Franck Camerel

Luminescent Ethynyl−Pyrene Liquid Crystals and Gels for Optoelectronic Devices

Two functional ethynyl-pyrene derivatives have been designed and synthesized by di- and tetra-substitutions of bromo pyrene derivatives with N-(4-ethynylphenyl)-3,4,5-tris(hexadecyloxy)benzamide fragments. The photoluminescence wavelength of the pyrene core can be tuned by the substitution pattern and the state of matter (solid, solution, gel, or liquid crystal). The disubstituted pyrene derivative 1 is not mesomorphic but produces robust and highly fluorescent gels in DMF, toluene, and cyclohexane. The well-defined fibers and ropes of the gel states were characterized by SEM and laser scanning confocal microscopy, and extended over several micrometers. The gels were integrated as active layers in field-effect transistors, which provided good bulk electron and hole charge mobilities as well as light emission generation. The tetra-substituted pyrene derivative is not a gelator but displays a stable liquid crystalline phase with 2D hexagonal symmetry between 20 and 200 degrees C. The pronounced luminescence properties of the mesophase allow one to observe original mesophase textures with flower-like patterns directly by fluorescence microscopy without crossed-polarizers.

Swollen liquid-crystalline lamellar phase based on extended solid-like sheets

Ordering particles at the nanometre length scale is a challenging and active research area in materials science. Several approaches have so far been developed, ranging from the manipulation of individual particles to the exploitation of self-assembly in colloids. Nanometre-scale ordering is well known to appear spontaneously when anisotropic organic moieties form liquid-crystalline phases; this behaviour is also observed for anisotropic mineral nanoparticles resulting in the formation of nematic, smectic and hexagonal mesophases. Here we describe a lyotropic liquid-crystalline lamellar phase comprising an aqueous dispersion of planar solid-like sheets in which all the atoms involved in a layer are covalently bonded. The spacing of these phosphatoantimonate single layers can be increased 100-fold, resulting in one-dimensional structures whose periodicity can be tuned from 1.5 to 225 nanometres. These highly organized materials can be mechanically or magnetically aligned over large pH and temperature ranges, and this property can be used to measure residual dipolar couplings for the structure determination of biomolecules by liquid-state NMR. We also expect that our approach will result in the discovery of other classes of mineral lyotropic lamellar phases.

Unusual sculpting of dipeptide particles by ultrasound induces gelation.

A readily synthesized dipeptide shows unprecedented gelation behavior when dispersed and submitted to ultrasound in nonsolvents. SEM and FFEM revealed spectacular shape changes from a sheet-like material into a highly interconnected fiber network and ribbons while the dipeptide maintains an anti conformation inside β-sheets at the molecular scale.

Divergent Approach to a Large Variety of Versatile Luminescent Lanthanide Complexes

Using a regioselective strategy for nucleophilic aromatic substitution on polyfluoropyridines, a nonacoordinating precursor was designed that is adequately suited for complexation of lanthanide cations. Further functionalizations afforded numerous applications for near-IR emission, two-photon absorption spectroscopy, or the formation of luminescent gels.

Luminescent ionic liquid crystals from self-assembled BODIPY disulfonate and imidazolium frameworks

A series of modular mesogenic salts based on the combination of anionic 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (F-BODIPY) 2,6-disulfonate dyes and trialkoxybenzyl-functionalised imidazolium cations has been designed and synthesised. Each salt contains a rigid dianionic BODIPY core associated with two imidazolium cations functionalised by 1,2,3-trialkoxybenzyl (alkyl=n-C(8), n-C(12) or n-C(16)) units or, in one case, with imidazolium cations functionalised by a trialkylgallate (3,4,5-trialkoxybenzoate) unit in which the 3,5-dialkyl groups are terminated with a polymerisable acrylate entity. All these compounds were highly fluorescent in solution with quantum yields ranging from 54 to 62%. In the solid state, the width of the emission band observed at around 650 nm is a clear signature of aggregation. With the trialkoxybenzylimidazolium cations, polarised optical microscopy (POM) and X-ray scattering experiments showed that columnar mesophases were formed. Differential scanning calorimetry (DSC) studies confirmed the mesomorphic behaviour from room temperature to about 130 degrees C for salts with alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of the BODIPY unit was maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The salt containing the gallate-functionalised imidazolium cations showed no mesomorphism but the acrylate terminal units could be used to engender photoinitiated polymerisation thereby allowing the material to be immobilised on glass plates. The polymerisation process was followed by FTIR spectroscopy and the fixed and patterned films were highly fluorescent with a solid-state emission close to that of the complex in the solid state.

Supramolecular Luminescent Lanthanide Dimers for Fluoride Sequestering and Sensing

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C2 -symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high-resolution mass spectrometry. The X-ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the EuFEu bridging motive, π stacking interactions, and a four-component hydrogen-bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24 nM.

Ionic Liquid Crystals Formed by Self-Assembly around an Anionic Anthracene Core

We have designed and synthesised a series of modular, mesogenic complexes based on anthracene-2,6-disulfonate and trialkoxybenzyl-functionalised imidazolium cations. Each complex contains a central, rigid, dianionic anthracene core and two flexible monocations bearing paraffin chains anchored on imidazolium rings. Anthracene-2,6-disulfonate can be crystallised with various simple alkylammonium ions and, in the case of +N(CH3)2(C16H33)2, a crystal structure determination has shown that the long paraffinic chains are intercalated between the anthracene moieties. The dianion forms columnar mesophases with trialkoxybenzylimidazolium cations, as identified by polarising optical microscopy and X-ray scattering measurements. Differential scanning calorimetry studies confirmed mesomorphic behaviour from room temperature to about 200 degrees C for alkyl chains containing 8, 12 and 16 carbon atoms. The strong luminescence of anthracene is maintained in the mesophase and fluorescence measurements confirmed the presence of J aggregates in all cases. The new functional materials described herein provide an easy access to stable and luminescent mesomorphic materials engineered by an ionic self-assembly process.

Terpyridine-functionalized imidazolium ionic liquids

This paper reports the synthesis and the physical characterization of a new family of chelating ionic liquids carrying a terpyridine fragment suitable for metal extraction.

Probing magnetic interactions in columnar phases of a paramagnetic gold dithiolene complex

A novel radical gold dithiolene complex exhibits a hexagonal columnar mesophase, as confirmed by optical microscopy, DSC analysis and X-ray diffraction. The extent of delocalization of the spin density in such a complex was analyzed by EPR. Temperature dependent magnetization measurements reveal that the global magnetic moment is remarkably affected at the liquid-crystalline phase transition with a marked hysteresis signature, rare behavior among the few described paramagnetic discotic phases. In addition, these molecules were found to strongly aggregate in solution into one-dimensional fibers with a mean diameter of 60 nm extending over micrometres, leading to the formation of gel-like structures. These fibers are stable and can be isolated on surfaces. The gelation of the system can also be detected by temperature-dependent magnetic measurements.

New organogelators based on cyclotriveratrylene platforms bearing 2-dimethylacetal-5-carbonylpyridine fragments

Gelators, compounds able to solidify solvents, and in particular hydrogelators are interesting soft materials. In this paper we have synthesized cyclotriveratrylene (CTV) platforms symmetrically end-substituted with pendent primary amines or nicotamic substituents. These non-amphiphilic structures induce self-assembly in a large variety of solvents forming robust and opaque gels. Cyclotriveratrylene gels have for the first time been formed and characterized using FT-IR and freeze fracture electron microscopy. Two hierarchical events are responsible for the gel structure. Individual fibres of 4–5 nm diameter are formed by aggregation of the functionalised CTV molecules. These fibres then further self-assemble into large ribbons several µm long and 20 to 40 nm wide. Within the ribbons the fine striations observed by FFEM are due to individual straight chains organized in a highly compacted state. Within the fibres the individual CTV molecules are held together by hydrogen bonding of the amide function as probed by infra-red spectroscopy.