Spontaneous assembly of silylethane-thiol derivatives on Au(111): a chemically robust thiol protecting group as the precursor for the direct formation of aromatic gold thiolate monolayers.

Self-assembled monolayers (SAMs) on gold were obtained by the direct absorption of a fully conjugated phenylenethienylene derivative () presenting robust silylethane-thiol protecting groups as anchoring agents. The thiol deprotection and SAM formation have been evidenced by quartz crystal microbalance (QCM) measurements and X-ray photoelectron spectroscopy (XPS), and have been compared to the SAM obtained from its thioacetate analog (5). The chemically robust silylethane-thiol protecting group appeared as a surprisingly effective anchoring agent for the preparation of aromatic SAMs on Au(111), suitable for subsequent post-functionalization.

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 π stacking 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 (<70 ps) diffusion of excitons from the bulk of the film toward the surface where they are quenched, thus providing evidence of antenna effects.

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.

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 π orbitals 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 π system, 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.

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.

Comparison of fluorescence and QCM technologies: example of explosives detection with a pi-conjugated thin film.

A pi-conjugated compound was synthesized as a sensitive material for explosives detection. The detection of vapors of 2,4-dinitrotoluene was demonstrated with quartz crystal microbalance (QCM) and fluorescence transduction methods. The fluorescence intensity monitoring shows a higher sensitivity and selectivity than the monitoring of the QCM frequency. Both methods appear to be synergic when used simultaneously as the sensor helps to discriminate interferent vapors from nitroaromatics.

Acetylenic spacers in phenylene end-substituted oligothiophene core for highly air-stable organic field-effect transistors.

Two thiophene-phenylene semiconductors, bis(2-phenylethynyl) end-substituted oligothiophenes (diPhAc-nTs, n = 2, 3), were synthesized and studied with respect to their optical, electrochemical, structural and electrical properties. The optical and electrochemical properties of the oligomers in solution were investigated by UV-vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. High vacuum evaporated thin films were investigated by optical absorption, X-ray diffraction and AFM, and implemented as p-type semiconducting layers into organic thin-film transistors (OTFTs). A comparative study in solution and in the solid state with distyryl-oligothiophenes (DSnTs, n = 2, 3) reveals the great influence of acetylenic (-C[triple bond]C-) vs. olefinic (-C=C-) spacers in thiophene-phenylene derivatives on electronic structure, physical properties, and device efficiencies. Substituting olefinic for acetylenic pi-spacers in terthiophene-based conjugated semiconductors leads to one of incontrovertible attributes of OTFTs for low cost applications, a high mobility at low substrate temperature (T(sub)) i.e. typically 25 degrees C. Fine-tuning in the HOMO/LUMO levels by reducing the HOMO level introduces increased air-oxidation strength of thin films where OTFTs provide exactly the same hole mobility value after 100 days in air. All the results suggested that introduction of carbon-carbon triple bonds provided an efficient route to highly air-stable organic thin film transistors.

Electroactive nanorods and nanorings designed by supramolecular association of pi-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.

Self-organized ureido substituted diacetylenic organogel. Photopolymerization of one-dimensional supramolecular assemblies to give conjugated nanofibers.

The introduction of the urea function as structure directing agent of diacetylene organogels (DA-OGs) has been achieved. Despite the urea function being one of the most frequently used structure directing agents for the formation of organogels, it has never been exploited in the fabrication and photopolymerization of DA-OGs. The self-association of ureas involving two hydrogen bonds is much stronger than that of urethanes or amides, and the resulting supramolecular assemblies are completely insoluble. In this context, 1,1'-(hexa-2,4-diyne-1,6-diyl)bis(3-(10-(triethoxysilyl)decyl)urea) 2 was synthesized. Compound 2 was soluble owing to the triethoxysilane function that we recently used in the fabrication of a silylated bis-urea-stilbene organogel. It formed an organogel, and its photopolymerization was studied in cyclohexane. The loss of the gel state and the formation of a red solution resulting from the polymerization were found to be the result of the constraints introduced by the urea function in close vicinity to the polymerizable function. To obtain an ureido substituted diacetylenic organogelator affording a blue highly conjugated polydiacetylene (PDA) without a sol-gel transition, a propylene spacer was introduced to move the urea function away from the polymerizable function (derivative 3). The thermochromism exhibited by the latter in the solid state was studied. Using the same setup and the same sample, UV-vis and FTIR spectra were simultaneously recorded as a function of the temperature to highlight a relation between color changes and urea association mode changes. The data showed that the reversible thermochromic transition must be associated with a reversible supramolecular modification and, conversely, that irreversible chromic transitions are the result of irreversible structural modifications. The chromic effects of the acidic hydrolysis-polycondensation of the trialkoxysilyl groups to form a siloxane network were studied on a thin film of 3. In the same way, solvent effects on the color of the organogels of 3 were also investigated. Correlations could be established between the different stimuli. These results provide a deeper understanding of the precise molecular mechanism of the blue to red transition and of the reversibility of the purple to red transition generally encountered in PDA thermochromism.

Nanostructuration of phenylenevinylenediimide-bridged silsesquioxane: from electroluminescent molecular J-aggregates to photoresponsive polymeric H-aggregates.

A new approach to control molecular aggregation of pi-conjugated chromophores in the solid state has been investigated. Our strategy was to use a modifiable bulky fragment which should induce a J-aggregation and offer the possibility to reach an H-aggregation upon its chemical modification by lateral slip of pi-conjugated molecules. The chosen fragment for that purpose was the hydrolyzable triethoxysilane function (Si(OEt)3). Our objective was to design and synthesize electroluminescent or solar cell hybrid organic-inorganic materials by the sol-gel process applied to a bifunctionalized silane. With this intention, the synthesis of the sol-gel processable phenylenevinylenediimide silsesquioxane 6 was accomplished and the study of spin-coated thin films of the pure silane precursor subjected or not to the sol-gel process has been carried out. Optical properties of 6 are consistent with the formation of J-aggregates in the solid state due to the steric hindrance introduced by the triethoxysilane units. Conversely, the spectroscopic behavior observed for the hybrid film 6F is attributed to an H-aggregation corresponding to a "card pack" orientation of the distyrylbenzeneimide chromophores in the compressed silicate network. Morevover, 6 and 6F also exhibited different electronic behaviors: light-emitting diodes exhibited high brightness with the native precursor 6 and almost no light output with the sol-gel processed silsesquioxane 6F. Photovoltaic cells showed the opposite behavior with low photocurrent generation in the precursor case and higher photocurrents with the sol-gel processed layers. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of the oligosiloxane precursor.

Vapour diffusion hydrolysis of a self-assembled silylated organogel, the OG-HG transcription process: a new way to cast and handle fluorescent silsesquioxane.

A new method of hydrolysis-polycondensation of silsesquioxanes, the OG-HG (Organo Gel-Hybrid Gel or Organised Gel-Hydrolysed Gel) process allows the transcription of the supramolecular architecture of organogels to corresponding hybrid silsesquioxanes.

New chiral pi-conjugated polymers based on a (1R,2R)-diiminocyclohexane chiral unit with weak interchain pi stacking.

New chiral pi-conjugated polymers consisting of alternating conjugated segments and (1R,2R)-diiminocyclohexane units with C2 symmetry were prepared by a palladium-catalyzed coupling reaction, they exhibited very high specific optical rotations ([alpha] up to -3000 degrees) and strong Cotton effects ([theta] 10(6) deg cm2 mol-1).