Françoise Serein-Spirau

Electroactive Nanorods and Nanorings Designed by Supramolecular Association of π-Conjugated Oligomers

In investigations into the design and isolation of semiconducting nano-objects, the synthesis of a new bisureido pi-conjugated organogelator has been achieved. This oligo(phenylenethienylene) derivative was found to be capable of forming one-dimensional supramolecular assemblies, leading to the gelation of several solvents. Its self-assembling properties have been studied with different techniques (AFM, EFM, etc.). Nano-objects have successfully been fabricated from the pristine organogel under appropriate dilution conditions. In particular, nanorods and nanorings composed of the electroactive organogelator have been isolated and characterized. With additional support from an electrochemical study of the organogelator in solution, it has been demonstrated by the EFM technique that such nano-objects were capable of exhibiting charge transport properties, a requirement in the fabrication of nanoscale optoelectronic devices. It was observed that positive charges can be injected and delocalized all along an individual nano-object (nanorod and nanoring) over micrometers and, remarkably, that no charge was stored in the center of the nanoring. It was also observed that topographic constructions in the nanostructures prevent transport and delocalization. The same experiments were performed with a negative bias (i.e., electron injection), but no charge delocalization was observed. These results could be correlated with the nature of 1, which is a good electron-donor, so it can easily be oxidized, but can be reduced only with difficulty.

Synthesis and characterization of thienylene–phenylene copolymers with oligo(ethylene oxide) side chains

A series of novel soluble conjugated regioregular copolymers (poly(2,5-dialkoxy-p-phenylene thienylenes)) was prepared via palladium-catalyzed Stille coupling reactions. The attachment of ethylene oxide chains to the phenylene rings ((OCH2CH2)nOR; n n = 2, R = Me; n n = 2, R = Bu; n n = 3, R = Me) provided to the polymer increased solubility in polar solvents improved adhesion properties on glass substrates and complexing properties towards lithium ions. The structure of the polymers was established by FTIR, 1H and 13C NMR spectroscopy and elemental analysis. The physical properties were studied by X-ray diffraction, cyclic voltammetry, UV–Vis absorption and fluorescence, and Raman spectroscopy. The electrochemical properties of the copolymers compare favorably to those of the active materials used in electro-optical devices such as LED, LEC and electrochromic devices. Compared to related phenylene–thienylene copolymers the reported new materials showed lower values of the band gap and of the oxidation potential which is indicative of a highly conjugated π-system.

One pot synthesis of fluorescent π-conjugated materials: immobilization of phenylene–ethynylene polyelectrolytes in silica confined ionogels

Phenylene–ethynylene monomer and copolymer bearing pendant imidazolium groups were synthesized in order to obtain luminescent entities highly soluble in ionic liquids (ILs). Their stabilities under non-aqueous sol–gel conditions (i.e. using formic acid) allowed their incorporation into silica in the form of ionogels. This was performed in a one step synthesis and the resulting materials were found to be easily processable as monoliths and showed interesting luminescent properties. Given the high solubility of 2,4-dinitrotoluene in the investigated IL, (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), the relevance of these ionogels as luminescent sensors was tested.

Generating long supramolecular pathways with a continuous density of states by physically linking conjugated molecules via their end groups.

Self-assembly of conjugated 2,5-dialkoxy-phenylene-thienylene-based oligomers on epitaxial monolayer graphene was studied in ultrahigh vacuum by low-temperature scanning tunneling microscopy (STM). The formation of long one-dimensional (1D) supramolecular chain-like structures has been observed, associated to a physical linking of their ends which involved the rotation of the end thiophene rings in order to allow π-π stacking of these end-groups. dI/dV maps taken at an energy corresponding to the excited states showed a continuous electronic density of states, which tentatively suggests that within such molecular chains conjugation of electrons is preserved even across physically linked molecules. Thus, in a self-organization process conjugation may be extended by appropriately adapting conformations of neighboring molecules. Our STM results on such self-organized end-linked molecules potentially represent a direct visualization of J-aggregates.

Comprehensive analysis of fragment orbital interactions to build highly π-conjugated thienylene-substituted phenylene oligomers.

π-Conjugated thienylene-phenylene oligomers with fluorinated and dialkoxylated phenylene fragments have been designed and prepared to understand the interactions in fragment orbitals, the influence of the substituents (F, OMe) on the HOMO-LUMO gap, and the role of intramolecular non-covalent cumulative interactions in the construction of π-conjugated nanostructures. Their strong conjugation was also evidenced in the gas phase by UV photoelectron spectroscopy and theoretical calculations. These results can be explained by the crucial role of the relative energetic positions of the πu2005orbitals of the dimethoxyphenylene, which was used to model the dialkoxyphenylene entity, in determining the π/π(*) orbital levels of the fluorinated phenylene entity. Dialkoxyphenylenes raise the HOMO orbitals, whereas fluorinated phenylenes lower the LUMO orbitals in the oligomers. In addition, the presence of S⋅⋅⋅F and H⋅⋅⋅F interactions in the fluorinated phenylene-thienylene compounds add to the S⋅⋅⋅O interactions in the mixed targets and contribute to the full conjugation in the oligomer, inducing weak inter-ring angles between the involved aromatic cycles. These results, which showed extended conjugation of the πu2005system, were corroborated by a narrow HOMO-LUMO gap (according to DFT calculations) and by a relatively strong maximum wavelength (as obtained by TD-DFT calculations and experimental UV/Vis measurements). The crystallographic data of two mixed thienylene-(fluorinated and dialkoxylated phenylene) five-ring oligomers agree with the above results and show the formation of quasi-planar conformations with non-covalent S⋅⋅⋅O, H⋅⋅⋅F, and S⋅⋅⋅F interactions. These studies in the solid and gas phases show the relevance of associating dialkoxyphenylene and fluorinated phenylene fragments with thiophene to lead to oligomers with improved electronic delocalization for electronic or optoelectronic devices.

Controlling bandgap energy and multivibronic modes of a poly(2,5-thiophene-1,4-dialkoxyphenylene) derivative by gamma photons.

In this work, the influence of γ radiation on electronic, structural, and vibrational properties of a poly(2,5-thiophene-1,4-dialkoxyphenylene) derivative is studied by optical absorption and photoluminescence. A Gaussian fit of emission spectra within Franck-Condon vertical transitions formalism was carried out in order to understand how vibronic coupling is affected by the dose, because an unexpected luminescence behavior was observed. Aiming to understand the ionizing radiation-matter interaction processes, we employed a molecular modeling procedure, through the use of a semiempirical method (AM1) applied to conjugated oligomers conformational structure and equilibrium geometries, to clarify the defects induction for the used doses. From AM1 optimized structures, electronic transitions were calculated by ZINDO/S-CI semiempirical method to measure the chain scission degree. Moreover, with the results presented in this work, it is possible to come up with a new physical-chemical route to treat and increase conjugated polymers efficiency. Finally, we believe that the present paper contributes to the literature about defects on conjugated polymers.

Efficient Sensing of Explosives by Using Fluorescent Nonporous Films of Oligophenyleneethynylene Derivatives Thanks to Optimal Structure Orientation and Exciton Migration

The fluorescence of thin films of a diimine-substituted phenyleneethynylene compound can be efficiently quenched by nitroaromatic vapors, which is not the case for the unsubstituted parent compound. Thin-film porosity is usually considered to be an essential factor for efficient quenching, but in the present case the origin of the quenching is completely different, as both films are nonporous and hermetic to 2,4-dinitrotoluene (DNT) molecules. The molecular organization in the two crystallized thin films offers a low level of πu2005stacking for both compounds, but the orientation of the phenylenethynylene fluorophore differs markedly with respect to the surface of the films. For the substituted compound, the fluorophore is almost parallel to the surface, thus making it readily available to molecules of a nitroaromatic quencher. This rationale is also observed in the case of a related compound bearing methoxy side chains instead of the long octyloxy moieties. Fluorescence-lifetime experiments show that the efficient quenching process in the nonporous crystallized films of the substituted compound is due to a fast (<70u2005ps) diffusion of excitons from the bulk of the film toward the surface where they are quenched, thus providing evidence of antenna effects.

Structural and Electronic Properties of New Materials Based on Thiophene and Phenylene

Theoretical study on the geometries and electronic properties of new conjugated compounds based on thiophene and phenylene was carried out. The theoretical ground-state geometries and electronic structures of the studied molecules were obtained using the density functional theory (DFT) method at B3LYP level with 6-31G(d) basis set. The electronic properties were determined by ZINDO/s, CIS/3-21G(d), and TD//B3LYP/3-21G(d) calculations performed on the B3LYP/6-31(d) optimized geometries. The effects of the ring structure and the substituents on the geometries and electronic properties of these materials were discussed. The results of this study indicate how the electronic properties can be tuned by the backbone ring or side group and suggest these compounds as good candidates for opto-electronic applications.

One- and two-photon absorption and emission properties of an oligo(phenylenethienylene)s series†

The photophysical and nonlinear absorption properties of an oligo(phenylenethienylene)s series (nTBT) are investigated in this article. The length of the chromophore is gradually increased from one to four phenylenethienylene repeating units in order to evaluate the effects of the electronic delocalization on the two-photon absorption cross sections (δ). According to the excitation anisotropy measurements and quantum chemical calculations, two electronic transitions with distinctive symmetries, 1Ag → 1Bu and 1Ag → 2Ag, are present in the low energy region of the linear absorption spectrum. The lowest-energy transition 1Ag → 1Bu is one-photon allowed but two-photon forbidden and implies an electronic charge delocalization all along the oligomer segment whereas the weakly-allowed 1Ag → 2Ag transition exhibits a transition moment perpendicular to the average plane of the chromophore. The latter transition mainly contributes to the two-photon absorption ability of the oligomers. All derivatives are poorly solvatochromic and the breakdown of the mirror symmetry rule observed between absorption and fluorescence spectra at room temperature has been attributed to a photoinduced geometrical relaxation leading to a very efficient planarization process of the oligomer irrespective of its size. Increasing the oligomer length results in a slight shift of the two-photon absorption band (∼1300 cm(-1)) and in a drastic increase of δ from 2 ± 1 GM up to 802 ± 160 GM for 1TBT and 4TBT respectively. Based on a three-level model, it was found that main contributions to the strong increase of δ stem from the transition moments Mge and Mee which are multiplied by a factor of 2.8 and 5 when going from 1TBT to 4TBT.

Photomodulation of DNA-templated supramolecular assemblies

A new type of DNA ligand that contains a phosphate-binding group and a photoresponsive azobenzene moiety is reported. When the azobenzene is in trans configuration, the ligand binds to the minor groove of a double-stranded DNA, whereas it partially desorbs upon trans-cis isomerisation with light. The ability to photoswitch the ligand upon interaction with DNA is evidenced by (chir)optical signatures, and deciphered by the differences of binding geometry, stability, and dynamics of the DNA/ligand complexes for the two isomers. We exploit these properties to photomodulate DNA-templated self-assembly, through the incorporation of another π-stacking DNA ligand, which together with the photoresponsive ligand form mixed supramolecular complexes along DNA. Our study demonstrates that well-designed photoresponsive DNA binders can be used to modulate multicomponent supramolecular DNA assemblies.