Michèle Veber

Evidence of nematic, hexagonal and rectangular columnar phases in thermotropic ionic liquid crystals

Dithiolium salts, with amphipathic character, are compounds of choice for investigations of the influence of an ionic feature upon mesomorphic properties. In this way, salts bearing a branched chain have been studied by SAXS. In spite of their rod-like shape, they exhibit only columnar mesophases, the supramolecular organization of which is close to that of cylindrical inverted micelles. Moreover, the nematic columnar phase, characterized by the loss of lateral positional correlations of the columns themselves, is one of the first examples of such behaviour in the case of thermotropic liquid crystals.

First example of a pyrylium salt dimerisation in solution

A pyrylium salt is capable of forming dimers in non-polar solvents at room temperature. The dimer, corresponding to a head-to-tail configuration in which the monomer dipole moments are parallel, has been characterized by its electronic absorption spectra, fluorescence spectra and its fluorescence lifetime. The dimer–monomer equilibrium constant is found to be 2.7 × 105 dm3 mol–1.

Mesomorphic self-assembly of 1,3-Diacylaminobenzenes

Abstract Some new examples of hydrogen-bonded 1,3-diacylaminobenzene mesogens are presented. This odd type of polymeric self-hydrogen bonded supramolecular association allows a nematic and/or a columnar order to be obtained. Such a mesogenic hydrogen-bonded arrangement (MHB) should be able to stabilize diverse mesomorphic molecular edifices such as layers, columns and tubes.

Synthesis and X-ray study of new columnar heteroaromatic salts

Abstract The synthesis of 2,4,6-triarylpyrylium tetrafluoroborates substituted by six long alkyloxy chains (n = 8 and 12) is described. These salts exhibit a discotic mesophase from room temperature to 200°C. X-ray diffraction experiments on powder and oriented samples show that this phase is an ordered hexagonal columnar phase, D ho.

Photophysical properties of discogenic triaryl pyrylium salts Excimer migration in columnar liquid crystals

Abstract The photophysical properties of a homologous series of 2, 4-6-triaryl pyrylium tetrafluoroborates substituted by six alkoxy chains (CnH2n+1O, n = 2, 3, 4, 5, 8, 12) are reported. in dilute solution, the electronic absorption and fluorescence spectra do not depend on the length of the alkoxy chains while both fluorescence lifetimes and fluorescence quantum yields increase when the chain length increases. Monomer and excimer fluorescence is observed with the pure compounds; the steady state and time resolved emission spectra and the decay kinetics which do not depend on the chain length suggest that the columnar structure exist with very short lateral chains. Excimer lifetimes determined in the liquid-crystalline phases follow an exponential law as a function of the reciprocal temperature: τ ∝ exp (E M/RT). The activation energy for excimer migration is 0.046 eV, 0.061 eV and 0.087 eV respectively for n = 5, 8 and 12.

Non-symmetrical dithiolium salts Mesomorphic properties

Abstract A series of non-symmetrical dithiolium salts has been synthesized. Classical characterization methods (DSC and optical microscopy) show that according to the number and nature of the substituents, these dithiolium salts display crystalline or smectic A phases. These S A phases have been studied by X-ray diffraction and several models are proposed to describe the molecular organization in the mesophase. In some cases, it was not possible on the basis of X-ray diffraction data alone, to determine which model should apply. In these cases, however, a comparison with similar molecules shows that one of these models is more probable than the others.

Photophysical properties of a hexadodecyloxy-substituted triarylpyrylium salt: self-association in solution

The photophysical properties of a triarylpyrylium salt bearing six dodecyloxy peripheral chains and able to form organized assemblies (columnar liquid crystals, Langmuir–Blodgett films) have been studied in dichloromethane and n-heptane solutions. It is shown that self-association occurs in n-heptane, the aggregate size increasing with concentration. Aggregate growth induces a hypsochromic shift in the absorption spectra and a bathochromic shift in the fluorescence spectra. The continuous variation of the spectroscopic properties with the size of the aggregates proves the existence of very strong long-range interactions, due to the ionic nature of the chromophores. Both absorption and fluorescence probably involve localized excited states.

Selective Adsorption of Proteins on Single-Wall Carbon Nanotubes by Using a Protective Surfactant

The dispersion of highly hydrophobic carbon materials such as carbon nanotubes in biological media is a challenging issue. Indeed, the nonspecific adsorption of proteins occurs readily when the nanotubes are introduced in biological media; therefore, a methodology to control adsorption is in high demand. To address this issue, we developed a bifunctional linker derived from pyrene that selectively enables or prevents the adsorption of proteins on single-wall carbon nanotubes (SWNTs). We demonstrated that it is possible to decrease or completely suppress the adsorption of proteins on the nanotube sidewall by using proper functionalization (either covalent or noncovalent). By subsequently activating the functional groups on the nanotube derivatives, protein adsorption can be recovered and, therefore, controlled. Our approach is simple, straightforward, and potentially suitable for other biomolecules that contain thio or amino groups available for coupling.

DIMERISATION PROCESSES OF TRIARYL PYRYLIUM SALTS

Abstract The present paper deals with dialkylamino 2,4,6-triaryl pyrylium tetrafluoroborates studied in solution at room temperature. It is intended to clarify the dimerisation process described previously. The photophysical properties of the didodecylamino derivative, the synthesis of which is reported for the first time, are practically identical to those of the methyldodecylamino analogue, but its solubility in non-polar solvents is higher by two orders of magnitude allowing a better investigation of the dimers. The absorption spectra of both compounds, recorded as a function of concentration in solvents of low dielectric constant, exhibit isosbestic points. Different equilibria, involving the cationic chromophore and its counterion, are discussed. Two-dimensional 1 H-NMR experiments performed by off-resonance ROESY (Rotating frame nuclear Overhauser Effect SpectroscopY) confirm dimerisation. Electrical conductivity measurements show that in the monomer-dimer equilibrium related to the isosbestic point, the “monomer” corresponds to an ion-pair and the “dimer” is composed of two ion-pairs.

Vinamidines and vinamidinium salts. New thermotropic liquid crystals

Abstract Following studies concerning mesomorphic heteroaromatic salts (pyrylium, thiopyrylium, dithiolium), we present here new thermotropic liquid crystal materials based on 1,5diazapentadienium (vinamidinium) salts and on the pseudocyclic vinamidine system (1,5-diazapentadiene). The mesomorphic properties of these systems have been studied by microscopic observations, DSC and X-ray diffraction.