Chiral Perylene Diimides: Building Blocks for Ionic Self‐Assembly

A chiral perylene diimide building block has been prepared based on an amine derivative of the amino acid L ‐phenylalanine. Detailed studies were carried out into the self‐assembly behaviour of the material in solution and the solid state using UV/Vis, circular dichroism (CD) and fluorescence spectroscopy. For the charged building block BTPPP, the molecular chirality of the side chains is translated into the chiral supramolecular structure in the form of right‐handed helical aggregates in aqueous solution. Temperature‐dependent UV/Vis studies of BTPPP in aqueous solution showed that the self‐assembly behaviour of this dye can be well described by an isodesmic model in which aggregation occurs to generate short stacks in a reversible manner. Wide‐angle X‐ray diffraction studies (WXRD) revealed that this material self‐organises into aggregates with π–π stacking distances typical for π‐conjugated materials. TEM investigations revealed the formation of self‐assembled structures of low order and with no expression of chirality evident. Differential scanning calorimetry (DSC) and polarised optical microscopy (POM) were used to investigate the mesophase properties. Optical textures representative of columnar liquid–crystalline phases were observed for solvent‐annealed samples of BTPPP. The high solubility, tunable self‐assembly and chiral ordering of these materials demonstrate their potential as new molecular building blocks for use in the construction of chiro‐optical structures and devices.     

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.     

Homochiral [2.2]Paracyclophane Self‐Assembly Promoted by Transannular Hydrogen Bonding

[2.2]paracyclophane (pCp), unlike many π‐building blocks, has been virtually unexplored in supramolecular constructs. Reported here is the synthesis and characterization of the first pCp derivatives capable of programmed self‐assembly into extended cofacial π‐stacks in solution and the solid state. The design employs transannular (intramolecular) hydrogen bonds (H‐bonds), hitherto unstudied in pCps, between pseudo‐ortho‐positioned amides of a pCp‐4,7,12,15‐tetracarboxamide (pCpTA) to preorganize the molecules for intermolecular H‐bonding with π‐stacked neighbors. X‐ray crystallography confirms the formation of homochiral, one‐dimensional pCpTA stacks helically laced with two H‐bond strands. The chiral sense is dictated by the planar chirality (R_p or S_p) of the pCpTA monomers. A combination of NMR, IR, and UV/Vis studies confirms the formation of the first supramolecular pCp polymers in solution.     

Supramolecular Polymerization of C3‐Symmetric Organogelators: Cooperativity,Solvent, and Gelation Relationship

A systematic study of the influence of solvent and the size of C₃‐symmetric discotics on their supramolecular polymerization mechanism is presented. The cooperativity of the self‐assembly of the reported compounds is directly related to their gelation ability. The two series of C₃‐symmetric discotics investigated herein are based on benzene‐1,3,5‐tricarboxamides (BTAs) and oligo(phenylene ethynylene)‐based tricarboxamides (OPE? TAs) that are peripherally decorated with achiral ( 1 a and 2 a ) or chiral N‐(2‐aminoethyl)‐3,4,5‐trialkoxybenzamide units ( 1 b and 2 b ). The supramolecular polymerization of compounds 1 a , b and 2 a , b has been exhaustively investigated in a number of solvents and by using various techniques: variable‐temperature circular dichroism (VT‐CD) spectroscopy, concentration‐dependent ¹H NMR spectroscopy, and isothermal titration calorimetry (ITC). The supramolecular polymerization mechanism of compounds 2 is highly cooperative in solvents such as methylcyclohexane and toluene and is isodesmic in CHCl₃. Unexpectedly, chiral compound 1 b is practically CD‐silent, in contrast with previously reported BTAs. ITC measurements in CHCl₃ demonstrated that the supramolecular polymerization of BTA 1 a is isodesmic. These results confirm the strong influence of the π‐surface of the central aromatic core of the studied discotic and the branched nature of the peripheral side chains on the supramolecular polymerization. The gelation ability of these organogelators is negated in CHCl₃, in which the supramolecular polymerization mechanism is isodesmic.     

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.     

How to Distinguish Isodesmic from Cooperative Supramolecular Polymerisation

To study the supramolecular polymerisation mechanisms of a self‐assembling system, concentration‐ and temperature‐dependent measurements can both be used to probe the transition from the molecular dissolved state to the aggregated state. In this report, both methods are evaluated to determine their effectiveness in identifying and quantifying the self‐assembly mechanism for isodesmic and cooperative self‐assembling systems. It was found that for a rapid and unambiguous determination of the self‐assembly mechanism and its thermodynamic parameters, temperature‐dependent measurements are more appropriate. These studies allow the acquisition of a large data set leading to an accurate determination of the self‐assembly mechanism and quantification of the different thermodynamic parameters that describe the supramolecular polymerisation. For a comprehensive characterisation, additional concentration‐dependent measurements can be performed.     

Redox‐Active Peptide‐Functionalized Quinquethiophene‐Based Electrochromic π‐Gel

An electrochromic system based on a self‐assembled dipeptide‐appended redox‐active quinquethiophene π‐gel is reported. The designed peptide‐quinquethiophene consists of a symmetric bolaamphiphile that has two segments: a redox‐active π‐conjugated quinquethiophene core for electrochromism, and peptide motif for the involvement of molecular self‐assembly. Investigations reveal that self‐assembly and electrochromic properties of the π‐gel are strongly dependent on the relative orientation of peptidic and quinquethiophene scaffolds in the self‐assembly system. The colors of the π‐gel film are very stable with fast and controlled switching speed at room temperature.     

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.     

Mechanochemical Encapsulation of Fullerenes in Peptidic Containers Prepared by Dynamic Chiral Self‐Sorting and Self‐Assembly

Molecular capsules composed of amino acid or peptide derivatives connected to resorcin[4]arene scaffolds through acylhydrazone linkers have been synthesized using dynamic covalent chemistry (DCC) and hydrogen‐bond‐based self‐assembly. The dynamic character of the linkers and the preference of the peptides towards self‐assembly into β‐barrel‐type motifs lead to the spontaneous amplification of formation of homochiral capsules from mixtures of different substrates. The capsules have cavities of around 800 ų and exhibit good kinetic stability. Although they retain their dynamic character, which allows processes such as chiral self‐sorting and chiral self‐assembly to operate with high fidelity, guest complexation is hindered in solution. However, the quantitative complexation of even very large guests, such as fullerene C₆₀ or C₇₀, is possible through the utilization of reversible covalent bonds or the application of mechanochemical methods. The NMR spectra show the influence of the chiral environment on the symmetry of the fullerene molecules, which results in the differentiation of diastereotopic carbon atoms for C₇₀, and the X‐ray structures provide unique information on the modes of peptide–fullerene interactions.     

Metastable Chiral Azobenzenes Stabilized in a Double Racemate

The self‐assembly of chiral organic chromophores is gaining huge significance due to the abundance of supramolecular chirality found in natural systems. We report an interdigitated molecular assembly involving axially chiral octabrominated perylenediimide (OBPDI) which transfers chiral information to achiral aromatic moieties. The crystalline two‐component assemblies of OBPDI and electron‐rich aromatic units were facilitated through π‐hole???π donor–acceptor interactions, and the charge‐transfer characteristics in the ground and excited states of the OBPDI cocrystals were established through spectroscopic and theoretical techniques. The OBPDI cocrystals entail a remarkable homochiral segregation of P and M enantiomers of both molecular entities in the same crystal system, leading to twisted double‐racemic arrangements. Synergistically engendered cavities with the stored chiral information of the twisted OBPDI stabilize higher‐energy P/M enantiomers of trans‐azobenzene through non‐covalent interactions.     

X‐Shaped Polycyclic Aromatic Hydrocarbons: Optical Properties and Tunable Assembly Ability

Although a number of synthetic methodologies have been developed to prepare stable polycyclic aromatic hydrocarbons (PAHs), much less research has been devoted to functionalizing the peripheries of molecules to tune the self‐assembly ability or introduce functional groups without altering their photophysical properties. Herein, we report twisted “X”‐shaped molecules prepared through annulation of hexacene with benzoanthracene on the zigzag edge, and an investigation of their photophysical properties and self‐assembly properties. The shape‐complementary “X”‐shaped molecules prefer to dimerize, while the π‐extension would lead to one‐dimensional π‐stacking. Our findings give some insights into the design of stable PAHs without disturbing the electronic structures.     

NMR Spectroscopic Study of the Self‐Aggregation of 3‐Hexen‐1,5‐diyne Derivatives

The self‐assembly of polycatenar molecules derived from 1,6‐diphenyl‐3,4‐dipropyl‐3‐hexen‐1,5‐diyne has been studied in detail by solution NMR spectroscopy. The analysis of the concentration‐ and temperature‐dependent evolution of the chemical shifts and the diffusion coefficients in [D₁₂]cyclohexane agrees well with an isodesmic model of association in this solvent. The association constants for the stacking and entropy and enthalpy of the process have been obtained. The driving force for the aggregation process is provided by a negative enthalpy (ΔH), which is partially compensated by a negative entropy (ΔS). A structural study of the self‐assembly in solution has been carried out with the help of NOESY NMR spectroscopic experiments.     

Mirror helices and helicity switch at surfaces based on chiral triangular-shape oligo(phenylene ethynylenes)

The self‐assembly of triangular‐shaped oligo(phenylene ethynylenes) (OPEs), peripherally decorated with chiral and linear paraffinic chains, is investigated in bulk, onto surfaces and in solution. Whilst the X‐ray diffraction data for the chiral studied systems display a broad reflection centered at 2θ ～20° (λ=Cu_Kα), the higher crystallinity of OPE 3 , endowed with three linear decyl chains, results in a diffractrogram with a number of well‐resolved reflections that can be accurately indexed as a columnar packing arranged in 2D oblique cells. Compounds (S)‐ 1 a and (R)‐ 1 b —endowed with (S)‐ and (R)‐3,7‐dimethyloctyloxy chains—transfer their chirality to the supramolecular structures formed upon their self‐assembly, and give rise to helical nanostructures of opposite handedness. A helicity switch is noticeable for the case of chiral (S)‐ 2 decorated with (S)‐2‐methylnonyloxy chains which forms right‐handed helices despite it possesses the same stereoconfiguration for their stereogenic carbons as (S)‐ 1 a that self‐assembles into left‐handed helices. The stability and the mechanism of the supramolecular polymerization in solution have been investigated by UV/Vis experiments in methylcyclohexane. These studies demonstrate that the larger the distance between the stereogenic carbon and the aromatic framework is, the more stable the aggregate is. Additionally, the self‐assembly mechanism is conditioned by the peripheral substituents: whereas compounds (S)‐ 1 a and (R)‐ 1 b self‐assemble in a cooperative manner with a low degree of cooperativity, the aggregation of (S)‐ 2 and 3 is well described by an isodesmic model. Therefore, the interaction between the chiral coil chains conditions the handedness of the helical pitch, the stability of the supramolecular structure and the supramolecular polymerization mechanism of the studied OPEs.     

Theoretical investigation of the interactions between the π‐systems of molecular organic semiconductors and an analysis of the contributions of repulsion and electrostatics

A concept for the interactions between π‐systems is necessary to understand a number of phenomena in modern material sciences such as supramolecular properties and self‐assembly. In the present article, we investigate the intermolecular interaction energies between organic semiconductors with extended π‐systems using SAPT (symmetry‐adapted perturbation theory), LMO‐EDA (localized molecular orbital energy decomposition analysis), DFT‐D (density functional theory including dispersion corrections), and force‐field approaches. Both apolar organic molecules such as acenes and highly polarized π‐systems of merocyanines and squaraines were used to probe the influence of electrostatics on the shape of the potential energy surfaces (PES) governing the geometric structures of aggregates. Our results reveal that the shapes of the PESs result from variations in the short‐range, highly specific repulsion forces even for highly polar molecules. Using distributed quadrupoles, we show that it is nevertheless possible to mimic the intermolecular potentials with electrostatics. This is also possible with van‐der‐Waals potentials and a simple overlap‐based force‐field ansatz based on the overlap between p‐orbitals. © 2016 Wiley Periodicals, Inc.     

Pathway Control in Cooperative vs. Anti‐Cooperative Supramolecular Polymers

Controlling the nanoscale morphology in assemblies of π‐conjugated molecules is key to developing supramolecular functional materials. Here, we report an unsymmetrically substituted amphiphilic Pt^II complex 1 that shows unique self‐assembly behavior in nonpolar media, providing two competing anti‐cooperative and cooperative pathways with distinct molecular arrangement (long‐ vs. medium‐slipped, respectively) and nanoscale morphology (discs vs. fibers, respectively). With a thermodynamic model, we unravel the competition between the anti‐cooperative and cooperative pathways: buffering of monomers into small‐sized, anti‐cooperative species affects the formation of elongated assemblies, which might open up new strategies for pathway control in self‐assembly. Our findings reveal that side‐chain immiscibility is an efficient method to control anti‐cooperative assemblies and pathway complexity in general.     

Corannulene‐Fused Anion‐Responsive π‐Conjugated Molecules that Form Self‐Assemblies with Unique Electronic Properties

The fusion of bowl‐shaped π‐conjugated corannulene units to anion‐responsive π‐conjugated dipyrrolyldiketone‐boron complexes resulted in new molecular materials with a unique self‐assembly capability. The bowl‐fused receptor with aliphatic tails could form both supramolecular gels and mesophases through π‐stacking interactions and also exhibited anion‐responsive characteristics. The presence of the π‐bowl unit not only afforded enhanced self‐assembly capability both in solution and in the mesophases, as evidenced by gelation experiments and phase‐transition profiles, but also enhanced intrinsic charge‐carrier mobility.     

Self‐Assembly of Ureido‐Pyrimidinone Dimers into One‐Dimensional Stacks by Lateral Hydrogen Bonding

Ureido‐pyrimidinone (UPy) dimers substituted with an additional urea functionality self‐assemble into one‐dimensional stacks in various solvents through lateral non‐covalent interactions. ¹H NMR and DOSY studies in CDCl₃ suggest the formation of short stacks (<10), whereas temperature‐dependent circular dichroism (CD) studies on chiral UPy dimers in heptane show the formation of much larger helical stacks. Analysis of the concentration‐dependent evolution of chemical shift in CDCl₃ and the temperature‐dependent CD effect in heptane suggest that this self‐assembly process follows an isodesmic pathway in both solvents. The length of the aggregates is influenced by substituents attached to the urea functionality. In sharp contrast, UPy dimers carrying an additional urethane group do not self‐assemble into ordered stacks, as is evident from the absence of a CD effect in heptane and the concentration‐independent chemical shift of the alkylidene proton of the pyrimidinone ring in CDCl₃.     

Chiral Induction and Amplification in Supramolecular Systems at the Liquid–Solid Interface

Chiral induction and amplification in surface‐confined supramolecular monolayers are investigated at the liquid–solid interface. Scanning tunneling microscopy (STM) proves that achiral molecules can self‐assemble into globally chiral patterns through a variety of approaches, including induction by chiral solvents or by a novel chiral amplification method. Our study demonstrates the aptness of both approaches, which have already been applied to (supramolecular) polymers in solution, to create chiral supramolecular monolayers at the liquid–solid interface.     

Hooking Together Sigmoidal Monomers into Supramolecular Polymers

Supramolecular polymers show great potential in the development of new materials because of their inherent recyclability and their self‐healing and stimuli‐responsive properties. Supramolecular conductive polymers are generally obtained by the assembly of individual aromatic molecules into columnar arrays that provide an optimal channel for electronic transport. A new approach is reported to prepare supramolecular polymers by hooking together sigmoidal monomers into 1D arrays of π‐stacked anthracene and acridine units, which gives rise to micrometer‐sized fibrils that show pseudoconductivities in line with other conducting materials. This approach paves the way for the design of new supramolecular polymers constituted by acene derivatives with enhanced excitonic and electronic transporting properties.     

Self‐Assembly of π‐Conjugated Gelators into Emissive Chiral Nanotubes: Emission Enhancement and Chiral Detection

A series of new π‐conjugated gelators that contain various aromatic rings (phenyl, naphthyl, 9‐anthryl) and amphiphilic L ‐glutamide was designed, and their gel formation in organic solvents and self‐assembled nanostructures was investigated. The gelators showed good gelation ability in various organic solvents that ranged from polar to nonpolar. Those gelator molecules with small rings such as phenyl and naphthyl self‐assembled into nanotube structures in most organic solvents and showed strong blue emission. However, the 9‐anthryl derivative formed only a nanofiber structure in any organic solvent, probably owing to the larger steric hindrance. All of these gels showed enhanced fluorescence in organogels. Furthermore, during the gel formation, the chirality at the L ‐glutamide moiety was transferred to the nanostructures, thus leading to the formation of chiral nanotubes. One of the nanotubes showed chiral recognition toward the chiral amines.