Supramolecular Assembly of C3 Peptidic Molecules into Helical Polymers

Self‐assembly of C₃ discotic molecules bearing dipeptide pendants into helical supramolecular polymers is investigated. The dipeptides are constituted from glycine and alanine with altered sequence, aiming at modulating the steric hindrance and examining the steric effects on the assembly. This steric hindrance effect is further illustrated with a dipeptide formed from glycine and valine, which carries a much larger isopropyl side unit. Their supramolecular polymerization is examined in various organic solvents and at different temperatures. The assembly morphology is directly visualized with atomic force microscopy. It is found that small changes in the dipeptide motifs in combination with solvent structure and the solution concentrations lead to different expression of the supramolecular assembly.

     

Impact of NDI-Core Substitution on the pH-Responsive Nature of Peptide-Tethered Luminescent Supramolecular Polymers

The pH-responsive nature of two self-assembled NDI-peptide amphiphile conjugates is reported. The diethoxy substituted NDI showed a pH-dependent assembly behaviour, as expected. In contrast, the isopropylamino- and ethoxy-substituted NDI based supramolecular polymer was stable at acidic and basic aqueous conditions. This finding highlights how subtle changes in the molecular design of π-stacked chromophore-peptide conjugates have a drastic impact on their equilibrium structure and ultimately functional properties.     

Chiral amplification in macromolecular helicity assisted by noncovalent interaction with achiral amines and memory of the helical chirality

Poly[(4-carboxyphenyl)acetylene] (poly-1) exhibits an intense induced circular dichroism (ICD) in the UV-visible region upon complexation with excess (R)-1-(1-naphthyl)ethylamine ((R)-2), owing to the formation of a predominantly single-handed helical conformation of the polymer backbone. In the presence of a small amount of (R)-2, poly-1 showed a very weak ICD due to the lack of a single-handed helical conformation. However, we have found that the co-addition of the excess bulky, achiral 1-naphthylmethylamine (5) with a small amount of (R)-2 caused a dramatic increase in the ICD magnitude, comparable to the full ICD induced by excess (R)-2. This indicates that an almost single-handed helix can be induced on poly-1 upon complexation with a small amount of (R)-2 assisted by achiral 5. Furthermore, the induced single-handed helical poly-1 could be successfully memorized by the replacement of (R)-2 and 5 with achiral 2-aminoethanol or n-butylamine.     

Influence of Metal,Ligand and Solvent on Supramolecular Polymerizations with Transition‐Metal Compounds: A Theoretical Study

The nature of intermolecular interactions governing supramolecular polymerizations is very important for controlling their cooperativity. In order to address this problem, supramolecular columns made of Pt^II and Pd^II complexes of oligo(phenylene ethynylene)‐based pyridine (OPE) and tetrazolylpyridine ligands (TEP) were investigated through the dispersion‐corrected PM6 method. Aromatic, CH–π, M–Cl and metallophilic interactions helped stabilize the supramolecules studied, and their geometries and associated cooperativities were in excellent agreement with experimental data. The OPE ligand and/or the presence of Pt^II led to stronger metallophilic interactions and also to cooperative supramolecular polymerizations, which clearly suggests that metallophilic interactions are a key factor for controlling cooperativity. The results indicate that sequential monomer addition is in general less spontaneous than the combination of two larger preformed stacks. The present theoretical investigations contribute to the further understanding of the relation between the thermodynamics of supramolecular polymerizations and the nature of different synthons.     

Multiple‐Stimulus‐Responsive Supramolecular Gels and Regulation of Chiral Twists: The Effect of Spacer Length

A new class of homologous gelators, LG₁₂‐(CH₂)_n‐BSA, composed of bipyridinyl groups, L ‐glutamic moieties having double dodecyl chains, and linked alkyl spacers with different lengths were synthesized. It was found that these gelators could immobilize medium‐polarity solvents readily and the behaviors of these gels showed a dependence on the spacer length. Of all the gels, the LG₁₂‐(CH₂)₁₁‐BSA gels exhibited self‐healing property and multiple‐stimulus responsibility, such as heating, shaking, and sonication. The investigation of CD spectra indicated that the supramolecular chirality, which was attributed to the chiral transfer from the chiral center to the assemblies, was also closely related to the length of methylene spacers. The longer the alkyl spacers, the weaker the transmitted supramolecular chirality. Only LG₁₂‐(CH₂)₁‐BSA gelators, which had the shortest spacers, formed right‐handed nanoscale chiral twists owing to crowded hydrogen bonding interactions. Moreover, the high‐polarity solvent DMF was found to be able to regulate the chiral twist as well as its pitch length readily.     

Elucidating the Supramolecular Copolymerization of N- and C-Centered Benzene-1,3,5-Tricarboxamides: The Role of Parallel and Antiparallel Packing of Amide Groups in the Copolymer Microstructure

An in-depth study of the supramolecular copolymerization behavior of N- and C-centered benzene-1,3,5-tricarboxamides (N- and C-BTAs) has been conducted in methylcyclohexane and in the solid state. The connectivity of the amide groups in the BTAs differs, and mixing N- and C-BTAs results in supramolecular copolymers with a blocky microstructure in solution. The blocky microstructure results from the formation of weaker and less organized, antiparallel hydrogen bonds between N- and C-BTAs. In methylcyclohexane, the helical threefold hydrogen-bonding network present in C- and N-BTAs is retained in the mixtures. In the solid state, in contrast, the hydrogen bonds of pure BTAs as well as their mixtures organize in a sheet-like pattern, and in the mixtures long-range order is lost. Drop-casting to kinetically trap the solution microstructures shows that C-BTAs retain the helical hydrogen bonds, but N-BTAs immediately adopt the sheet-like pattern, a direct consequence of the lower stabilization energy of the helical hydrogen bonds. In the copolymers, the stability of the helical aggregates depends on the copolymer composition, and helical aggregates are only preserved when a high amount of C-BTAs is present. The method outlined here is generally applicable to elucidate the copolymerization behavior of supramolecular monomers both in solution as well as in the solid state.     

Self-Assembly of Chiral Cyclohexanohemicucurbit[n]urils with Bis(Zn Porphyrin): Size,Shape, and Time-Dependent Binding

In order to investigate the ability of bis(zinc octaethylporphyrin) (bis–ZnOEP) to discriminate cyclohexanohemicucurbit[n]urils (cycHC[n]) of different shapes and sizes, the self-assembly of barrel-shaped chiral cycHC[n] with bis–ZnOEP was studied by various spectroscopic methods (absorption, fluorescence, circular dichroism (CD), and NMR). While the binding of 6-membered cycHC[6] induced a tweezer-like conformation followed by the formation of anti-form of bis–ZnOEP upon further addition of cycHC[6], the interaction of 8-membered cycHC[8] is more complex and proceeds through the featured syn-to-anti conformational change of bis–ZnOEP and further intermolecular self-assembly via multiple noncovalent associations between cycHC[8] and bis–ZnOEP. Whilst bis–porphyrins are known to be effective chemical sensors able to differentiate various guests based on their chirality via induced CD, their ability to sense small differences in the shape and size of relatively large macrocycles, such as chiral cycHC[6] and cycHC[8], is scarcely examined. Both studied complexes exhibited characteristic induced CD signals in the region of porphyrin absorption upon complexation.     

CD Spectroscopic Study on the Molecular Recognition of Chiral Salen－Metal Complexes

The molecular recognition behavior of the chiral salen-metal complexes towards guest molecules, such as imidazole derivatives and amino-acid ester, was systematically investigated by means of circular dichroism (CD) spectra. The coordination numbers of the host-guest complexes as well as the recognition capability of the salen-metal complexes were explained by character and intensity analyses of the CD spectra.     

Diels-Alder Reactivity of a Chiral Anthracene Template with Symmetrical and Unsymmetrical Dienophiles: A DFT Study

In this work, we used Density Functional Theory calculations to assess the factors that control the reactivity of a chiral anthracene template with three sets of dienophiles including maleic anhydrides, maleimides and acetoxy lactones in the context of Diels-Alder cycloadditions. The results obtained here (at the M06-2X/6-311++G(d,p) level of theory) suggest that the activation energies for maleic anhydrides and acetoxy lactones are dependent on the nature of the substituent in the dienophile. Among all studied substituents, only −CN reduces the energy barrier of the cycloaddition. For maleimides, the activation energies are independent of the heteroatom of the dienophile and the R group attached to it. The analysis of frontier molecular orbitals, charge transfer and the activation strain model (at the M06-2X/TZVP level based on M06-2X/6-311++G(d,p) geometries) suggest that the activation energies in maleic anhydrides are mainly controlled by the amount of charge transfer from the diene to the dienophile during cycloaddition. For maleimides, there is a dual control of interaction and strain energies on the activation energies, whereas for the acetoxy lactones the activation energies seem to be controlled by the degree of template distortion at the transition state. Finally, calculations show that considering a catalyst on the studied cycloadditions changes the reaction mechanism from concerted to stepwise and proceed with much lower activation energies.     

Liquid Crystallinity and Supramolecular Organization of Regioisomeric Isatin Derivatives: An Experimental and Theoretical Study

Spatially organized chromophores can be beneficial for advanced applications like for example, organic solar cells, laser technology or non‐linear optic devices as well as supramolecular photochemistry. Of particular interest are non‐static ordered forms of molecular organization as for example, liquid crystals. With this in mind we synthesised four new regioisomeric isatin derivatives by Suzuki–Miyaura coupling of 4‐dodecyloxyphenylboronic acid with all four possible regioisomers of bromoisatin. Liquid crystalline properties are found for 5‐(4‐dodecyloxyphenyl)isatin, while the other regioisomers do not display a mesomorphic behaviour. The synthesis, physicochemical investigations including polarization microscopy, differential scanning calorimetry and X‐ray investigations are discussed and accompanied with density functional theory calculations with respect to the target molecules and their possible H‐bonded aggregates. Two distinct setups of supramolecular assemblies for such isatin derivatives are discussed and a model for the mesophase is proposed.     

Molecular Assembly of an Achiral Phosphine and a Chiral Primary Amine: A Highly Efficient Supramolecular Catalyst for the Enantioselective Michael Reaction of Aldehydes with Maleimides

A combined catalyst system of a cinchonidine‐derived primary amine and triphenylphosphine (CD‐NH₂/PPh₃) exhibited high catalytic performance in the Michael reaction of aldehydes with maleimides, thereby affording the corresponding functionalized aldehydes in excellent yields (up to 99 %) and enantioselectivities (>99 % ee). More interestingly, the significance of the phosphine in enhancing the enantioselectivities in the chiral‐primary‐amine‐catalyzed Michael reaction was revealed. Furthermore, we explored the origin of the reaction mechanism in the Michael addition promoted by the dual organocatalytic system. On the basis of experimental results and spectroscopic analysis, such as UV/Vis, fluorescence emission (FL), NMR, and circular dichroism (CD) spectroscopy, as well as ESI‐MS, we found that the molecular assembly of phosphine and primary amine played a crucial role in this enantioselective reaction, in which a possible supramolecular complex was formed as an effective chiral catalyst through noncovalent molecular interactions of a cinchona alkaloid‐derived primary amine with triphenylphosphine.     

A Chiral-Label-Free SERS Strategy for the Synchronous Chiral Discrimination and Identification of Small Aromatic Molecules

A versatile and robust chiral discrimination strategy for small aromatic compounds is of significant importance in multidisciplinary fields. However, most current methods lack either the sufficient sensitivity for recognizing the chirality of the target molecules or their molecular specificity information. We have developed a versatile and chiral-label-free surface-enhanced Raman scattering (SERS)-based chiral discrimination sensing system for small aromatic molecules, where the molecular structural specificity and enantioselectivity can be given synchronously in a single SERS spectrum. More than 10 types of chiral aromatic molecules were successfully identified by using this system. This work has enormous potential for recognizing chiral products effectively in sophisticated systems, especially in the fields of chiral synthesis and chiral catalysis.     

Chiral Triazolium Salts and Ionic Liquids: From the Molecular Design Vectors to Their Physical Properties through Specific Supramolecular Interactions

An exhaustive experimental study based on X‐ray diffraction analysis, NMR, FTIR‐ATR (attenuated total reflection), and Raman spectroscopy as well as theoretical calculations is reported in order to understand how the non‐covalent intermolecular contacts are fundamental to explain structure–property relationships and allowing us to correlate a basic macroscopic property (i.e., the melting point, T_m) with the structural variables of a family of enantiopure 1,4‐dialkyl‐1,2,4‐triazolium salts. The effect of different structural vectors such as the ring size, the spatial disposition of the substituent, the substitution on the oxygen atom, the nature of the anion, or the N4 alkylation of the triazole on the intermolecular interactions of these chiral salts of a well‐defined 3D structure is reported. The non‐covalent intermolecular contacts mainly implicating the triazolium H3 proton are fundamental to explain structure–property relationships and, therefore, the physical properties of these new chiral salts, rather than simple anion–cation interactions. Overall, our findings highlight the importance of the specific supramolecular interactions for the understanding of the physical properties of triazolium salts and ionic liquids.     

A Supramolecular Strategy for the Synthesis of Cyclic Oligomers and Polymers by Ring Expansion

Ring-opening metathesis polymerization (ROMP) of strained macrocycles is a key method to prepare diverse polymers. However, lack of ring strain in most macrocycles is an impediment to polymerization. In this paper, the polymerization/oligomerization of unstrained macrocycles was achieved using a supramolecular approach, leading selectively to cyclic products. Diphenyl thiourea and other guest molecules were used as additives to the ROMP reaction of unstrained macrocycles. An intermediate host-guest complex leads to the stabilization of the open form of the macrocycle after treatment with Grubbs catalysts, thereby favoring polymerization by inhibiting the ring-closing reaction back to the monomer. This proof-of-concept enables ring-expansion polymerization of unstrained macrocycles leading to cyclic polymers with molecular weights up to 6700 Da.     

Comparative Study of the Enantioseparation of Uniconazole and Imazalil by HPLC on Various Cellulose Chiral Columns

Abstract

This paper reported the enantioseparation of uniconazole and imazalil using four different self‐prepared cellulose derivative columns, namely, CTB, CTMB, CTPC, and CDMPC. The mobile phase composition including the type of the alcohol modifier and the ethanol concentration in hexane changed, and the influence on the retention factor (k′); for each enantiomer, the separation factor (α) and the resolution (R_S) was studied. The influence of the structures of the analytes on the chiral separation was investigated. The results showed that uniconazole obtained the best resolution of 2.16 on CTPC, while imazalil obtained the best separation on CTMB, with the maximal resolution of 4.83. It was concluded that the structure of solutes and CSP play a key role in the chiral recognition. And the chiral attraction between them is the predominant factor.     

Supramolecular coordination chemistry: the synergistic effect of serendipity and rational design

Supramolecular coordination compounds bear exceptional advantages over their organic counterparts. They are available in one-pot reactions and in high yields and display physical properties that are generally inaccessible with organic species. Moreover, their weak, reversible, noncovalent bonding interactions facilitate error checking and self-correction. This Review emphasizes the achievements in supramolecular coordination chemistry initiated by serendipity and their materialization based on rational design. The recognition of similarities in the synthesis of different supramolecular assemblies allows prediction of potential results in related cases. Supramolecular synthesis obeys guidelines comparable to the "lead sheet" used by small jazz ensembles for improvisation and therefore more often leads to unpredicted results. The combination of detailed symmetry considerations with the basic rules of coordination chemistry has only recently allowed for the design of rational strategies for the construction of a variety of nanosized systems with specified size and shape.     

Mechanistic Study of the Solvent-Dependent Formation of Extended and Stacked Supramolecular Polymers Composed of Bis(imidazolylporphyrinatozinc) Molecules

Bis(imidazolylporphyrinatozinc) molecules linked through a 1,3-butadiynylene moiety respond to the solvents they are dissolved in to afford exclusively extended (E) or stacked (S) supramolecular polymers. This system is expected to be a solvation/desolvation indicator. However, the principles underlying the solvent-dependent formation of the two types of polymers and the mechanism of the transformation between them are unclear. The formation of the polymers is considered to depend on the two types of complementary coordination bonds that can be formed and the π–π interactions between the porphyrins. In this study, the contributions and solvent dependence of both the coordination bonds and the π–π interactions have been investigated. The results clearly indicate that the coordination bonds are weakly or little solvent-dependent, and that the π–π interactions function effectively only in the inner porphyrins of the S-polymer and are strongly solvent-dependent. Thermodynamic analysis revealed that the formation of the E- or S-polymer in solution is determined by the total energies and the type of solvent used. The transformation of the E- to S-polymer was investigated by gel permeation chromatography. The kinetics of the transformation were also determined. The role of the terminal imidazolylporphyrinatozinc moieties was also investigated: The results indicate that the transformation from the E- to S-polymer occurs by an exchange mechanism between the polymers, induced by attack of terminal free imidazolyl groups on a polymer to zinc porphyrins on other polymers.