=490 nm and d=3.6 nm). These measured values are in good agreement with the value L_w=755 nm obtained from fitting the temperature‐dependent CD data by using a recently developed equilibrium model. This experimental verification validates our common practice for determining the length of BTA‐based supramolecular polymers from model fits to experimental CD data. The ability of thioamides to induce cooperative supramolecular polymerisation makes them effective and broadly applicable in supramolecular chemistry.     

Single chain self‐assembly of well‐defined heterotelechelic polymers generated by ATRP and click chemistry revisited

Well‐defined heterotelechelic poly(styrene) carrying thymine/diaminopyridine (DAP) (M_n,SEC = 9300, PDI = 1.04) and Hamilton wedge (HW)/cyanuric acid (CA) (M_n,SEC = 8200, PDI = 1.04) bonding motifs are prepared via a combination of controlled/living radical polymerization and copper catalyzed azide/alkyne “click” chemistry and are subsequently self‐assembled as single chains to emulate—on a simple level—the self‐folding behavior of natural biomacromolecules. Hydrogen nuclear magnetic resonance (¹H NMR) in deuterated dichloromethane and dynamic light scattering analyses provides evidence for the hydrogen bonding interactions between the α‐thymine and ω‐DAP as well as α‐CA and ω‐HW chain ends of the heterotelechelic polymers leading to circular entropy driven single chain self‐assembly. This study demonstrates that the choice of NMR solvent is important for obtaining well‐resolved NMR spectra of the self‐assembled structures. In addition, steric effects on the HW can affect the efficiency of the self‐assembly process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011