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4. Technologies de l’information

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L’approche bottom-up permise par la chimie moléculaire apporte une réponse à la miniaturisation toujours plus poussée à laquelle doit faire face la microélectronique. Dans ce contexte, nous explorons le potentiel de POMs électroactifs dans le domaine des mémoires moléculaires par piégeage de charges et pour de nouveaux dispositifs de traitement de l’information (F. Volatron, G. Izzet, A. Proust). Les POMs peuvent aussi jouer le rôle de commutateurs redox afin de moduler les interactions magnétiques dans des assemblages binaires POMs/nanocristaux magnétiques, formant ainsi de nouveaux matériaux pour le stockage magnétique (F. Volatron, A. Proust). Le stockage magnétique sous-tend également la conception et l’étude d’assemblages moléculaires multifonctionnels incorporant des molécules à haut spin, des molécules aimants, des molécules photo-commutables et/ou des interrupteurs magnétiques (V. Marvaud).

Mots clés : mémoires moléculaires à piégeage de charges, fonctionnalization de surface, molécules à haut spin, molécules photo-commutables, interrupteurs magnétiques, super-réseaux binaires POMs / nanocristaux

- Surface Organization of Polyoxometalate Hybrids Steered by a 2D Supramolecular PTCDI​/Melamine Network
A. Lombana, C. Rinfray, F. Volatron, G. Izzet, N. Battaglini, S. Alves, P. Decorse, P. Lang, A. Proust, J. Phys. Chem. C (2016), 120(5), 2837-2845.

A 2D supramol. honeycomb network built on hydrogen bonding of perylene-​3,​4,​9,​10-​tetracarboxylic acid diimide (PTCDI) and 1,​3,​5-​triazine-​2,​4,​6-​triamine (Melamine) has been self-​assembled on Au(111) by a soln. processed method. The ability of the porous network to host functional mol. oxides or polyoxometalates (POMs) has been investigated using a functionalized species [PW11O39Ge(p-​C6H4-​C≡C-​C6H4-​NHC(O) (CH2)​4(-​CH(CH2)​2S-​S-))​]​4-​ (KWGe[S-​S]​)​ : this inorg.​/org. hybrid built on a Keggin-​type POM core and an org. tether incorporating a thioctic acid function has been designed to enhance the host-​guest interaction by the formation of covalent Au-​S bonds. XPS anal. confirmed the presence of the POMs that are covalently held onto the surface. Probed by STM operating under ambient conditions, the spatial organization of the POMs display some reminiscence of the org. template, while monitoring the POM deposition at various immersion times by PM-​IRRAS showed that the POM raising at the substrate is fostered.

- Binary Superlattices from Mo132 Polyoxometalates and Maghemite Nanocrystals : Long-​Range Ordering and Fine-​Tuning of Dipole Interactions
R. Breitwieser, T. Auvray, F. Volatron, Florence ; C. Salzemann, A.-T. Ngo, P.-A. Albouy, A. Proust, C. Petit, Small 2016, 12(2), 220-228

The successful coassembly of spherical 6.2 nm maghemite (γ-​Fe2O3) nanocrystals and giant polyoxometalates (POMs) such as 2.9 nm Mo132 is demonstrated. To do so, colloidal solns. of oleic acid-​capped γ-​Fe2O3 and long-​chain alkylammonium-​encapsulated Mo132 dispersed in CHCl3 are mixed together and supported self-​organized binary superlattices were obtained upon the solvent evapn. on immersed substrates. Both electronic microscopy and small angles x-​ray scattering data reveal an AB-​type structure and an enhanced structuration of the magnetic nanocrystals (MNCs) assembly with POMs in octahedral interstices. Therefore, Mo132 acts as an efficient binder constituent for improving the nanocrystals ordering in 3-​dimensional films. In the case of didodecyldimethylammonium (C12)​-​encapsulated POMs, the long-​range ordered binary assemblies were obtained while preserving the nanocrystals magnetic properties due to weak POMs-​MNCs interactions. However, POMs of larger effective diam. can be employed as spacer blocks for MNCs as shown by using Mo132 capped with dioctadecyldimethylammonium (C18) displaying longer chains. In that case, POMs can also be used for fine-​tuning the dipolar interactions in γ-​Fe2O3 nanocrystal assemblies.

- Electron transfer properties of a monolayer of hybrid polyoxometalates on silicon
F. Volatron, J.-M. Noël, C. Rinfray, P. Decorse, C. Combellas, F. Kanoufi, A. Proust, J. Mater. Chem. C, 2015, 3(24), 6266-6275

As electroactive molecules, polyoxometalates (POMs) have potential in charge trapping or resistive molecular memories, yet scarcely investigated until very recently. Since charge–discharge processes as well as transport properties are dependent upon the organization of the thin layers, we chose to explore a covalent approach and we prepared a diazonium post-functionalized Keggin-type polyoxometalate [PW11O39Ge(p-C6H4–CC–C6H4–N2+)]3− that was subsequently anchored on hydrogenated n-type Si(100) surfaces. A flat and homogeneous hybrid POM monolayer is obtained and characterized by AFM, ellipsometry and XPS techniques. Vertical and lateral electron transfers are studied by cyclic voltammetry and scanning electrochemical microscopy (SECM). If the electron transfer between the POM layer and the silicon surface is quite slow (kETvert = 5 s−1), SECM suggests that the monolayer displays a good lateral conductivity. Interestingly, SECM experiments evidence the influence of the organization of the layer on the lateral charge transfer and show the possibility to accumulate negative charges within the POM monolayer.

- Photomagnetic molecular and extended network Langmuir–Blodgett films based on cyanide bridged molybdenum–copper complexes
N. Bridonneau, J. Long, J.-L. Cantin, J. von Bardeleben, D. R. Talham, V. Marvaud, RSC Adv., 2015,5, 16696-16701.

Two types of cyanide bridged molybdenum–copper photomagnetic films have been obtained : the first one is based on a molecular [MoCu6] complex, the other being a two-dimensional [MoCu2] coordination network. Both systems employ surfactant functionalized ligands and films were deposited on Melinex substrates using the Langmuir–Blodgett technique. All systems, including monolayer films, showed full retention of the intrinsic photomagnetic properties known for analogous solids as demonstrated by EPR spectroscopy.

- Control of the Grafting of Hybrid Polyoxometalates on Metal and Carbon Surfaces : Toward Submonolayers
S. Gam Derouich, C. Rinfray, G. Izzet, J. Pinson, J.-J. Gallet, F. Kanoufi, A. Proust, C. Combellas, Langmuir, 2014, 30, 2287−2296.

A Keggin-type POM is attached to gold or glassy carbon surfaces by electro(chemical) or peptidic
coupling. In addition to demonstrating the robust attachment of the POMs (by electrochemistry, XPS, and IRRAS), the surface concentration, layer thickness, and rate constant for electron transfer from the surface to the POMs have been measured. The use of such complementary techniques is mandatory to characterize the modi !ed electrodes properly. Whatever the grafting method, experimental conditions are found to allow monolayer or submonolayer coverage. Besides covalently grafted species, additional electrostatically bonded POMs are present in the film. Cathodic polarization allows removing them to get a grafted !lm that is stable with time and potential, which is a requirement in the design of molecular memories.

- Electron Transfer to Covalently Immobilized Keggin Polyoxotungstates on Gold
M. Yaqub, J. J. Walsh, T. E. Keyes, A. Proust, C. Rinfray, G. Izzet, T. McCormac, R. J. Forster, Langmuir 2014, 30, 4509−4516.

Spontaneously adsorbed monolayers have been formed on gold electrodes using a Keggin polyoxotungstate with covalently attached alkanethiol linkers of two different lengths. Films of both polyoxotungstates show two well- defined reduction processes associated with the polyoxotungstate centers where the ionic liquid, [BMIM][BF4], acts as supporting electrolyte. The surface coverages are both less than that expected for a close-packed monolayer. For the short and long linkers, the voltammetric response can be described in terms of the Butler− Volmer response involving a surface confined species using standard heterogeneous electron transfer rate constants of 170 and 140 s−1 for the first reduction and 150 and 100 s−1 for the second reduction processes, respectively. The rate of electron transfer to a solution phase redox probe, ferrocyanide, is significantly more sensitive to the length of the linker than the rate of electron transfer to the tungstate centers. This behavior probably arises due to potential-induced changes in the film structure.

- Electrografting of Diazonium Functionalized Polyoxometalates : Synthesis, Immobilization and Electron Transfer Characterization from Glassy Carbon
C. Rinfray, G. Izzet, J. Pinson, S. Gam Derouich, J.-J. Ganem, C. Combellas, F. Kanoufi, A. Proust, Chem. Eur. J., 2013, 19, 13838 – 13846.

POMs at work. Polyoxometalates (POMs) are attractive candidates for the rational design of multi-level charge storage materials since they display reversible multi-step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid CMOS/molecular devices, provided that a fine control of their immobilization on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin-type POM [PW11O39{Ge(p-C6H4-CC-C6H4-N2+)}]3- bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilized POM was thoroughly investigated and compared to that of the free POM in solution.

- Organosilyl/germyl polyoxotungstate hybrids for covalent grafting onto silicon surfaces : towards molecular memories
N. Joo, S. Renaudineau, G. Delapierre, G.Bidan, L.-M. Chamoreau, R. Thouvenot, P. Gouzerh, A. Proust, Chem. Eur. J., 2010, 16, 5043-5051.

A hybrid organosilyl/-germyl Keggin polyoxotungstate was covalently grafted to an n-type silicon wafer (see figure) and the electrochemical behavior of the thus modified electrode was investigated.