S. Valiyaveettil

Expression of chirality by achiral coadsorbed molecules in chiral monolayers observed by STM

The supramolecular packing mode of physisorbed monolayers built up by chiral isophthalic acid derivatives and coadsorbed achiral solvent molecules was imaged at the liquid/graphite interface with scanning tunneling microscopy (STM). The picture on the right shows the submolecularly resolved STM image of an enantiomorphous domain composed of the R enantiomer of the isophthalic acid derivative studied and 1-heptanol molecules; the latter express the chirality of the monolayer. Upon adsorption a racemic mixture is separated into enantiomorphous domains.

Molecular organization of azobenzene derivatives at the liquid/graphite interface observed with scanning tunneling microscopy

Physisorbed monolayers of azobenzene derivatives were studied with a scanning tunneling microscope at the liquid/graphite interface. Three different compounds, namely, 4,4′-di(dodecyloxy) azobenzene (C12(AZO)C12), 5-[ω-(4′-dodecyloxy-4-azobenzene-oxy)dodecyloxy] isophthalic acid (C12(AZO)C12ISA), and 4,4′-bis(ω-[3,5-bis(carboxylato) phenyl-1-oxy] dodecyloxy) azobenzene (ISAC12(AZO)C12ISA) have been investigated. In all cases monolayers could be observed with submolecular resolution at the liquid/graphite interface, allowing one to identify the azobenzene, as well as the other parts of the molecules. For each monolayer structure a molecular model could be composed with a good correspondence to the experimental data. Differences in the observed monolayer structures could be related to the chemical structure of the investigated compounds. The introduction of an isophthalic acid (ISA) headgroup has a profound influence on the monolayer structure because of its capability of hydrogen bond formation with other ...

Supramolecular mesomorphic structures based on 2,5-dialkoxyterephthalic acid derivatives

Self-assembly of 2,5-di-n-alkoxyterephthalic acids (CnOTpA) and 2,5-di-n-hexylterephthalic acid (C6TpA) as bifunctional hydrogen bond donors with bifunctional acceptors 1,3-diaminopropane (Dap), piperazine (Pip) and 4,4′-bipyridine (Bipy) has been investigated, both in the crystal lattice and in the mesophase. Stoichiometric mixtures of compounds CnOTpA with diamine Dap form mesophases with a lamellar-type structure and constitute, to the best of our knowledge, the first examples of the generation of mesophases from 2,5-di-n-alkoxyterephthalic acids and bifunctional diamines. The liquid crystalline behaviour of these mixtures was characterized using X-ray scattering, DSC and polarizing microscopy. The self-assembly of complexes C6TpA·Bipy and C6TpA·Pip in the crystal lattice was characterized using single crystal X-ray structural analysis. In the crystal lattice of C6TpA·Bipy, only hydrogen bonding interactions between the neutral acid and the base were observed. However, in the crystal structure of cocrystals of C6TpA with Pip, an ionic interaction was observed due to proton transfer from the acid group to the basic nitrogen atoms of the base.

Multiphase self-assembly of 5-alkoxyisophthalic acid and its applications

A summary of our recent studies on multiphase self-assembly of 5-alkoxyisophthalic acid (CnISA) as a versatile molecular building block is described. An interplay of weak interactions such as hydrogen bonding and van der Waals forces (alkyl chain crystallization) is the key factor in achieving and controlling the multiphase self-assembly of CnISAs. Self-assembly of pure acids and its stoichiometric mixtures with bifunctional diazines in the solid state is described with typical examples. Supramolecular structures observed at the solid–solution interface (2D crystals) show characteristic similarities and differences with the structures observed in the 3D crystal lattices. CnISAs with n>16 exhibit thermotropic behavior. An overview of the important features of the mesophase is also discussed. The self-assembly in solution leads to a cyclic structure similar to the one observed in the single crystal lattice of CnISAs with shorter alkyl chains (n=6–10). Towards designing a functional supramolecular system, properly functionalized CnISAs are used and the analysis of the results from their photophysical studies at the solid–solution interface is discussed.

Self-assembly of 5-alkoxyisophthalic acids: alkyl chain length dependence for the formation of channel-type and sheet-type structures

The effect of alkyl substituants at the C(5) position of isophthalic acid in the self-assembly of the title compounds is discussed with the aid of seven new crystal structures. The formation of channel-type (6 ⩽ n < 12) and sheet-type (n ⩾ 12) structures from 5-alkoxyisophthalic acids (CnISAs) in the crystal lattice is described. One of the reasons proposed for the formation of the cyclic and sheet-type structures observed in the crystal lattice from the title compound is the length of the alkyl chain (n = number of carbon atoms in the alkyl chain). The tolerance limit of the channel-type crystal lattice towards polar functional groups is also discussed. The cyclic structures that are observed may also be converted into a ribbon-type form under appropriate conditions.

Synthesis and structural study of poly(isophthalamide)s

The synthesis of a series of soluble 5-alkoxy substituted isophthalamides is described. Condensation of 1,3-diaminobenzene or 2,6-diaminopyridine with alkoxy substituted isophthaloylchlorides yields the title polyaramides. The influence of the alkyl chain length on the supramolecular structure of the polymer is investigated by X-ray diffraction experiments. Initial results indicate a lamellar type solid lattice for the polymers studied here.

Design and Control of the Structure of Polymers and Molecular Aggregates in the Solid Lattice: Synthetic and Self-Assembly Approach

The design and synthesis of oligomers and polymers with a well-defined structure has shown considerable success in recent years [1,2]. However, to fine tune the structure of a macromolecule and to obtain the desired electronic, photonic and thermal properties which are often the direct result of macroscopic order of molecules in the solid lattice, remains to be a challenge [3]. Two approaches are pursued towards designing polymers with well-defined structure. The first method rests on the careful choice of a monomer with multiple functional groups. This is particularly important for the synthesis of defect-free double stranded ladder-type polymers. The second method explores the self-assembly of small molecules and polymeric materials to form supramolecular structures. Here, the weak intermolecular interactions, such as hydrogen bonding and van der Waals forces, are used to tune the structural organization of the self-complementary molecular components. Both of these approaches have been exploited by many research groups [4,5].

SUPRAMOLECULAR STRUCTURES FORMED FROM HYDROGEN-BONDED NETWORKS OF 5-ALKOXYISOPHTHALIC ACID

A molecular model based on 5-alkoxyisophthalic acid and bifunctional aromatic bases for developing supramolecular structures is described; the molecular self-assembly of the title compounds in the crystal lattice and its modification with added diamines is characterised by single crystal analyses.