Mounir Maaloum

Macroscopic contraction of a gel induced by the integrated motion of light-driven molecular motors

Making molecular machines that can be useful in the macroscopic world is a challenging long-term goal of nanoscience. Inspired by the protein machinery found in biological systems, and based on the theoretical understanding of the physics of motion at the nanoscale, organic chemists have developed a number of molecules that can produce work by contraction or rotation when triggered by various external chemical or physical stimuli. In particular, basic molecular switches that commute between at least two thermodynamic minima and more advanced molecular motors that behave as dissipative units working far from equilibrium when fuelled with external energy have been reported. However, despite recent progress, the ultimate challenge of coordinating individual molecular motors in a continuous mechanical process that can have a measurable effect at the macroscale has remained elusive. Here, we show that by integrating light-driven unidirectional molecular rotors as reticulating units in a polymer gel, it is possible to amplify their individual motions to achieve macroscopic contraction of the material. Our system uses the incoming light to operate under far-from-equilibrium conditions, and the work produced by the motor in the photostationary state is used to twist the entangled polymer chains up to the collapse of the gel. Our design could be a starting point to integrate nanomotors in metastable materials to store energy and eventually to convert it.

Light-triggered self-construction of supramolecular organic nanowires as metallic interconnects

The construction of soft and processable organic material able to display metallic conduction properties-a large density of freely moving charges-is a major challenge for electronics. Films of doped conjugated polymers are widely used as semiconductor devices, but metallic-type transport in the bulk of such materials remains extremely rare. On the other hand, single-walled carbon nanotubes can exhibit remarkably low contact resistances with related large currents, but are intrinsically very difficult to isolate and process. Here, we describe the self-assembly of supramolecular organic nanowires between two metallic electrodes, from a solution of triarylamine derivative, under the simultaneous action of light and electric field triggers. They exhibit a combination of large conductivity values (>5 × 10(3) S m(-1)) and a low interface resistance (<2 × 10(-4) Ω m). Moreover, the resistance of nanowires in series with metal interfaces systematically decreases when the temperature is lowered to 1.5 K, revealing an intrinsic metallic behaviour.

Hierarchical Self-Assembly of Supramolecular Muscle-Like Fibers

An acid-base switchable [c2]daisy chain rotaxane terminated with two 2,6-diacetylamino pyridine units has been self-assembled with a bis(uracil) linker. The complementary hydrogen-bond recognition patterns, together with lateral van der Waals aggregations, result in the hierarchical formation of unidimensional supramolecular polymers associated in bundles of muscle-like fibers. Microscopic and scattering techniques reveal that the mesoscopic structure of these bundles depends on the extended or contracted states that the rotaxanes show within individual polymer chains. The observed local dynamics span over several length scales because of a combination of supramolecular and mechanical bonds. This work illustrates the possibility to modify the hierarchical mesoscopic structuring of large polymeric systems by the integrated actuation of individual molecular machines.

Healable Supramolecular Polymers as Organic Metals

Organic materials exhibiting metallic behavior are promising for numerous applications ranging from printed nanocircuits to large area electronics. However, the optimization of electronic conduction in organic metals such as charge-transfer salts or doped conjugated polymers requires high crystallinity, which is detrimental to their processability. To overcome this problem, the combination of the electronic properties of metal-like materials with the mechanical properties of soft self-assembled systems is attractive but necessitates the absence of structural defects in a regular lattice. Here we describe a one-dimensional supramolecular polymer in which photoinduced through-space charge-transfer complexes lead to highly coherent domains with delocalized electronic states displaying metallic behavior. We also reveal that diffusion of supramolecular polarons in the nanowires repairs structural defects thereby improving their conduction. The ability to access metallic properties from mendable self-assemblies extends the current understanding of both fields and opens a wide range of processing techniques for applications in organic electronics.

Conformational analysis and estimation of the persistence length of DNA using atomic force microscopy in solution

The worm-like chain model describes the mechanical properties of semi-flexible polymers by introducing a certain correlation length along the contour. This correlation length is called the persistence length. Using atomic force microscopy in solution, we performed measurements of the persistence length of DNA molecules. We found good agreement between the theoretical model and experimental data. However, the measured persistence length values in solution differ from those found by several authors using the same technique but in dry air. In order to determine the contribution of the electrostatic persistence length to the total persistence length, we varied the salt concentration. We found a large discrepancy between the Odijk, Skolnick and Fixman theory and our measurements. The effect of divalent ions and the omission in the theory of the dependence of non-electrostatic persistence length on salt concentration are qualitatively invoked.

DNA flexibility on short length scales probed by atomic force microscopy.

Unusually high bending flexibility has been recently reported for DNA on short length scales. We use atomic force microscopy (AFM) in solution to obtain a direct estimate of DNA bending statistics for scales down to one helical turn. It appears that DNA behaves as a Gaussian chain and is well described by the wormlike chain model at length scales beyond 3 helical turns (10.5 nm). Below this threshold, the AFM data exhibit growing noise because of experimental limitations. This noise may hide small deviations from the Gaussian behavior, but they can hardly be significant.

Light-controlled morphologies of self-assembled triarylamine-fullerene conjugates.

A family of triarylamine-fullerene conjugates has been synthesized and shown to self-assemble upon light stimulation in chlorinated solvents. This light-induced process primarily involves excitation of triarylamine derivatives, which then oxidize and stack with their neutral counterparts to form charge transfer complexes in the form of p-conducting channels, while fullerenes are consequently enforced in coaxial n-conducting columnar arrangements. These supramolecular heterojunctions can be organized over very long distances in micrometric fibers when a controlled amount of photons is provided from a white light source to initiate the process. Surprisingly, when sunlight or UV light is used instead, the nanostructuration leads to monodisperse spherical objects due to the nature of the nucleation-growth process involved in the stacks formation. This control over the supramolecular morphology of organic self-assemblies using the nature of light is of general interest for the design of functional responsive materials.

Atomic force microscopy study of DNA flexibility on short length scales: smooth bending versus kinking

The apparently anomalous flexibility of DNA on short length scales has attracted a lot of attention in recent years. We use atomic force microscopy (AFM) in solution to directly study the DNA bending statistics for small lengths down to one helical turn. The accuracy of experimental estimates could be improved due to a large data volume and a refined algorithm for image processing and measuring bend angles. It is found that, at length scales beyond two helical turns (7 nm), DNA is well described by the harmonic worm-like chain (WLC) model with the bending persistence length of 56 nm. Below this threshold, the AFM data are also described by the WLC model assuming that the accuracy of measured bend angles is limited by the physical width of the double helix. We conclude that the double helical DNA behaves as a uniform elastic rod even at very short length scales. Strong bends due to kinks, melting bubbles and other deviations from the WLC model are statistically negligible.

Control over nanostructures and associated mesomorphic properties of doped self-assembled triarylamine liquid crystals.

We have synthesized a series of triarylamine-cored molecules equipped with an adjacent amide moiety and dendritic peripheral tails in a variety of modes. We show by (1) H NMR and UV/Vis spectroscopy that their supramolecular self-assembly can be promoted in solution upon light stimulation and radical initiation. In addition, we have probed their molecular arrangements and mesomorphic properties in the bulk by integrated studies on their film state by using differential scanning calorimetry (DSC), variable-temperature polarizing optical microscopy (VT-POM), variable-temperature X-ray diffraction (VT-XRD), and atomic force microscopy (AFM). Differences in the number and the disposition of the peripheral tails significantly affect their mesomorphic properties associated with their lamellar- or columnar-packed nanostructures, which are based on segregated stacks of the triphenylamine cores and the lipophilic/lipophobic periphery. Such structural tuning is of interest for implementation of these soft self-assemblies as electroactive materials from solution to mesophases.

Supramolecular Organic Nanowires as Plasmonic Interconnects

Metallic nanostructures are able to interact with an incident electromagnetic field at subwavelength scales by plasmon resonance which involves the collective oscillation of conduction electrons localized at their surfaces. Among several possible applications of this phenomenon, the theoretical prediction is that optical circuits connecting multiple plasmonic elements will surpass classical electronic circuits at nanoscale because of their much faster light-based information processing. However, the placement and coupling of metallic elements smaller than optical wavelengths currently remain a formidable challenge by top-down manipulations. Here, we show that organic supramolecular triarylamine nanowires of ≈1 nm in diameter are able to act as plasmonic waveguides. Their self-assembly into plasmonic interconnects between arrays of gold nanoparticles leads to the bottom-up construction of basic optical nanocircuits. When the resonance modes of these metallic nanoparticles are coupled through the organic nanowires, the optical conductivity of the plasmonic layer dramatically increases from 259 to 4271 Ω(-1)·cm(-1). We explain this effect by the coupling of a hot electron/hole pair in the nanoparticle antenna with the half-filled polaronic band of the organic nanowire. We also demonstrate that the whole hybrid system can be described by using the abstraction of the lumped circuit theory, with a far field optical response which depends on the number of interconnects. Overall, our supramolecular bottom-up approach opens the possibility to implement processable, soft, and low cost organic plasmonic interconnects into a large number of applications going from sensing to metamaterials and information technologies.