Empty helical nanochannels with adjustable order from low-symmetry macrocycles

Continuous Nanospace in Nanoporous Liquid Crystal Investigated by 129 Xe NMR Spectroscopy

Continuous nanopores within fluid materials could be used for novel chemical events such as the accommodation of guest molecules, unique arrays of the entrapped molecules, and chemical reactions in a dynamic molecular assembly. Columnar liquid crystals composed of a one-dimensionally stacked assembly of shape-persistent macrocycles form nanochannels owing to the intrinsic nanospace in the column. However, the existence of substantial nanoporosity has not been confirmed experimentally thus far. In this study, for the first time in the literature, we confirmed the presence of discrete and spatiotemporally continuous voids in a liquid-crystalline material. In 129 Xe NMR spectroscopy of liquid crystalline columnar assembly of imine-bridged shape-persistent macrocycles under Xe atmosphere, the NMR signals of the Xe atoms entrapped in the liquid-crystalline macrocycle depended on the gas pressure and phase-transition temperatures. These results indicate that the encapsulation of Xe gas molecules within the discrete and oriented nanospaces of nanoporous liquid crystals is different from the homogeneous dissolution of the solute in an ordinary solution.

Charge Mobility in Discotic Liquid Crystals

Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

Figure-eight arylene ethynylene macrocycles: facile synthesis and specific binding behavior toward Hg2+

Two figure-eight arylene ethynylene macrocycles (AEMs) were synthesized from non-helical precursors and the figure-eight shape was clearly imaged by STM.

Construction of Supramolecular Liquid-Crystalline Metallacycles for Holographic Storage of Colored Images

Design and construction of new functionalized supramolecular coordination complexes (SCCs) via coordination-driven self-assembly strategy is highly important in supramolecular chemistry and materials science. Herein, we present a family of well-defined metallacycles decorated with mesogenic forklike dendrons through the strategy of coordination-driven self-assembly. Due to the existence of mesogenic forklike dendrons, the obtained metallacycles displayed the smectic A liquid crystal phase at room temperature while their precursors exhibited the rectangular columnar liquid crystal phase. Interestingly, by taking advantage of the electrostatic interactions between the positively charged metallacycle and the negatively charged heparin, the doping of heparin induced a significant change of the liquid-crystalline behaviors of metallacycles. More importantly, the prepared liquid-crystalline metallacycles could be further applied for holographic storage of colored images. Notably, the rhomboidal metallacycle and hexagonal metallacycle gave rise to different holographic performances although they featured a similar liquid crystal phase behavior. Therefore, this research not only provides the first successful example of supramolecular liquid-crystalline metallacycles for holographic storage of colored images but also opens a new door for supramolecular liquid-crystalline metallacycles toward advanced optical applications.

Luminescent Metallacycle-Cored Liquid Crystals Induced by Metal Coordination

Two rhomboidal metallacycles based on metal-coordination-driven self-assembly are presented. Because metal-coordination interactions restrict the rotation of phenyl groups on tetraphenylethene units, these metallacycles were emissive both in solution and in solid state, and their aggregation-induced emission properties were well-retained. Moreover, the rhomboidal metallacyclic structures offer a platform for intermolecular packing beneficial for the formation of liquid crystalline phases. Therefore, although neither of building blocks shows mesogenic properties, both thermotropic and lyotropic (in DMF) mesophases were observed in one of metallacycles, indicating that mesophases could be induced by metal-coordination interactions. This study not only reveals the mechanism for the formation of cavity-cored liquid crystals, but also provides a convenient approach to preparing supramolecular luminescent liquid crystals, which will serve as good candidates for chemo sensors and liquid crystal displays.

Regio- and stereoselective synthesis of spiropyrrolidine-oxindole and bis-spiropyrrolizidine-oxindole grafted macrocycles through [3 + 2] cycloaddition of azomethine ylides

A convenient and efficient method for the regioselective macrocyclization of triazole bridged spiropyrrolidine-oxindole, and bis-spiropyrrolizidine-oxindole derivatives was accomplished through intra and self-intermolecular [3 + 2] cycloaddition of azomethine ylides. The chalcone isatin precursors 9a-i required for the click reaction were obtained from the reaction of N-alkylazidoisatin 4 and propargyloxy chalcone 8a-i which in turn were obtained by the aldol condensation of propargyloxy salicylaldehyde 6 and substituted methyl ketones 7a-i. The regio- and stereochemical outcome of the cycloadducts were assigned based on 2D NMR and confirmed by single crystal XRD analysis. High efficiency, mild reaction conditions, high regio- and stereoselectivity, atom economy and operational simplicity are the exemplary advantages of the employed macrocyclization procedure.

Shape-Persistent π-Conjugated Macrocycles with Aggregation-Induced Emission Property: Synthesis, Mechanofluorochromism, and Mercury(II) Detection

Shape-persistent conjugated macrocycles are fundamentally important because of their unique structure and properties. Herein, a series of π-conjugated macrocycles with a shape-persistent architecture, an adaptive backbone, and aggregation-induced emission (AIE) properties are synthesized via oxidative coupling of acetylene-terminated tetraphenylethylene precursor with a half-ring topology and following transformation from butadiynylene linkers into thienylene ones. Characterization by NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry provided unambiguous proofs for the macrocyclic structures. In particular, the free rotation of aromatic rings in the rigid macrocyclic backbone was validated by two-dimensional NMR spectroscopy, variable-temperature NMR measurements, and theoretical calculations. Moreover, these shape-persistent macrocyclic chromophores all exhibited obvious AIE phenomena and remarkable mechanofluorochromism behaviors with a red-shifted luminescence upon grinding and blue-shifted emission after solvent annealing. Also, the introduction of S atoms into the macrocyclic frameworks endowed the macrocyclic luminogen the capability to selectively detect mecury(II) ions in aqueous media among other metal ions.

Free Space in Liquid Crystals-Molecular Design, Generation, and Usage

In the last 50 years, an important aim of molecular and materials design has been the generation of space for the uptake of guest molecules in macrocycles and cryptands, in dendrimers as monomolecular containers, and recently in porous networks like metal-organic and covalent organic frameworks. Such molecular, oligomeric, and polymeric materials can be applied for sensing, separation, catalysis, drug delivery, and gas storage, among others. The common goal is the recognition of molecules and their uptake into and release from an appropriate space. Typically, completely empty space is unfavorable in crystalline materials. Therefore, the elimination of molecules from the cavities is often accompanied by the collapse of the cavities, that is, by a change in the molecular conformation. In contrast to this solid matter, in which the cavities are rationally designed by covalent or coordinative bonds, liquid crystals (LCs) are fluid materials with high molecular mobility. Thus, the proposal of empty space in LCs is certainly a scientific provocation. However, various recent publications on columnar mesophases claim the existence of pores with low electron density or even completely empty space on the basis of X-ray and solid-state NMR studies. Although the latter may be debated, there are many examples in which LCs take up dopants such as polymerizable monomers in disclination lines, perdeuterated chains in the interstices between columns, or electron acceptors to fill mesogens with incommensurate building blocks, which eventually stabilize the LC phases. It seems that in LC science the generation and usage of free space has been studied only occasionally and were lucky discoveries rather than investigations based on rational design. This Account summarizes the research on the formal generation of void in LCs and highlights that rational design of molecules can lead to unconventional mesophases by efficient filling of the provided space, as was shown with shuttlecock mesogens and discotic mesogens related to the concept of complementary polytopic interactions. The topic was recently further developed by the investigation of shape-persistent star mesogens. Despite the formally empty space between their arms, they all form columnar liquid crystals. Such shape-persistent oligo(phenylenevinylene) molecules fill the void and efficiently nanosegregate by helical packing in columns and deformation of the molecular scaffold at the expense of the torsional energy. This inspired us to fill the intrinsic free space by guest molecules either via supramolecular or covalent bonds or just by physical mixing in order to avoid the increase in torsional energy and to stabilize the structure. This strategy led to complex filled liquid-crystalline matter with high structural control and may in the future be used for the design of organic electronic materials that are easily alignable for device applications.

Mixed Macrocycles Derived from 2,6-Diformylpyridine and Opposite Enantiomers of trans-1,2-Diaminocyclopentane and trans-1,2-Diaminocyclohexane

The condensation reaction of 2,6-diformylpyridine with an equimolar mixture of opposite enantiomers of trans-1,2-diaminocyclopentane and trans-1,2-diaminocyclohexane using a dynamic combinatorial chemistry approach has been examined. In nonmetal-templated reactions, depending on reaction conditions, mixed 2 + 1 + 1 macrocyclic imine or bigger mixed 4 + 2 + 2 imine macrocycle are formed selectively. The 2 + 1 + 1 imine used as a precursor in the templated by Cd^II ions produces a library of enlarged chiral mixed imines coordinated with metal cations among which the hexanuclear Cd^II complex of 6 + 3 + 3 imine was isolated and characterized. All macrocyclic imine compounds have been reduced to the corresponding macrocyclic amines, which have been further transformed into their hydrochlorides. Each macrocyclic compound has been obtained as two enantiomers. For imine macrocycles and for the hydrochloride derivatives of macrocyclic amines, their X-ray crystal structures have been determined. In particular, the crystals of protonated 4 + 2 + 2 macrocyclic amine, which contains two types of diastereomeric cations differing in terms of inverted twists of pyridine moieties, and hexanuclear Cd^II complex of 6 + 3 + 3 imine, which gives a deeper insight into the expansion reaction, have been investigated. A heterochiral self-sorting of 2 + 2 and 2 + 1 + 1 macrocyclic imines has been confirmed by a competition reaction of 2,6-diformylpyridine, racemic trans-1,2-diaminocyclopentane, and racemic trans-1,2-diaminocyclohexane and theoretical calculations.

A Liquid-Crystalline Phenylene-Based Shape-Persistent Molecular Spoked Wheel

Molecular spoked wheels with an all-phenylene backbone and different alkoxy side chain substitution patterns were synthesized using a cobalt-catalyzed [2 + 2 + 2] cycloaddition and subsequent template-directed cyclization via Yamamoto coupling. The two-dimensional organization of the molecules at the solid/liquid interface was investigated by means of scanning tunneling microscopy, allowing imaging of the molecular structure with submolecular resolution. With the right proportion of the flexible alkyl corona to the rigid core, mesomorphic behavior of one compound could be observed over a wide temperature range.

Photomodulation of fluoride ion binding through anion-π interactions using a photoswitchable azobenzene system

The discovery of photoswitchable azobenzene-systems that undergo trans-to-cis photoisomerisation was a milestone in supramolecular chemistry. Such photoswitches have possible applications in data storage, stimuli responsive delivery systems, and molecular machines due to fast and selective switching. However, the light induced cis isomer of azobenzene is rather unstable and reverts thermally and photochemically to the thermodynamically stable trans configuration. We report, for the first time, controlled photoswitching of an azo-naphthalenediimide (azo-NDI) which can be achieved upon binding of fluoride ions through anion-π interaction. This NDI-F-NDI "sandwich" stabilises the cis configuration through the generation of an NDI(•-) radical anion, and a dianionic, NDI(2-) species that becomes unusually stable in the cis form. The sandwiched cis form reverts to the trans form only upon decomplexation of F(-). A model pollutant was successfully degraded using the photogenerated NDI-F-NDI sandwich. This opens a wide range of applications in molecular and supramolecular nanotechnology.

The use of a selective saturation pulse to suppress t1 noise in two-dimensional (1)H fast magic angle spinning solid-state NMR spectroscopy

A selective saturation pulse at fast magic angle spinning (MAS) frequencies (60+kHz) suppresses t1 noise in the indirect dimension of two-dimensional (1)H MAS NMR spectra. The method is applied to a synthetic nucleoside with an intense methyl (1)H signal due to triisopropylsilyl (TIPS) protecting groups. Enhanced performance in terms of suppressing the methyl signal while minimising the loss of signal intensity of nearby resonances of interest relies on reducing spin diffusion--this is quantified by comparing two-dimensional (1)H NOESY-like spin diffusion spectra recorded at 30-70 kHz MAS. For a saturation pulse centred at the methyl resonance, the effect of changing the nutation frequency at different MAS frequencies as well as the effect of changing the pulse duration is investigated. By applying a pulse of duration 30 ms and nutation frequency 725 Hz at 70 kHz MAS, a good compromise of significant suppression of the methyl resonance combined with the signal intensity of resonances greater than 5 ppm away from the methyl resonance being largely unaffected is achieved. The effectiveness of using a selective saturation pulse is demonstrated for both homonuclear (1)H-(1)H double quantum (DQ)/single quantum (SQ) MAS and (14)N-(1)H heteronuclear multiple quantum coherence (HMQC) two-dimensional solid-state NMR experiments.

Synthesis of Spiroligomer-Containing Macrocycles

We demonstrate the synthesis and characterization of the solution conformations of a collection of functionalized spiroligomer-based macrocycles. These macrocycles contain 14 independently controllable stereocenters and four independently controllable functional groups on a highly preorganized scaffold. These molecules are being developed to display complex, preorganized surfaces for binding proteins and to create enzyme-like active sites. In this work, we demonstrate the convergent synthetic approach to this new class of macrocycles and demonstrate that the conformational properties of these molecules can be changed by altering the configuration stereocenters within the backbone.

Conjugated microporous polymers with dimensionality-controlled heterostructures for green energy devices

Dimensionality for conjugated micro-porous polymers (CMP-nD, n = 0, 1, 2) is proven to be of great importance for tailoring their photophysical properties. Moreover, CMP-nD can further be converted into boron and nitrogen co-doped porous carbons (nDBN, n = 0, 1, 2) with maintained 0D, 1D, and 2D nano-structures and highly efficient catalytic performance.

Benzodithiophene based π-conjugated macrocycles: synthesis, morphology and electrochemical characterization

A 7,8-didodecyloxybenzo[1,2-b:4,3-b']dithiophene (BdT-Dod) containing a macrocycle was synthesized from a thiophene capped BdT-Dod comonomer through a Ti(iv) mediated McMurry reaction and characterized by (1)H NMR, (13)C NMR and MALDI-TOF mass spectrometry. Additionally, the morphological characterization was performed by AFM and SEM to investigate the self-aggregation properties. The macrocycle underwent self-assembly in the solid state to form fibers on the Si/SiO2 surface with a length in the μm range and a thickness of about 400 nm.

Extremely strong tubular stacking of aromatic oligoamide macrocycles

As the third-generation rigid macrocycles evolved from progenitor 1, cyclic aromatic oligoamides 3, with a backbone of reduced constraint, exhibit extremely strong stacking with an astoundingly high affinity (estimated lower limit of Kdimer > 1013 M-1 in CHCl3), which leads to dispersed tubular stacks that undergo further assembly in solution. Computational study reveals a very large binding energy (-49.77 kcal mol-1) and indicates highly cooperative local dipole interactions that account for the observed strength and directionality for the stacking of 3. In the solid-state, X-ray diffraction (XRD) confirms that the aggregation of 3 results in well-aligned tubular stacks. The persistent tubular assemblies of 3, with their non-deformable sub-nm pore, are expected to possess many interesting functions. One such function, transmembrane ion transport, is observed for 3.

The influence of intraannular templates on the liquid crystallinity of shape-persistent macrocycles

A series of shape-persistent phenylene–ethynylene–naphthylene–butadiynylene macrocycles with different extraannular alkyl groups and intraannular bridges is synthesized by oxidative Glaser-coupling of the appropriate precursors. The intraannular bridges serve in this case as templates that reduce the oligomerization even when the reaction is not performed under pseudo high-dilution conditions. The extraannular as well as the intraannular substituents have a strong influence on the thermal behavior of the compounds. With branched alkyl chains at the periphery, the macrocycles exhibit liquid crystalline (lc) phases when the interior is empty or when the length of the alkyl bridge is just right to cross the ring. With a longer alkyl or an oligoethylene oxide bridge no lc phase is observed, most probably because the mesogene is no longer planar.

Tunable porous organic crystals: structural scope and adsorption properties of nanoporous steroidal ureas

Previous work has shown that certain steroidal bis-(N-phenyl)ureas, derived from cholic acid, form crystals in the P6(1) space group with unusually wide unidimensional pores. A key feature of the nanoporous steroidal urea (NPSU) structure is that groups at either end of the steroid are directed into the channels and may in principle be altered without disturbing the crystal packing. Herein we report an expanded study of this system, which increases the structural variety of NPSUs and also examines their inclusion properties. Nineteen new NPSU crystal structures are described, to add to the six which were previously reported. The materials show wide variations in channel size, shape, and chemical nature. Minimum pore diameters vary from ~0 up to 13.1 Å, while some of the interior surfaces are markedly corrugated. Several variants possess functional groups positioned in the channels with potential to interact with guest molecules. Inclusion studies were performed using a relatively accessible tris-(N-phenyl)urea. Solvent removal was possible without crystal degradation, and gas adsorption could be demonstrated. Organic molecules ranging from simple aromatics (e.g., aniline and chlorobenzene) to the much larger squalene (M(w) = 411) could be adsorbed from the liquid state, while several dyes were taken up from solutions in ether. Some dyes gave dichroic complexes, implying alignment of the chromophores in the NPSU channels. Notably, these complexes were formed by direct adsorption rather than cocrystallization, emphasizing the unusually robust nature of these organic molecular hosts.

Solid-state NMR in macromolecular systems: insights on how molecular entities move

The function of synthetic and natural macromolecularsystems critically depends on the packing and dynamics of the individual components of a given system. Not only can solid-state NMR provide structural information with atomic resolution, but it can also provide a way to characterize the amplitude and time scales of motions over broad ranges of length and time. These movements include molecular dynamics, rotational and translational motions of the building blocks, and also the motion of the functional species themselves, such as protons or ions. This Account examines solid-state NMR methods for correlating dynamics and function in a variety of chemical systems. In the early days, scientists thought that the rotationalmotions reflected the geometry of the moving entities. They described these phenomena as jumps about well-defined axes, such as phenyl flips, even in amorphous polymers. Later, they realized that conformational transitions in macromolecules happen in a much more complex way. Because the individual entities do not rotate around well-defined axes, they require much less space. Only recently researchers have appreciated the relative importance of large angle fluctuations of polymers over rotational jumps. Researchers have long considered that cooperative motions might be at work, yet only recently they have clearly detected these motions by NMR in macromolecular and supramolecular systems. In correlations of dynamics and function, local motions do not always provide the mechanism of long-range transport. This idea holds true in ion conduction but also applies to chain transport in polymer melts and semicrystalline polymers. Similar chain motions and ion transport likewise occur in functional biopolymers, systems where solid-state NMR studies are also performed. In polymer science, researchers have appreciated the unique information on molecular dynamics available from advanced solid-state NMR at times, where their colleagues in the biomacromolecular sciences have emphasized structure. By contrast, following X-ray crystallographers, researchers studying proteins using solution NMR introduced the combination of NMR with computer simulation before that became common practice in solid-state NMR. Today's simulation methods can handle partially ordered or even disordered systems common in synthetic polymers. Thus, the multitechnique approaches employed in NMR of synthetic and biological macromolecules have converged. Therefore, this Account will be relevant to both researchers studying synthetic macromolecular and supramolecular systems and those studying biological complexes.

Coexistence of helical morphologies in columnar stacks of star-shaped discotic hydrazones

Discotic hydrazone molecules are of particular interest as they form discotic phases where the discs are rigidified by intramolecular hydrogen bonds. Here, we investigate the thermotropic behavior and solid-state organizations of three discotic hydrazone derivatives with dendritic groups attached to their outer peripheries, containing six, eight, and ten carbons of linear alkoxy chains. On the basis of two-dimensional wide angle X-ray scattering (2DWAXS), the elevated temperature liquid crystalline (LC) phases were assigned to a hexagonal columnar (Colh) organization with nontilted hydrazone discs for all three compounds. With WAXS, advanced solid-state nuclear magnetic resonance (SSNMR) techniques, and ab initio computations, the compounds with six and ten carbons of achiral alkoxy side chains were further subjected to studies at 25 °C, revealing complex crystalline phases with rigid columns and flexible side chains. This combined approach led to models of coexisting helical columnar stacking morphologies for both systems with two different tilt/pitch angles between successive hydrazone molecules. The differences in tilt/pitch angles between the two compounds illustrate that the columns with short alkoxy chains (six carbons) are more influenced by the presence of other stacks in their vicinity, while those with long side chains are less tilted due to a larger alkoxy (ten carbons) buffer zone. The formation of different packing morphologies in the crystalline phase of a columnar LC has rarely been reported so far, which suggests the possibility of complex stacking structures of similar organic LC systems, utilizing small molecules as potential materials for applications in organic electronics.

Probing the relation between charge transport and supramolecular organization down to ångström resolution in a benzothiadiazole-cyclopentadithiophene copolymer

Molecular modeling shows that longitudinal displacement of the backbones by a couple of ångströms has a profound impact on the electronic coupling mediating charge transport in a conjugated copolymer. These changes can be probed by monitoring the calculated X-ray scattering patterns and NMR chemical shifts as a function of sliding of the polymer chains and comparing them to experiment.