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.

Layering transitions and Schlieren textures in Langmuir films of two organic radicals.

Two paramagnetic radicals have been investigated in terms of their film-forming properties at the air-water interface. Although the radicals failed to display any mesomorphic behavior in the bulk, they were found prone to built-up multilayer films on the Langmuir trough. The molecules seem to dimerize in the upper layers of the films that exhibit striking Schlieren textures when observed with Brewster angle microscopy. These Schlieren textures, together with the ability to form multilayers, indicate that the molecules came close to displaying smectic mesomorphism. A tentative model of the layers' structure is proposed, and a suggestion for synthesizing new molecules with actual mesomorphism is offered. The presented results show that the study of the behavior of molecules at the air-water interface can shed a new light on their behavior in the bulk and help in the design of new magnetic mesogens.

Smectic ordering in liquid-crystal-aerosil dispersions. I. X-ray scattering.

Comprehensive x-ray scattering studies have characterized the smectic ordering of octylcyanobiphenyl (8CB) confined in the hydrogen-bonded silica gels formed by aerosil dispersions. For all densities of aerosil and all measurement temperatures, the correlations remain short range, demonstrating that the disorder imposed by the gels destroys the nematic (N) to smectic-A (SmA) transition. The smectic correlation function contains two distinct contributions. The first has a form identical to that describing the critical thermal fluctuations in pure 8CB near the N-SmA transition, and this term displays a temperature dependence at high temperatures similar to that of the pure liquid crystal. The second term, which is negligible at high temperatures but dominates at low temperatures, has a shape given by the thermal term squared and describes the static fluctuations due to random fields induced by confinement in the gel. The correlation lengths appearing in the thermal and disorder terms are the same and show a strong variation with gel density at low temperatures. The temperature dependence of the amplitude of the static fluctuations further suggests that nematic susceptibility becomes suppressed with increasing quenched disorder. The results overall are well described by a mapping of the liquid-crystal-aerosil system onto a three-dimensional XY model in a random field with disorder strength varying linearly with the aerosil density.

Hydrogen-bonded silica gels dispersed in a smectic liquid crystal: a random field XY system.

The effect on the nematic to smectic-A transition in octylcyanobiphenyl (8CB) due to dispersions of hydrogen-bonded silica (aerosil) particles is characterized with high-resolution x-ray scattering. The particles form weak gels in 8CB creating a quenched disorder that replaces the transition with the growth of short-range smectic correlations. The correlations include thermal critical fluctuations that dominate at high temperatures and a second contribution that quantitatively matches the static fluctuations of a random field system and becomes important at low temperatures.

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.