Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H Interactions in Self‐Assembled 1,5‐Dichloroanthraquinone Layers on Au(111)

The role of halogen bonds in self‐assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5‐dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl???H and O???H hydrogen bonds with marginal contributions from Cl‐related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br‐ and I‐related halogen bonds, Cl‐related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.     

Chiral J‐Aggregates of Atropo‐Enantiomeric Perylene Bisimides and Their Self‐Sorting Behavior

Herein we report on structural, morphological, and optical properties of homochiral and heterochiral J‐aggregates that were created by nucleation–elongation assembly of atropo‐enantiomerically pure and racemic perylene bisimides (PBIs), respectively. Our detailed studies with conformationally stable biphenoxy‐bridged chiral PBIs by UV/Vis absorption, circular dichroism (CD) spectroscopy, and atomic force microscopy (AFM) revealed structurally as well as spectroscopically quite different kinds of J‐aggregates for enantiomerically pure and racemic PBIs. AFM investigations showed that enantiopure PBIs form helical nanowires of unique diameter and large length‐to‐width ratio by self‐recognition, while racemic PBIs provide irregular‐sized particles by self‐discrimination of the enantiomers at the stage of nucleation. Steady‐state fluorescence spectroscopy studies revealed that the photoluminescence efficiency of homochiral J‐aggregated nanowires (47±3 %) is significantly higher than that of heterochiral J‐aggregated particle‐like aggregates (12±3 %), which is explained in terms of highly ordered molecular stacking in one‐dimensional nanowires of homochiral J‐aggregates. Our present results demonstrate the high impact of homochirality on the construction of well‐defined nanostructures with unique optical properties.     

Self‐Assembly of α‐Helical Polypeptides Driven by Complex Coacervation

Reported is the ability of α‐helical polypeptides to self‐assemble with oppositely‐charged polypeptides to form liquid complexes while maintaining their α‐helical secondary structure. Coupling the α‐helical polypeptide to a neutral, hydrophilic polymer and subsequent complexation enables the formation of nanoscale coacervate‐core micelles. While previous reports on polypeptide complexation demonstrated a critical dependence of the nature of the complex (liquid versus solid) on chirality, the α‐helical structure of the positively charged polypeptide prevents the formation of β‐sheets, which would otherwise drive the assembly into a solid state, thereby, enabling coacervate formation between two chiral components. The higher charge density of the assembly, a result of the folding of the α‐helical polypeptide, provides enhanced resistance to salts known to inhibit polypeptide complexation. The unique combination of properties of these materials can enhance the known potential of fluid polypeptide complexes for delivery of biologically relevant molecules.     

Kinetic Control of the Supramolecular Chirality of Porphyrin J‐Aggregates

The aggregation of achiral sulfonatophenyl‐ and phenyl‐meso‐substituted diprotonated porphyrins to chiral J‐aggregates is a hierarchical noncovalent polymerization process preceded by a critical nucleation stage. This allows significant enantiomeric excesses by the formation of a few primary nuclei and the control of their growth by the effect that flows (imperfect mixing) have on the secondary nucleation of the J‐aggregate particles. In addition, the results strongly suggest that when only one species of aggregate predominates, the CD signals of the three excitonic bands in the visible region (around 420, 490, and 700 nm) show the same sign. Thus, differences on their relative sign would be due to the presence of different species.     

Selective Synthesis of Single‐ and Multi‐Walled Supramolecular Nanotubes by Using Solvophobic/Solvophilic Controls: Stepwise Radial Growth via “Coil‐on‐Tube” Intermediates

Novel hexa‐peri‐hexabenzocoronene (HBC) derivatives, ^FHBC and ^FHBC*, which carry perfluoroalkyl segments on one side of the HBC core and long alkyl tails on the other, were synthesized. Their perfluoroalkyl segments are highly solvated in C₆F₆ (solvophilic effect) and do not assemble, whereas in CH₂Cl₂, they are excluded (solvophobic effect) and assemble together consequently. For example, the use of C₆F₆ and CH₂Cl₂ as assembling media for ^FHBC leads to the selective formation of single‐ and multi‐walled nanotubes, respectively. When a higher monomer concentration is applied in CH₂Cl₂, multi‐walled nanotubes with a larger number of walls result. ^FHBC in CH₂Cl₂ self‐assembles rather slowly, thereby allowing for the observation of coil‐on‐tube structures, which are possible intermediates for the stepwise radial growth of the nanotubular wall. Casting of the multi‐walled nanotubes onto a quartz plate yields a superhydrophobic thin film with a water contact angle of 161±2°.     

Dynamic Supramolecular Polymers Based on Benzene‐1,3,5‐tricarboxamides: The Influence of Amide Connectivity on Aggregate Stability and Amplification of Chirality

N‐Centred benzene‐1,3,5‐tricarboxamides (N‐BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid‐state behaviour and self‐assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X‐ray diffraction revealed that the chiral N‐BTA derivatives with branched 3,7‐dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Col_ho. In contrast, N‐BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable‐temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N‐BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N‐BTAs revealed a cooperative self‐assembly behaviour of the N‐BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants‐and‐soldiers as well as the majority‐rules principles were operative in stacks of N‐BTAs. In fact, the self‐assembly of N‐BTAs resembles closely that of their carbonyl (C?O)‐centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self‐assembly of N‐ and C?O‐BTAs were analysed by density functional theory (DFT) calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of N‐BTAs. The lower interaction energy is due to the higher energy penalty for rotation around the Ph? NH bond compared to the Ph? CO bond and the diminished magnitude of dipole–dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N‐BTAs and C?O BTAs.     

Remote Control of the Planar Chirality in Peptide‐Bound Metallomacrocycles and Dynamic‐to‐Static Planar Chirality Control Triggered by Solvent‐Induced 310‐to‐α‐Helix Transitions

The dynamic planar chirality in a peptide‐bound Ni^II‐salphen‐based macrocycle can be remotely controlled. First, a right‐handed (P)‐3₁₀‐helix is induced in the dynamic helical oligopeptides by a chiral amino acid residue far from the macrocyclic framework. The induced planar chirality remains dynamic in chloroform and acetonitrile, but is almost completely locked in fluoroalcohols as a result of the solvent‐induced transition of the peptide chains from a 3₁₀‐helix to a wider α‐helix, which freezes the rotation of the pendant peptide units around the macrocycle.     

Herringbone and Helical Self‐Assembly of π‐Conjugated Molecules in the Solid State through CH/π Hydrogen Bonds

Self‐organization of organic molecules through weak noncovalent forces such as CH/π interactions and creation of large hierarchical supramolecular structures in the solid state are at the very early stage of research. The present study reports direct evidence for CH/π interaction driven hierarchical self‐assembly in π‐conjugated molecules based on custom‐designed oligophenylenevinylenes (OPVs) whose structures differ only in the number of carbon atoms in the tails. Single‐crystal X‐ray structures were resolved for these OPV synthons and the existence of long‐range multiple‐arm CH/π interactions was revealed in the crystal lattices. Alignment of these π‐conjugated OPVs in the solid state was found to be crucial in producing either right‐handed herringbone packing in the crystal or left‐handed helices in the liquid‐crystalline mesophase. Pitch‐ and roll‐angle displacements of OPV chromophores were determined to trace the effect of the molecular inclination on the ordering of hierarchical structures. Furthermore, circular dichroism studies on the OPVs were carried out in the aligned helical structures to prove the existence of molecular self‐assembly. Thus, the present strategy opens up new approaches in supramolecular chemistry based on weak CH/π hydrogen bonding, more specifically in π‐conjugated materials.     

Tuning the Chirality of Block Copolymers: From Twisted Morphologies to Nanospheres by Self‐Assembly

New advances into the chirality effect in the self‐assembly of block copolymers (BCPs) have been achieved by tuning the helicity of the chiral‐core‐forming blocks. The chiral BCPs {[N?P(R)‐O₂C₂₀H₁₂]_200?x[N?P(OC₅H₄N)₂]_x}‐b‐ [N?PMePh]₅₀ ((R)‐O₂C₂₀H₁₂=(R)‐1,1′‐binaphthyl‐2,2′‐dioxy, OC₅H₄N=4‐pyridinoxy (OPy); x=10, 30, 60, 100 for 3 a – d , respectively), in which the [N?P(OPy)₂] units are randomly distributed within the chiral block, have been synthesised. The chiroptical properties of the BCPs ([α]_D vs. T and CD) demonstrated that the helicity of the BCP chains may be simply controlled by the relative proportion of the chiral and achiral (i.e., [N?P(R)‐O₂C₂₀H₁₂] and [N?P(OPy)₂], respectively) units. Thus, although 3 a only contained only 5 % [N?P(OPy)₂] units and exhibited a preferential helical sense, 3 d with 50 % of this unit adopted non‐preferred helical conformations. This gradual variation of the helicity allowed us to examine the chirality effect on the self‐assembly of chiral and helical BCPs (i.e., 3 a – c ) and chiral but non‐helical BCPs (i.e., 3 d ). The very significant influence of the helicity on the self‐assembly of these materials resulted in a variety of morphologies that extend from helical nanostructures to pearl‐necklace aggregates and nanospheres (i.e., 3 b and 3 d , respectively). We also demonstrate that the presence of pyridine moieties in BCPs 3 a – d allows specific decoration with gold nanoparticles.     

Chemistry of Anthracene–Acetylene Oligomers XXV: On‐Surface Chirality of a Self‐Assembled Molecular Network of a Fan‐Blade‐Shaped Anthracene–Acetylene Macrocycle with a Long Alkyl Chain

An anthracene cyclic dimer with two different linkers and a dodecyl group was synthesized by means of coupling reactions. The calculated structure had a planar macrocyclic π core and a linear alkyl chain. Scanning tunneling microscopy observations at the 1‐phenyloctane/graphite interface revealed that the molecules formed a self‐assembled monolayer that consisted of linear striped bright and dark bands. In each domain, the molecular network consisted of either Re or Si molecules that differed in the two‐dimensional chirality about the macrocyclic faces, which led to a unique conglomerate‐type self‐assembly. The molecular packing mode and the conformation of the alkyl chains are discussed in terms of the intermolecular interactions and the interactions between the molecules and the graphite surface with the aid of MM3 simulations of a model system.     

Streptococcal Hyaluronate Lyase Reveals the Presence of a Structurally Significant CH⋅⋅⋅O Hydrogen Bond

Carbon‐donated hydrogen bonds (CDHBs) are weak forms of hydrogen bonding (0.5–1.0 kcal mol^?1) that are difficult to detect, and thus their roles in the structure and functionality of chemical systems often go unrecognized. Utilizing a computational approach, the existence of a structurally significant CDHB in the medically relevant protein Streptococcus pneumoniae hyaluronate lyase (SpnHL) is affirmed. The structure of a tetrapeptide fragment model containing the CDHB was optimized with second‐order perturbation theory. From this, a CDHB with bond distance and angle consistent with previously discovered CDHBs and comparable to neighboring traditional HBs in the fragment model was found. The CDHB competes with another donor T253 OH, whereby the two alternate in strength between protein conformations, imbuing αHelix 3 appreciable flexibility. The CDHB seems to exist in spite of torsional and steric strain on the donor methyl group. It is postulated that the CDHB could aid in either counteracting the macrodipole of αHelix 3 or protecting the A249 CO from destabilizing interactions with the adjacent solvent. Employing the energy gradients from the optimization, the torque generated by the fragment model was computed, which accurately predicts the direction of rotation of αHelix 3 observed from experiment. A strongly correlated motion between αHelix 3 and αHelices 2, 4, and 5 was noted, which the interactions of the fragment model help drive by generating a torque much larger than necessary to rotate just αHelix 3. Considering these results, we conclude that CDHBs should be considered as possible beneficial components of chemical and biological phenomena.     

Synthesis and Aggregation Behavior of meso‐Sulfinylporphyrins: Evaluation of S‐Chirality Effects on the Self‐Organization to S–Oxo‐Tethered Cofacial Porphyrin Dimers

The synthesis and aggregation behavior of meso‐sulfinylporphyrins are described. The copper‐catalyzed C–S cross‐coupling reaction of a meso‐iodoporphyrin with benzenethiol and n‐octanethiol has proved to be an efficient method for the synthesis of meso‐sulfanylporphyrins, which are oxygenated by m‐chloroperbenzoic acid to produce the corresponding meso‐sulfinylporphyrins. Optically active zinc meso‐sulfinylporphyrins were successfully isolated by means of optical resolution of the racemates on a chiral HPLC column. Zinc sulfinylporphyrins readily undergo self‐organization through S–oxo–zinc coordination to form cofacial porphyrin dimers in solution, in which the hetero‐ and homodimers are present as a diastereomeric mixture. The aggregation modes of the S–oxo‐tethered porphyrin dimers were fully characterized by ¹H NMR, IR, and UV/Vis spectroscopy as well as DFT calculations on their model compounds, thus revealing that the self‐aggregation behavior depends on the combination of S chirality. The absolute configurations at the sulfur center can be determined by the exciton‐coupled CD method. The observed self‐association constant for the S–oxo‐tethered dimerization of (S)‐phenylsulfinylporphyrin in toluene is larger than that in dichloromethane, which reflects the difference in dipole moments between the homodimer and the monomer. In cyclic and differential pulse voltammetry, the first oxidation process of the cofacial dimers is split into two reversible steps, which indicates that the initially produced π radical cations are delocalized efficiently between the two porphyrin rings. The present findings demonstrate the potential utility of meso‐sulfinyl groups as promising ligands for investigating the effects of peripheral chirality on the structures and optical and electrochemical properties of metal‐assisted porphyrin self‐assemblies.     

Cyclotrimerization‐Induced Chiral Supramolecular Structures of 4‐Ethynyltriphenylamine on Au(111) Surface

Cyclotrimerization‐induced chiral supramolecular structures of 4‐ethynyltriphenylamine (ETPA) have been synthesized on the Au(111) surface through alkyne‐based reactions. Whereas the ETPA molecules adsorbed on the Au(111) surface remain inert and form a close‐packed self‐assembled structure at room temperature, the combination of scanning tunneling microscopy observations and theoretical calculations unambiguously reveal that the ETPA molecules cyclotrimerize to form new trimer‐like species—1,3,5‐tris[4‐(diphenylamino)phenyl]benzene (TPAPB)—after annealing at 323 K. Further annealing drives these cyclotrimerized TPAPB molecules to form chiral hexagonal supramolecular structures with an extraordinary self‐healing ability.     

Twisted Morphologies and Novel Chiral Macroporous Films from the Self‐Assembly of Optically Active Helical Polyphosphazene Block Copolymers

A series of optically active helical polyphosphazene block copolymers of general formula R? [N?P(O₂C₂₀H₁₂)]_n‐b‐[N?PMePh]_m (R‐ 7 a – c ) was synthesized and characterized. The polymers were prepared by sequential living cationic polycondensation of N‐silylphosphoranimines using the mono‐end‐capped initiator [Ph₃P?N?PCl₃][PCl₆] ( 5 ) and exhibit a low polydispersity index (ca. 1.3). The temperature dependence of the specific optical activity ([α]_D) of R‐ 7 a , b relative to that for the homopolymers R‐[N?P(O₂C₂₀H₁₂)]_n (R‐ 8 a ) and the R/S analogues (R/S‐ 7 a , b ), revealed that the binaphthoxy–phosphazene segments induce a preferential helical conformation in the [N?PMePh] blocks through a “sergeant‐and‐soldiers” mechanism, an effect that is unprecedented in polyphosphazenes. The self‐assembly of drop‐cast thin films of the chiral block copolymer R‐ 7 b (bearing a long chiral and rigid R? [N?P(O₂C₂₀H₁₂)] segment) evidenced a transfer of helicity mechanism, leading to the formation of twisted morphologies (twisted “pearl necklace”), not observed in the nonchiral R/S‐ 7 b . The chiral R‐ 7 a and the nonchiral R/S‐ 7 a , self‐assemble by a nondirected morphology reconstruction process into regular‐shaped macroporous films with chiral‐rich areas close to edge of the pore. This is the first nontemplate self‐assembly route to chiral macroporous polymeric films with pore size larger than 50 nm. The solvent annealing (THF) of these films leads to the formation of regular spherical nanostructures (ca. 50 nm), a rare example of nanospheres exclusively formed by synthetic helical polymers.     

Controlled Stability of the Triple‐Stranded Helical Structure of a β‐1,3‐Glucan with a Chromophoric Aromatic Moiety at a Peripheral Position

We synthesized a semiartificial β‐1,3‐glucan, curdlan with dialkylaniline groups (CUR‐DA), that bears chromophoric aromatic groups at its peripheral positions. Spectroscopic studies as well as microscopic observations indicate that CUR‐DA adopts a triple‐stranded helical structure in water‐ or methanol‐rich solutions of dimethyl sulfoxide (DMSO). This triple‐stranded helical structure exhibits high thermal stability and resistance to base, attributes that are similar to those of the triple‐stranded helical structure of native β‐1,3‐glucans such as schizophyllan. Moreover, we found that the stability of the triple‐stranded helical structure can be easily modulated by solvent composition and metal‐ion (Zn²⁺) binding. As β‐1,3‐glucan polysaccharides are known to serve as “polymeric” hosts, including certain DNA molecules, carbon nanotubes, and conjugated polymers, and complexation occurs only with the single‐stranded structure, this information is very useful for the creation of these attractive polymeric composites, the controlled release of DNA, and so on.     

Concentration induced the interfacial self‐assembly polymorphism of 4, 4′‐dihexadecyloxy‐benzophenon by scanning tunneling microscopy

The concentration effect on a two‐dimensional (2D) self‐assembly of 4, 4′‐dihexadecyloxy‐benzophenon (DHB) has been investigated by scanning tunneling microscopy. The self‐assembly of DHB at the phenyloctane/graphite interface was concentration dependent. Under low concentration, the DHB molecules were adsorbed intactly on the graphite surface. With the increasing of concentration, one of side chains connecting the conjugated moiety stretched into the liquid phase. The coexistence of two self‐assembled structures was observed in a moderate concentration. The result indicated that the van der Waals interactions between the molecules and the graphite lattice were decreasing with the increasing concentration. After the samples were placed in ambient conditions over 24 h, a different self‐assembled structure was obtained on the gas/solid interface, in which the DHB molecules were adsorbed on the surface with only one of the side chains. Both the benzophenon core and the other side chain were extended to the gas phase. The results demonstrated that concentration played an important role in forming the 2D molecular self‐assembly and provided an efficient approach for the control of the DHB molecular nanostructure. Copyright © 2013 John Wiley & Sons, Ltd.     

Concentration controlled multilevel self‐assembly of 3‐armed poly(ethylene glycol)‐b‐poly(ε‐caprolactone) block copolymers investigated by AFM

Concentration dependent morphology of 3‐armed poly(ethylene glycol)‐b‐poly(ε‐caprolactone) copolymer aggregates in aqueous system was investigated by atomic force microscopy (AFM). The AFM results show that, at a low concentration, 4 × 10^?5 g/mL, spherical micelles occur, and unmicellized molecules are not distributed homogeneously in the copolymer aqueous solution. Unequal outspread clusters composed of wormlike aggregates are formed at a moderate copolymer concentration, 4 × 10^?4 g/mL, those wormlike aggregates are orderly packed in the clusters. At a high concentration of 0.05 g/mL, the copolymer aqueous system is indeed a gel at room temperature, outspread clusters of wormlike aggregates join together to forma network structure. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1412–1418, 2008