Photoreversible supramolecular polymerisation and hierarchical organization of hydrogen-bonded supramolecular co-polymers composed of diarylethenes and oligothiophenes

Diarylethene 1 equipped with two monotopic melamine hydrogen-bonding sites and oligothiophene-functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA-BB-type supramolecular co-polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co-monomers formed flexible quasi-one-dimensional SCPs in solution that hierarchically self-organized into helical nanofibers through H-aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H-aggregated state to non-aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen-bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UV-irradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π-conjugated oligomers.     

Thermally induced structural transformation in a hydrogen-bonded supramolecular single-crystal

The thermally induced structural transformation of a hydrogen-bonded crystal formed from an amphoteric molecule of 6-[2-methoxy-4-(pyridylazo)phenoxy]hexanoic acid MeO was studied using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction measurement (XRD). Crystal form of the hydrogen-bonded crystal was measured by single crystal four circle diffractometer (Mo-K_α radiation). As a result, the crystal of MeO was stabilized by many C–H⋅⋅⋅O hydrogen bonds, and the C–H⋅⋅⋅O hydrogen bonds were broken by thermal energy reversibly. After transformation the supramolecular architecture was composed of supramolecular polymer including free-rotation pentamethylene main chains.     

Structural rules for the chiral supramolecular organization of OPE-based discotics: induction of helicity and amplification of chirality

A systematic study on the structural rules that regulate the chiral supramolecular organization of oligo(phenylene ethynylene) (OPE)-based discotics is presented. This study is based on the chirooptical properties of two different series of triangular shape OPEs. The first of them is composed by OPE-based trisamides with a variable number of chiral side chains (compounds 1) that self-assemble following a cooperative mechanism. The CD experiments carried out with these desymmetrized trisamides demonstrate that only one stereogenic center is sufficient to achieve a helical organization with a preferred handedness. However, the ability to amplify the chirality decreases upon decreasing the number of stereocenters at the peripheral side chains. The second series is constituted by triangular shape OPEs with a variable number of ether and amide functional groups and constant absolute configuration of the stereogenic centers at all of the peripheral chains (compounds 2). These compounds do not self-assemble into helical aggregates as demonstrated by the corresponding CD studies. The amplification of chirality observed in the mixtures of some of the components of both series has been investigated. The combination of chiral trisamide 1d with chiral but nonhelical 2b or 2c does not produce an amplification of chirality most probably due to the mismatch between the stereogenic centers of both components. However, the combination of achiral trisamide 1a with chiral but nonhelical bisamide 2c generates, in a cooperative manner, helical structures with a preferred handedness in a process involving the transfer of helicity from 1a to 2c and the transfer of chirality from 2c to 1a. The structural features of the OPE discotics also exert a strong influence on the columnar aggregates. Thus, while achiral 1a bundles into thick filaments to form an organogel, the gelation ability of these triangular OPEs decreases upon increasing the number of stereogenic centers, being totally canceled for compounds 2 in which the amide functionalities are replaced by ether linkages. Finally, we have also registered AFM images of the helical aggregates obtained from the mixture of 1a+2c, which implies an efficient transfer of the chiral objects from solution to surfaces. The study presented herein increases the understanding of the structural rules that regulate the chiral supramolecular organization of discrete molecules in general and, more specifically, those based on π-conjugated oligomers.     

Ferrocenoyl glycylcystamine: organization into a supramolecular helicate structure

Extensive intermolecular hydrogen bonding in ferrocenoyl glycylcystamine gives rise to a novel ordered double helical arrangement with a helical pitch height of 14 A.     

Ferrocenylpyrazole—A versatile building block for hydrogen-bonded organometallic supramolecular assemblies

The crystal architectures of 5-ferrocenylpyrazole (1) and its metal complexes were investigated. Compound 1 can form non-solvated and chloroform-solvated crystals. In both cases, 1 forms a zigzag one-dimensional architecture via NH?N hydrogen bonds. The hydrogen bond exhibits a twofold disorder, which was shown to be static by solid-state ¹³C NMR. In the solvated crystal, the chloroform is released at 415 K, associated with melting of the crystal. The reaction of 1 with metal salts provided metal-centered ferrocenyl clusters [Zn(NO₃)₂(1)₄] (4), [Co(NO₃)₂(1)₄] (5), [CoCl₂(1)₄] (6), [Zn(NCS)₂(1)₂] (7), cis-[Pt(NH₃)₂(1)₂](PF₆)₂ (8), and trans-[Pt(NH₃)₂(1)₄](PF₆)₂ (9). In all of these complexes, 1 acts as a monodentate ligand. In 4, 5, and 7, the multinuclear units are joined via hydrogen bonds to form supramolecular chains. Two polymorphs were found for the crystals of 4. Both are composed of the same hydrogen-bonded chains, but their arrangements are different. 5-Ferrocenyl-1-tritylpyrazole (2) and 4-ferrocenyl-1-methylpyrazole (3) were also crystallographically characterized.     

Functional isocyanide metal complexes as building blocks for supramolecular materials: hydrogen-bonded liquid crystals

Gold, palladium and platinum complexes with an unusual isocyanide ligand containing a carboxylic acid function, [AuCl(CNC(6)H(4)COOH)], cis-[MI(2)(CNC(6)H(4)COOH)(2)] and trans-[MI(2)(CNC(6)H(4)COOH)(2)] (M = Pd, Pt) have been isolated. The carboxylic acid group of the coordinated isocyanide acts as a hydrogen donor for hydrogen-bonding and three series of stable hydrogen-bonded liquid crystalline metal complexes have been prepared with decyloxystilbazole. Although all the metal acid derivatives used are not mesomorphic, and decyloxystilbazole only shows an ordered Smectic E phase, four out of the five hydrogen-bonded decyloxystilbazole complexes studied display enantiotropic smectic A or nematic mesophases. The single crystal X-ray diffraction structure of trans-[PdI(2)(CNC(6)H(4)COOH)(2)].C(4)H(8)O(2) has been determined and confirms the formation of a supramolecular array in the solid state supported by hydrogen-bonding.     

A highly stable quadruply hydrogen-bonded heterocomplex useful for supramolecular polymer blends

The butyl urea of guanosine (UG) presents an ADDA hydrogen-bonding array that is complementary to the DAAD array of 2,7-diamido-1,8-naphthyridine (DAN). The stability of the DAN.UG complex was measured by fluorimetry using the fluorescence resonance energy transfer (FRET) from the naphthyridine ring to a coumarin 343 moiety linked covalently to the UG unit. The quadruply hydrogen-bonded complex is extremely stable with a measured association constant, Kassoc, of 3 x 108 M-1. Unlike related hydrogen-bonding modules, the guanosine urea, UG, contains a relatively fixed tautomeric form and only weakly self-associates (Kdimer = ca. 200 M-1). The DAN unit was linked to a styrene-based monomer and copolymerized with styrene to form a polymer (PS-DAN) containing a controlled number of the DAAD recognition units. Likewise, a methacrylate monomer containing the UG unit was copolymerized with butyl methacrylate to form a polymer (PBMA-UG). Blends formed from PS-DAN and PBMA-UG were characterized by DSC, SEC, and viscometry. The importance of selective heterocomplexation and weak self-association in forming the blended networks was demonstrated by using a ureidopyrimidinone (UPy) unit, which also forms strong heterocomplexes with DAN but is able to strongly self-associate.     

Simultaneously bound guests and chiral recognition: a chiral self-assembled supramolecular host encapsulates hydrophobic guests

Driven by the hydrophobic effect, a water-soluble, chiral, self-assembled supramolecular host is able to encapsulate hydrophobic organic guests in aqueous solution. Small aromatics can be encapsulated in the supramolecular assembly, and the simultaneous encapsulation of multiple species is observed in many cases. The molecular host assembly is able to recognize different substitutional isomers of disubstituted benzenes with ortho substitution leading to the encapsulation of two guests, but meta or para substitution leading to the encapsulation of only one guest. The scope of hydrophobic guest encapsulation is further explored with chiral natural products. Upon encapsulation of chiral molecules into the racemic host, diastereomeric host-guest complexes are formed with observed diastereoselectivities of up to 78:22 in the case of fenchone.     

Tailor-made naphthyridines: Self-assembling multiple hydrogen-bonded supramolecular architectures from dimer to helix

Stepwise changes of functional oxo and amino groups in 1,8-naphthyridines to modify the supramolecular architecture have been carried out. The first example of a naphthyridine helix has been found and its structure established by X-ray crystallography. The design is based on hydroxy and amido tautomeric naphthyridines which crystallize in dimers or catemers, one of them attaining helicity. The most stable tautomer present in all the compounds discussed in this paper, as well as the formation of hydrogen-bonded dimers or catemers, was established by X-ray crystallography and rationalized with theoretical calculations.     

Functional organic materials based on polymerized liquid-crystal monomers: supramolecular hydrogen-bonded systems

Functional organic materials are of great interest for a variety of applications. To obtain precise functional properties, well-defined hierarchically ordered supramolecular materials are crucial. The self-assembly of liquid crystals has proven to be an extremely useful tool in the development of well-defined nanostructured materials. We have chosen the illustrative example of photopolymerizable hydrogen-bonding mesogens to show that a wide variety of functional materials can be made from a relatively simple set of building blocks. Upon mixing these compounds with other reactive mesogens, nematic, chiral nematic, and smectic or columnar liquid-crystalline phases can be formed that can be applied as actuators, sensors and responsive reflectors, and nanoporous membranes, respectively.     

Ag+ labeling: a convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag+ labeling. The labeling technique with Ag+ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1(3).2(3)), double (3(3).2(6)), and tetrarosettes (4(3).2(12)) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag+ ion. A variety of functionalities has been identified that provide strong binding sites for Ag+, either acting in a cooperative way (pi-arene and pi-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2,000 and 8,000 Da, and most likely far beyond this limit.     

Nanochannels: hosts for the supramolecular organization of molecules and complexes

Nanochannels have been used as hosts for supramolecular organization for a large variety of guests. The possibilities for building complex structures based on 2D and especially 3D nanochannel hosts are larger than those based on 1D nanochannel hosts. The latter are, however, easier to understand and to control. They still give rise to a rich world of fascinating objects with very distinguished properties. Important changes are observed if the channel diameter becomes smaller than 10 nm. The most advanced guest-nanochannel composites have been synthesized with nanochannels bearing a diameter of about 1 nm. Impressive complexity has been achieved by interfacing these composites with other objects and by assembling them into specific structures. This is explained in detail. Guest-nanochannel composites that absorb all light in the right wavelength range and transfer the electronic excitation energy via FRET to well-positioned acceptors offer a unique potential for developing FRET-sensitized solar cells, luminescent solar concentrators, color-changing media, and devices for sensing in analytical chemistry, biology, and diagnostics. Successful 1D nanochannel hosts for synthesizing guest-host composites have been zeolite-based. Among them the largest variety of guest-zeolite composites with appealing photochemical, photophysical, and optical properties has been prepared by using zeolite L (ZL) as a host. The reasons are the various possibilities for fine tuning the size and morphology of the particles, for inserting neutral molecules and cations, and for preparing rare earth complexes inside by means of the ship-in-a-bottle procedure. An important fact is that the channel entrances of ZL-based composites can be functonalized and completely blocked, if desired, and furthermore that targeted functionalization of the coat is possible. Different degrees of organizational levels and prospects for applications are discussed, with special emphasis on solar energy conversion devices.     

Formation of a miscible supramolecular polymer blend through self-assembly mediated by a quadruply hydrogen-bonded heterocomplex

A supramolecular network polymer consisting of a pair of immiscible polymers, poly(butyl)methacrylate (PBMA) and polystyrene (PS), is described. A urea of guanosine (1, UG) and 2,7-diamido-1,8-naphthyridine (2, DAN), which form an exceptionally strong quadruply hydrogen-bonding complex, are displayed at 1-10 mol % along the main backbone of PBMA and PS, respectively. (1)H NMR studies show heterocomplexation between UG and DAN exclusively. This high-fidelity, high-affinity supramolecular connection of two different polymer coils at the molecular level produces a polymer blend. Blends containing different weight ratios of the polymers and mole percent of the recognition units were characterized by AFM and DSC experiments with no isolated domains observed and a single glass-transition temperature (T(g)). The T(g) is tunable by varying the weight ratio of the polymers in the blend. In addition, viscosity measurements, size-exclusion chromatography (SEC), and dynamic light-scattering (DLS) studies demonstrate the formation of a supramolecular network structure.     

Control of carbon nanotube handedness using a supramolecular chiral surface

Sorting diameter and handedness of carbon nanotubes still appears as an important challenge in nanotechnology. In this context, supramolecular structures formed by self-assembled chiral molecules deposited on well-defined metal surfaces can be used to discriminate the two isomers of carbon nanotubes. Calculations are carried out to determine the adsorption energy of nanotube enantiomers on alaninate coated Cu(110) surface. The results show a significant discrimination of the L and R handed isomers by such a surface and an additional selectivity in terms of small and large tube diameters.     

Influence of supramolecular organization on energy transfer properties in chiral oligo(p-phenylene vinylene) porphyrin assemblies

A comparative study on oligo(p-phenylene vinylene) (OPV)-appended porphyrins containing all trans-vinylene (either hydrophilic or lipophilic) or amide linkages (lipophilic) is presented. The type of supramolecular arrangement obtained in organic solvents proves to be strongly dependent on the nature of the covalent connection. In the case of all trans-vinylene linkages, a J-type intermolecular packing is obtained and the assemblies are only of moderate stability. Conversely, the supramolecular structures obtained from the amide-linked system display an H-type stacking arrangement of enhanced stability and chirality as a consequence of intermolecular hydrogen bonding along the stack direction, favorably interlocking the stacked building blocks. Interestingly, the observed differences in stability and organization are qualitatively illustrated by monitoring the sequential energy transfer process in both types of assemblies. Efficient intramolecular energy transfer from the OPVs (donors) to the respective porphyrin cores is followed by energy transfer from Zn-porphyrin (donor) to free-base porphyrin (acceptor) in both systems. However, the improved intermolecular organization for the amide-linked system increases the energy transfer efficiency along the stack direction. In addition, the water-soluble (OPV)-appended porphyrin system forms highly stable assemblies in an aqueous environment. Nevertheless, the poor energy transfer efficiency along the stack direction reveals a relative lack of organization in these assemblies.     

Strong stacking between FH--N hydrogen-bonded foldamers and fullerenes: formation of supramolecular nano networks

The stacking interactions between FH--N hydrogen-bonded foldamers 1-3, bis-foldamer 4, and tris-foldamer 5 and C(60) and C(70) are described. Compound 4 contains two folded units, which are connected by an isophthalamide linker, whereas 5 has a C(3)-symmetrical discotic structure, in which three folded units are connected by a benzene-1,3,5-tricarboxamide unit. UV/Vis, fluorescence, and NMR experiments have revealed that the foldamers or folded units strongly stack with fullerenes in chloroform. The (apparent) association constants of the respective complexes have been determined by a fluorescence titration method. The strong association is tentatively attributed to intermolecular cooperative fluorophenylpi and solvophobic interactions. A similar but weaker interaction has also been observed between an MeOH--N hydrogen-bonded foldamer and fullerenes. AFM studies have revealed that the surfaces of 3 and 4 show fibrous networks, while the surface of 5 shows particles. In sharp contrast, mixtures of 3 and 4 with C(60) have been shown to generate thinner separated fibrils, whereas a mixture of 5 and C(60) produces honeycomb-like nano networks, for which a columnar cooperative stacking pattern is proposed. The results demonstrate the usefulness of FH--N hydrogen-bonded folded structures in the construction of nanoscaled materials.     

Anion-templated supramolecular C3 assembly for efficient inclusion of charge-dispersed anions into hydrogen-bonded networks

The binding properties and conformational adaptability of a known nitrate/sulfate receptor N,N'-3-azapentane-1,5-bis[3-(1-aminoethylidene)-6-methyl-3H-pyran-2,4-dione] (L) toward various charge-dispersed monoanions (HSO(3)(-), ClO(4)(-), IO(4)(-), PF(6)(-), and SbF(6)(-)) are considered. These anions template the folding of three HL(+) species through a self-assembly process into a new hollow supramolecular trication. During the self-assembly, all strong hydrogen-bond donors of the podand become coordinatively saturated by interactions with the oxo functionalities from other HL(+) molecules. In that way, only the weak hydrogen-bond-donating groups in the exterior part of the receptor are accessible for anion binding. The investigated anions are accommodated in the hydrophobic pockets of the isomorphous hydrogen-bonded frameworks, which serve as a basis for selective crystallization from the highly competitive anion/solvent systems. This behavior is discussed in terms of size and geometry of the anions as well as the receptor's coordination capabilities to provide the most favorable surroundings for guest inclusion both in solution and in the solid state.     

Enantioselective formation of a dynamic hydrogen-bonded assembly based on the chiral memory concept

In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a(3).(CYA)(6) composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a-e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a(3).(CYA)(6) is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids D-3a and L-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a(3).(BuCYA)(6).(D-3a)(3) and (M)-1a(3).(BuCYA)(6).(L-3a)(3) with 90% diastereomeric excess. The diastereomeric excess in (M)-1a(3).(BuCYA)(6).(L-3a)(3) is "memorized" when L-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a(3).(BuCYA)(6) is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a(3).(BuCYA)(6) has a high kinetic stability toward racemization (E(a) = 119 kJ mol(-)(1), half-life of (M)-1a(3).(BuCYA)(6) is ca. 1 week at 20 degrees C).