Size-dependent properties of magnetic iron oxide nanocrystals.

The fine control of iron oxide nanocrystal sizes within the nanometre scale (diameters range from 2.5 to 14 nm) allows us to investigate accurately the size-dependence of their structural and magnetic properties. A study of the growth conditions of these nanocrystals obtained by thermal decomposition of an iron oleate precursor in high-boiling point solvents has been carried out. Both the type of solvent used and the ligand/precursor ratio have been systematically varied, and were found to be the key parameters to control the growth process. The lattice parameters of all the nanocrystals deduced from X-ray diffraction measurements are consistent with a structure of the type Fe3-xO4, i.e. intermediate between magnetite and maghemite, which evolves toward the maghemite structure for the smallest sizes (x=1/3). The evolution of the magnetic behavior with nanoparticle sizes emphasizes clearly the influence of the surface, especially on the saturation magnetization Ms and the magneto-crystalline anisotropy K. Dipolar interactions and thermal dependence have been also taken into account in the study on the nanoscale size-effect of magnetic properties.

Tilt-induced ferromagnetic ordering in anisotropic molecular monolayers.

It is shown that orientational ordering of anisotropic organic molecules with permanent magnetic dipoles in a tilted film should result in a macroscopic magnetisation in the plane of the film. The important requirement here is that the molecules are strongly biaxial, and the corresponding biaxial orientational order parameter in the tilted phase is sufficiently large. The molecules should also be characterised by a reduced symmetry of the magnetic core compared with existing "single molecular magnets". Possible symmetry groups of the molecular magnetic core, which allow for the existence of nonzero average magnetic moment, are discussed in detail. The tilt-induced ferromagnetic ordering of such molecules may be determined by nonmagnetic intermolecular interactions including, for example, quadrupole-quadrupole electrostatic interaction or dispersion interaction between molecules of particular symmetry. Magnetic intermolecular interactions are not important here, and as a result the induced ferromagnetic state may be stable in any temperature range where the corresponding tilted film is stable. These general conclusions, which form a theoretical foundation for the existence of novel fluid low-dimensional magnetic materials, are based on symmetry arguments and are supported by a simple mean-field molecular model. We also discuss how such induced ferromagnetic ordering may be observed in Langmuir-Blodgett films which seem to be the best candidates for preparing these magnetic materials.

Bent-core liquid crystals forming two- and three-dimensional modulated structures.

Two columnar phases of bent-core molecules have been observed, with a two-dimensional (2D) structure modulated in the plane perpendicular to the direction of the spontaneous polarization vector. The phases are switchable under an applied electric field, contrary to the commonly observed 2D modulated B1 phase. These new phases are built from broken smectic layers with orthogonal or tilted molecules. The evidence for a 3D structure in which the density modulations along and perpendicular to the spontaneous polarization vector co-exists is also given.

Rectangular to hexagonal columnar phase transition exhibited by a biforked mesogen.

We present the structural study of a biforked compound exhibiting a Colr-Colh phase transition. This study was performed by means of DSC, optical microscopy, X-ray diffraction, dilatometry and molecular modelling. The molecular packing within the two columnar mesophases of this compound is discussed. For the first time, it is shown that the clusters of adjacent molecules which form the elementary section of a column are tilted with respect to the columnar axis in the rectangular mesophase. From the whole set of experimental data, the number of molecules in a columnar section is found to be about 5 in the rectangular mesophase, and 4 in the hexagonal one. The transition between the two columnar mesophases is discussed with respect to its slow kinetics.

Amphiphilic diblock dendrimers: synthesis and incorporation in Langmuir and Langmuir-Blodgett films.

A new dendron with peripheral long alkyl chains and containing five C(60) units in the branching shell has been prepared and attached to a Fréchet-type dendron functionalized with ethylene glycol chains. The peripheral substitution of the resulting globular dendrimer with hydrophobic chains on one hemisphere and hydrophilic groups on the other provides the perfect hydrophobic/hydrophilic balance allowing the formation of stable Langmuir films. Furthermore, a perfect reversibility has been observed in successive compression/decompression cycles. The diblock structure of the dendrimer has been also crucial for the efficient transfer of the Langmuir films in order to obtain well-ordered multilayered Langmuir-Blodgett films. This approach appears particularly interesting since functional groups not well adapted for the preparation of Langmuir and Langmuir-Blodgett films such as fullerenes can be attached into the branching shell of the dendritic structure and, thus, efficiently incorporated in thin ordered films.

A mixed fullerene-ferrocene thermotropic liquid crystal: synthesis, liquid-crystalline properties, supramolecular organization and photoinduced electron transfer.

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.

Dendromesogens: liquid crystal organizations of poly(amidoamine) dendrimers versus starburst structures.

A new series of liquid crystalline poly(amidoamine) (PAMAM) dendrimers is described. These dendrimers are made by attaching to the 0-, 1-, 2-, 3-, and 4-generation of PAMAM-terminal promesogenic units that carry two decyloxy chains in the 3- and 4-positions of their peripheral aromatic ring. X-ray diffraction studies show that all the compounds display a hexagonal columnar mesophase. A high density of aliphatic chains imposes a curved interface with the promesogenic units that forces the molecules to adopt a radial conformation, and therefore, the columnar structure. A model for the supramolecular organization of the different generations within the columnar mesophase is proposed based on the variation of some of the structural parameters.

A liquid crystalline supramolecular complex of C60 with a cyclotriveratrylene derivative

Cyclotriveratrylene (CTV) derivatives substituted with 9 (1) or 18 (2) long alkyl chains have been prepared. Whereas no liquid crystalline behavior has been observed for 1, the CTV derivative 2 has mesomorphic properties. Indeed, at room temperature compound 2 exhibits a nematic phase characterized by cybotactic groups with a local lamello-columnar order. Both CTV derivatives 1 and 2 are able to form supramolecular complexes with C60 in the solid state. In both cases, the 2:1 host-guest species have been obtained as brown compounds. No liquid crystalline behavior was observed for the supramolecular complex [C60 is included in (1)2]. In contrast, observation of the brown product obtained from C60 and the CTV derivative 2 directly after preparation by polarized optical microscopy revealed a fluid birefringent phase at room temperature. When the sample is heated above 70 degrees C, the birefringence of the texture under the microscope disappears and the X-ray diffraction pattern is transformed into a pattern characteristic of a cubic phase. For the first time in thermotropic liquid crystals, the space group of this cubic phase can be assigned as I4(1)32.

Synthesis, structure and magnetic properties of tetrakis-mu-carboxylato-bis(dodecylnicotinato)dicopper(II) complexes; crystal and molecular structure of the decyl carboxylate derivative.

Dodecylnicotinate bis-adducts of binuclear copper carboxylates, of the general formula Cu2(O2CC(n-1)H(2n-1))4(C5H4NCOOC12H25)2, were synthesized for n = 10, 12, 14, 16, 18 and 20, and their crystal structure, thermal behavior and magnetic properties studied. The molecular structure of the decyl derivative has been determined from single-crystal X-ray diffraction data. The dimer is centrosymmetric with the CuII ions in a square-pyramidal coordination with four O-alkyl O atoms [average d(Cu-O) 1.960 (6) A] in the basal plane and the nicotine N atom at apical positions [d(Cu-N) 2.183 (3) A]. The copper ions, 2.615 (1) A apart, are bridged by four O-alkyl carboxylate groups. Both the n = 20 and n = 18 homologues exhibit lamellar phases, which can be related to the supramolecular arrangement found in the n = 10 derivative. The magnetic behavior of the decyl and octadecyl dimers was studied in the 2-300 K temperature range. They exhibit a strong intramolecular antiferromagnetic interaction (Cu-Cu superexchange coupling constant J = -347 cm(-1) for the decyl derivative), which can be attributed to a large overlap of the metal 3d orbitals and the oxygen lone pair orbitals of the linking carboxylate groups.

Investigations of Thin Films with Amphiphilic Dendrimers Bearing Peripheral Fullerene Subunits.

In spite of a molecular mass of 7704.6 g mol(-1), third-generation compound G3 (shown schematically; Z=C(8)H(17)) is able to form stable Langmuir films. In a systematic study, the amphiphilic properties of the corresponding dendrimers of first (G1) and second generation (G2), with one and two peripheral fullerene units, respectively, were investigated and a model could be proposed for the multilayer films obtained from G1.

Molecular theory of ferroelectric ordering in enantiomeric mixtures of smectic-C* liquid crystals.

A molecular theory of ferroelectric ordering in smectic-C* liquid crystals composed of left and right enantiomers is developed taking into account the effects of chiral discrimination. The recently observed nonlinear dependence of the spontaneous polarization on the enantiomeric excess is explained in the framework of a molecular model that takes into consideration the strong electrostatic interaction between effective atomic charges in the chiral centers of the two enantiomers. This nonlinear dependence is determined by a difference of interaction energies between the pairs of chiral molecules with equal and opposite handedness, respectively. A relation between the molecular structure of different ferroelectric smectics C* and the dependence of the polarization on the enantiomeric excess is discussed in detail.

[Seasonal, weekly and lunar cycles of birth. Statistical study of 12,035,680 births]. / Les rythmes saisonnier, hebdomadaire et lunaire des naissances. Etude statistique sur 12,035,680 naissances.

The authors have studied the seasonal, weekly and lunar rhythms which could characterize deliveries, based on 12,035,680 french births, between January 1, 1968 and December 31, 1982. The seasonal rhythm is the most remarkable. It is characterized by a maximum of newborn in May and a minimum in November; there is an increased number of births during the month of September. The weekly rhythm is characterized by a drop in the number of births especially on Sundays, but also on Saturdays, with a maximum on Tuesday. The number of births also fluctuate according to the lunar cycle. Their amplitude is low. They are however significant. From that standpoint, these results confirm other studies already conducted in France and the USA, in a much smaller scale.

[Births, fertility, rhythms and lunar cycle. A statistical study of 5,927,978 births]. / Naissances, fertilité, rythmes et cycle lunaire. Etude statistique sur 5 927 978 naissances.

Is there any relationship between the times when babies are born and the synodic lunar cycle? There are published works that show that there is such a relationship. We have looked at 5,927,978 French births occurring between the months of January 1968 and the 31st December 1974. Using Fourier's spectral analysis we have been able to show that there are two different rhythms in birth frequencies: --a weekly rhythm characterised by the lowest number of births on a Sunday and the largest number on a Tuesday: --an annual rhythm with the maximum number of births in May and the minimum in September-October. A statistical analysis of the distribution of births in the lunar month shows that more are born between the last quarter and the new moon, and fewer are born in the first quarter of the moon. The differences between the distribution observed during the lunar month and the theoretical distribution are statistically significant.