Hydrogen-Bonded Helical Organic Nanotubes

Microwave-assisted synthesis of naphthalenemonoimides and N-desymmetrized naphthalenediimides

Naphthalenemonoimides and N-desymmetrized naphthalenediimides were synthesized using a stepwise microwave-assisted protocol. The steric and electronic properties of aliphatic amines determined the outcome of the reactions, while in the amino acid series their ability to solubilize the naphthalene dianhydride starting material was crucial. Molecular modeling was used to rationalize the observed selectivity.

Gelation or molecular recognition; is the bis-(α,β-dihydroxy ester)s motif an omnigelator?

Understanding the gelation of liquids by low molecular weight solutes at low concentrations gives an insight into many molecular recognition phenomena and also offers a simple route to modifying the physical properties of the liquid. Bis-(α,β-dihydroxy ester)s are shown here to gel thermoreversibly a wide range of solvents, raising interesting questions as to the mechanism of gelation. At gelator concentrations of 5-50 mg ml⁻¹, gels were successfully formed in acetone, ethanol/water mixtures, toluene, cyclohexane and chloroform (the latter, albeit at a higher gelator concentration). A range of neutron techniques - in particular small-angle neutron scattering (SANS) - have been employed to probe the structure of a selection of these gels. The universality of gelation in a range of solvent types suggests the gelation mechanism is a feature of the bis-(α,β-dihydroxy ester) motif, with SANS demonstrating the presence of regular structures in the 30-40 Å range. A correlation between the apparent rodlike character of the structures formed and the polarity of the solvent is evident. Preliminary spin-echo neutron scattering studies (SESANS) indicated the absence of any larger scale structures. Inelastic neutron spectroscopy (INS) studies demonstrated that the solvent is largely unaffected by gelation, but does reveal insights into the thermal history of the samples. Further neutron studies of this kind (particularly SESANS and INS) are warranted, and it is hoped that this work will stimulate others to pursue this line of research.

Heterobimetallic metal-complex assemblies constructed from the flexible arm-like ligand 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene: structural versatility in the solid state

The bidentate ferrocenyl sandwich molecule 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene (3-BPFA) has been employed as an organometallic ligand in reactions with a series of transition metal salts to construct heterobimetallic architectures. X-ray crystallographic characterization reveals that the crystal packing of free ligand 3-BPFA induces spontaneous resolution of helical chains via intermolecular hydrogen bonds. By combining the flexibility from the arm-like molecule (3-BPFA) with the variation of the coordination property from different metal ions and/or the different counteranions, five different types of architectures are prepared: one octahedral coordination cage (copper(II) complex 1); two discrete pseudocapsules for combination of chlorine anions (nickel(II) complex 2 and cobalt(II) complex 3); two dimers with metal-metal interactions (silver(I) complexes 4 and 5); one macrocyclic complex (mercury(II) complex 6); and five two-dimensional mixed-metal-organic frameworks (M'-MOFs) (zinc(II), cadmium(II), and mercury(II) complexes 7-11). The structures of all complexes are characterized in detail by IR, elementary analysis, and single-crystal X-ray diffraction analysis. The factors inducing the structure variation among the complexes are discussed by taking account of the coordination geometry of different metal ions, the span angle between the two "arms", and the coordination mode of the 3-BPFA ligand.

Substituted heterocyclic naphthalene diimides with unexpected acidity. Synthesis, properties, and reactivity

Naphthalene bisimides (NDIs) with a heterocyclic 1,4-dihydro-2,3-pyrazinedione moiety have been synthesized from both 2,6-dibromonaphthalene and 2,3,6,7-tetrabromonaphthalene bisanhydrides by means of a stepwise protocol including imidization, nucleophilic displacement of the bromine atoms by ethane-1,2-diamine, in situ reductive dehalogenation, and further oxidation. These heterocycles (R = n-pentyl, cyclohexyl) are yellow dyes with green emission in organic solvent, where the acid form dominates. The orange nonfluorescent conjugate base can be generated quantitatively by CH(3)COONBu(4) addition in DMSO, where it exhibits a pK(a) = 7.63. The conjugate base becomes the only detectable species (by UV-vis spectroscopy), in water solution, even under acid conditions (pH 1). In aqueous DMSO the acid/base equilibrium is a function of the DMSO/water ratio. The unexpected acidity of these heterocyclic NDIs, which justifies the reactivity with CH(2)N(2), has been rationalized by DFT computational means [PBE0/6-31+G(d,p)] in aqueous solvent (PCM models) as a result of a strong specific solvation effect, modeled by the inclusion of three water molecules.

Self-assembling chiral gelators for fluorinated media

Formulations involving partially and fully fluorinated media represent a technological challenge given the lipophobic and hydrophobic nature of such liquids. The identification of self-associating materials with which to control the viscosity and solubilizing characteristics of fluorinated solvents is a particularly interesting area of research. It is shown here that the presence of the stereogenic centers inherent in a family of bis-(alpha,beta-dihydroxy ester)s is an essential requirement for the thermoreversible gelation of mixtures of partially fluorinated liquids 2H,3H-perfluoropentane (HPFP) and 1H,1H-heptafluorobutanol (HFB). Gelation is driven by hydrogen bonding, which induces a nonpreferred conformation around the bis-(alpha,beta-dihydroxy ester) structural motif. An analysis of the melting temperature yields an enthalpy of melting that is consistent with three to four hydrogen bonds, commensurate with the end-group structure of the gelator. Small-angle neutron scattering demonstrated the existence of the common fibrillar structures whose dimensions showed no obvious correlation with the molecular structure of the gelator.

A rigid sublimable naphthalenediimide cyclophane as model compound for UHV STM experiments

The design, synthesis and characterization of a rigid naphthalenediimide cyclophane as a model compound for ultrahigh vacuum (UHV) STM experiments are described together with first self-assembly investigations on an Au(111) substrate displaying the formation of densely packed parallel rows of molecules.