Philippe Gerbier

Radical para-Benzoic Acid Derivatives: Transmission of Ferromagnetic Interactions through Hydrogen Bonds at Long Distances

Investigation of the transmission of magnetic interactions through hydrogen bonds has been carried out for two different benzoic acid derivatives which bear either a tert-butyl nitroxide (NOA) or a poly(chloro)triphenylmethyl (PTMA) radical moiety. In the solid state, both radical acids formed dimer aggregates by the complementary association of two carboxylic groups though hydrogen bonding. This association ensured that atoms with most spin density are separated from one another by more than 15 A. Thus, no competing through-space magnetic exchange interactions are expected in these dimers and, hence, they provide good models to investigate whether noncovalent hydrogen bonds play a role in the long-range transmission of magnetic interactions. The nature of the magnetic exchange interaction and their strengths within similar dimer aggregates in solution was assessed by electron spin resonance (ESR) spectroscopy. In the case of radical NOA, low-temperature ESR experiments showed a weak ferromagnetic interaction between the two radicals in the dimer aggregates (which have the same geometry as in the solid state). In contrast, the corresponding solution ESR study performed with radical PTMA did not lead to any conclusive results, as aggregates were formed by noncovalent interactions other than hydrogen bonds. However, the bulkiness of the poly(chloro)triphenylmethyl radical prevented interdimer contacts in the solid state between regions of high spin density. Hence, solid-state measurements of the alpha phase of PTMA radical provided evidence of the intradimer interaction to confirm the transmission of a weak ferromagnetic interaction through the carboxylic acid bridges, as found for the NOA radical. Moreover, crystallization of the PTMA radical in presence of ethanol to form the beta phase of PTMA radical prevented the dimer formation; this resulted in the suppression of this interaction and provides further evidence of the magnetic exchange mechanism through noncovalent hydrogen bonds at long distances.

Chiral, single-molecule nanomagnets: synthesis, magnetic characterization and natural and magnetic circular dichroism

The first three chiral dodecamanganese clusters that behave as single-molecule magnets are reported. All reveal natural optical activity, which is stronger for the 2-chloropropionate derivative than for either the (S)-6-methoxy-α-methyl-2-naphthaleneacetate or the (S)-2-phenylbutyric acetate compounds. For the cluster with 2-chloropropionate moieties at its periphery, the magnetic circular dichroism was investigated and found to display large optical hysteresis, which depends on the direction in which the magnetic field direction is swept.

Characterisation of nanoscopic (Mn12O12(O2CR)16(H2O)4) single-molecule magnets: physicochemical properties and LDI- and MALDI-TOF mass spectrometry{

The syntheses of the two new single molecule magnets, [Mn12O12(O2CR)16(H2O)4]S (R = CHCHCH3, S = H2O (2) and R = C6H4C6H5, S = 2C6H5C6H4COOH (3)) and X-ray crystal structure of the first are described. Complex 2 crystallizes in the orthorhombic space group Ibca, which at 198 K has a = 21.2208(4), b = 21.2265(4), c = 42.2475(6) A, and Z = 8. Frequency-dependent out-of-phase ac signals are seen for both complexes, which indicates that these two complexes function as single-molecule magnets. Dc magnetization measurements for both complexes also exhibit hysteresis in the magnetization vs. external magnetic field plots with regular steps characteristic of quantum mechanical tunnelling of the direction of magnetization. The techniques of LDI- and MALDI-TOF mass spectrometry have been investigated to prospect their utility for the chemical characterization of Mn12 clusters. The technique is applied to known clusters as well as to two new compounds, and characteristic signals are found, especially predominant being that of the [Mn12O12(O2CR)14] cation or anion, showing the potential interest of this technique for those preparing this type of compounds.

An organometallic route to zinc phosphonates and their intercalates

An organometallic non-aqueous route to zinc phosphonates and to their intercalates has been studied. Various phosphonic acids react with dimethylzinc in THF media to afford the corresponding layered zinc phosphonates Zn(O 3 PR 1 ) (R 1 = Me, Ph, 2- and 3-thienyl, thiophen-3-ylmethyl) with evolution of methane. The presence of a primary n-alkylamine in the reaction mixture allows the one-pot formation of 2D-layered intercalated phases of general formula Zn(O 3 PR 1 )·RNH 2 [R 2 = Bu n , Pen n (= n-pentyl)] whereas a more bulky amine such as cyclohexylamine (Hex c NH 2 ) lead to the formation of 1D polymeric chains of formula Zn(O 3 PPh)·2Hex c NH 2 .

Synthesis and characterization of a new chiral nanomagnet

The dodecanuclear complexes formed between manganese ions and carboxylate anionic ligands are the single molecule magnets (SMMs) with greatest synthetic accessibility and richness of magnetic properties. In this work, we describe a recently synthesized chiral SMM which presents both spontaneous magnetization and optical activity. This objective has been targeted because of the possibility of observing new phenomena related with the synergy between structural chirality, optical activity, and magnetic ordering.

Poly[(silylene)diacetylene]/fine metal oxide powder dispersions : Use as precursors to silicon-based composite ceramics

A synthetic route to silicon-based composite ceramics that employs organosilicon polymer/metal oxide precursors is described. The precursor materials were obtained by dispersing metal oxides in the poly[(silylene)diacetylenes] 1. Subsequent pyrolysis of the dispersions above 1400 °C under various experimental conditions afforded β-SiC-based metal carbides and metal nitrides, metal silicides and Si3N4-based composites of defined compositions, in high ceramic yields. Under atmospheres of argon or of nitrogen, the precursor-to-ceramic conversion involved two critical transformations: i) the thermal cross-linking of 1 leading to an irreversible encapsulation of the oxide particle inside the polymeric matrix and ii) the carbothermal reduction of the oxide constituents by the carbon resulting from the degradation of polymer 1 to produce either the final carbide or the final nitride under the pyrolysis conditions. Moreover, it is shown that the encapsulation of the oxide particles inside a reactive matrix led to the formation of particle-tailored micro-scale reactors in which the reduction process took place with high efficiency.

2,5-Thiophene substituted spirobisiloles - synthesis, characterization, electrochemical properties and performance in bulk heterojunction solar cells†

New spirobisilole molecules containing thienyl (DTSBS) and bis-thienyl (BBTSBS) electron donor groups have been synthesized for use as the donor material in bulk-heterojunction solar cells together with PCBM. The spectroscopic, electrochemical and thermal properties of DTSBS and BBTSBS have been investigated. All these new spirobisiloles exhibit excellent thermal stability. Cyclic voltammetry measurements revealed reversible and irreversible oxidation and irreversible reduction processes. The highest occupied and lowest unoccupied molecular orbital (HOMO/LUMO) energy levels were determined from electrochemical measurements and DFT calculations. The HOMO/LUMO energy levels were estimated to lie in the range of −5.2 to −2.3 eV for DTSBS and −4.9 to −2.6 eV for BBTSBS. For both compounds, electropolymerization processes occur at potentials higher than 1.5 V leading to low band gap electrogenerated polymers. Spin-coating-deposited bulk-heterojunction solar cells fabricated with the novel spirobisiloles as donor and PCBM as acceptor displayed open-circuit voltages up to 0.4 V, short-circuit currents around 0.5 mA cm−2, and power conversion efficiencies approaching 0.1%.

Supported thin flexible polymethylhydrosiloxane permeable films functionalised with silole groups: new approach for detection of nitroaromatics

Novel re-useable luminescent sensors highly sensitive to quenching by nitroaromatic compounds in both solution and the vapour phase have been developed by covalently bonding a new allyl-substituted silole into substrate-supported crosslinked polymethylhydrosiloxane (PMHS) thin films by Pt-catalyzed hydrosilylation of the SiH groups.

Novel metal-template assembled highly-functionalized cyanoporphyrazine ytterbium and vanadium complexes for potential photonic and optoelectronic applications

A novel facile synthetic route to the metal-template assembly of a tetrapyrrollic framework from tetracyanoethylene (TCNE) and tricyanovinylbenzene (TCNVB) structural units through reaction of TCNE and TCNVB with metal π-sandwich complexes has been developed. The reactions occur under extremely mild conditions, the porphyrazine macrocycles being assembled in high yield from TCNE and TCNVB building-blocks by VO2+ or Yb3+-template synthesis. The redox behaviour of the novel complexes has been investigated. The vanadyl octacyanoporphyrazine complex was found to be a rare example of a highly-absorbing dye combining significant electron-acceptor properties with a band gap unusually narrow for an organic semiconductor (ca. 1.1 eV). The preparation is described of a novel highly emissive ytterbium complex with a proposed unusual structure obtained by reaction of TCNVB with bis(indenyl)ytterbium(II) in THF. The analytical, spectral and electrochemical investigations of the obtained ytterbium complex indicate it to be a binuclear adduct with Yb(TCNVB)3 species in which a single doubly-reduced TCNVB molecule bridges two Yb3+ cations. The formation of a disordered polynuclear coordination polymer network including a macrocyclic structure and metal cations bridged through the nitrile nitrogen atoms is proposed. The complex is readily soluble and is compatible with a variety of polymeric matrices giving doped polymeric glasses and films which are highly luminescent in the biologically relevant optical window covering the visible and near-infrared (NIR) range (640–1000 nm). In addition, doped polymeric glasses and films highly emissive at the telecommunication wavelength (1540 nm), including a composition consisting of the novel ytterbium complex and an equimolar ratio of the novel ytterbium complex and a per se non-luminescent erbium chelate, have been obtained. The complex is found to be an extraordinarily strong sensitizer of NIR Er3+ emission.

Towards a Better Understanding of Photo-excited Spin Alignment Processes Using Silole Diradicals.

The synthesis of two nitroxide-based diradicals connected to a 2,3,4,5-tetraphenylsilole (TPS) unit, especially designed to present high spin photo-excited states, is reported. While the bisnitronylnitroxide (NN) silole-based diradical, TPSNN, is unstable and experiences a spontaneous fragmentation of its imidazolinic ring into a iso-butylammomium salt, the corresponding bisiminonitroxide (IN), TPSIN, is stable both in solution and in the solid state. This diradical crystallizes in the triclinic P-1 space group with a = 10.984(1), b = 11.474(1), c = 17.492(1) A, α = 81.10(1), β = 89.01(1), and γ = 65.71(2)°. Ground state magnetic properties of TPSIN have been investigated by means of SQUID and ESR measurements: the diradical displays weak intramolecular antiferromagnetic interactions (J/kB ≈ −1 K), in agreement with its topology and with the molecular packing observed in its crystal structure. In order to investigate the magnetic photo-excited states of TPSIN, time-resolved ESR experiments (TRESR) have been performed on this radical species. Despite the presence of both an appropriate topology for the diradical and a triplet photo-excited state for the TPS coupler, no TRESR signal was observed for this molecule within the timescale of the measurement. In addition to the work already published in this field, this result clearly indicates that besides the radical nature, the π-topological requirements and the need of photo-tunable spin-states for the coupler, the flexibility of the molecule also plays a crucial role in the achievement of photo-induced spin alignment processes.