Peter J. Collings

Handbook of liquid crystals

This four-volume reference aims to provide information on the basic principles of both low- and high-molecular weight materials, as well as the synthesis, characterization, modification, and applications (such as in computer displays or as structural materials) of all types of liquid crystals. Volume 1, for example, deals with the basic physical and chemical principles of liquid crystals, including structure-property relationships, nomenclature, phase behaviour, characterization methods, and general synthesis and application strategies.

Resonance light scattering: a new technique for studying chromophore aggregation

Light scattering experiments are usually performed at wavelengths away from absorption bands, but for species that aggregate, enhancements in light scattering of several orders of magnitude can be observed at wavelengths characteristic of these species. Resonance light scattering is shown to be a sensitive and selective method for studying electronically coupled chromophore arrays. The approach is illustrated with several examples drawn from porphyrin and chlorin chemistry. The physical principles underlying resonance light scattering are discussed, and the advantages and limitations of the technique are reviewed.

Liquid Crystals: Fundamentals

Liquid crystals: main types and classification distribution functions and order parameters physical properties of liquid crystals nematic liquid crystals nematic liquid crystals - elastostatics and nematodynamics smectic liquid crystals liquid crystals of disc-like molecules polymer liquid crystals chiral liquid crystals lyotropic liquid crystals defects and textures in liquid crystals.

Aggregation Kinetics Of Extended Porphyrin And Cyanine Dye Assemblies

The kinetics of J-aggregate formation has been studied for two chromophores, tetrakis-4-sulfonatophenylporphine in an acid medium and pseudoisocyanine on a polyvinylsulfonate template. The assembly processes differ both in their sensitivity to initiation protocols and in the reaction profiles they produce. The porphyrins assembly kinetics, for example, displays an induction period unlike that of the cyanine dye. Two kinetic models are presented. For the porphyrin, an autocatalytic pathway in which the formation of an aggregation nucleus is rate-determining appears to be applicable; for the pseudoisocyanine dye, an equation derived for diffusion-limited aggregation of a fractal object satisfactorily fits the data. These models are shown to be useful for the analysis of kinetic data obtained for several biologically important aggregation processes.

Depolarized Resonance Light Scattering by Porphyrin and Chlorophyll a Aggregates

A quantum mechanical model is developed for the observed resonance enhancement of light scattering by aggregates of electronically interacting chromophores. Aggregate size, monomer oscillator strength, extent of electronic coupling, and aggregate geometry are all important determinants of intensity in resonance light scattering (RLS) spectra. The theory also predicts the value of the depolarization ratio (rho(v)(90)) of RLS for a given aggregate geometry. These results are used to interpret the RLS depolarization ratios of four aggregates: tetrakis(4-sulfonatophenyl)porphine aggregated at low pH (rho(v)(90) = 0.17 at 488 nm), trans-bis(N-methylpyridinium-4-yl)-diphenylporphinato copper(II) aggregated in 0.2 M NaCl solution (rho(v)(90) = 0.13 at 450 nm) and on calf thymus DNA (rho(v)(90) = 0.20 at 454 nm), and chlorophyll a aggregates in formamide/water (rho(v)(90) = 0.23 and 0.32 at 469 and 699 nm, respectively). The analysis is consistent with a J-aggregate geometry for all four systems. Furthermore, the specific values of rho(v)(90) allow us to estimate the orientation of the monomer transition dipoles with respect to the long axis of the aggregate. We conclude that depolarized resonance light scattering spectroscopy is a powerful probe of the geometric and electronic structures of extended aggregates of strong chromophores.

Interactions of copper(II) porphyrins with DNA

Abstract The interactions of three cationic water soluble copper(II) porphyrins, differing in peripheral substituents, with calf thymus DNA are described. Tetrakis(N-methylpyridinium-4-yl)porphinecopper(II) behaves as a simple intercalator under the conditions investigated, whereas tetrakis(4-N,N′,N″-trimethylanilinium)porphinecopper(II), binds externally, with some limited aggregation under high drug load conditions. In contrast, trans-bis(N-methylpyridinium-4-yl)diphenylporphinecopper(II) (t-CuPagg), like the free-base t-H2Pagg from which it is derived, is capable of forming extended electronically coupled arrays while bound to the DNA template. These arrays have been investigated using a combination of extinction spectroscopy, circular dichroism, RLS and resonance Raman spectroscopy. They are found to contain 105–106 porphyrin units, arranged in long, narrow organized structures. The kinetics of assembly of t-CuPagg is reported on three DNAs: ct DNA, poly(dG–dC)2 and poly(dA–dT)2. A non-conventional autocatalytic model first proposed for t-H2Pagg assembly formation is successful at fitting these data, permitting direct comparisons of kinetic parameters for the two porphyrins. It is found that the catalytic rate constant (kc) is considerably smaller for t-CuPagg than for t-H2Pagg under comparable conditions, and that the template rigidity fosters assembly formation. We also report the resonance Raman spectra of t-H2Pagg/ct DNA and t-CuPagg/ct DNA complexes. Aggregation on the DNA template changes the intensity pattern of the porphyrins resonance Raman spectra, with some low- and high-frequency bands becoming strongly enhanced upon aggregation. We conclude that aggregation-enhanced resonance Raman spectroscopy is a useful probe of aggregation in porphyrin–DNA complexes that also gives detailed information about structural changes that accompany the aggregation process.

Chiral symmetry breaking and surface faceting in chromonic liquid crystal droplets with giant elastic anisotropy

Significance Lyotropic chronomic liquid crystals (LCLCs) are water-based systems consisting of planar molecules that form aligned stacks in the nematic phase that develop two-dimensional crystalline order upon cooling to the columnar phase. They are characterized by an unusually small resistance to twist distortions. This work explores the interplay of giant elastic anisotropy and geometrical frustration imposed by boundary conditions in droplets, demonstrating, in particular, spontaneous formation in the nematic phase of chiral patterns from achiral building blocks and of central line defects and surface faceting in the columnar phase. Because LCLCs are water-loving, these findings about the combined effects of anisotropic elasticity, confinement, and frustration take steps toward tapping applications for liquid crystals in aqueous environments. Confined liquid crystals (LC) provide a unique platform for technological applications and for the study of LC properties, such as bulk elasticity, surface anchoring, and topological defects. In this work, lyotropic chromonic liquid crystals (LCLCs) are confined in spherical droplets, and their director configurations are investigated as a function of mesogen concentration using bright-field and polarized optical microscopy. Because of the unusually small twist elastic modulus of the nematic phase of LCLCs, droplets of this phase exhibit a twisted bipolar configuration with remarkably large chiral symmetry breaking. Further, the hexagonal ordering of columns and the resultant strong suppression of twist and splay but not bend deformation in the columnar phase, cause droplets of this phase to adopt a concentric director configuration around a central bend disclination line and, at sufficiently high mesogen concentration, to exhibit surface faceting. Observations of director configurations are consistent with Jones matrix calculations and are understood theoretically to be a result of the giant elastic anisotropy of LCLCs.

Molecular aggregation and chromonic liquid crystals

Almost 30 years ago, when the study of chromonic liquid crystals was in its infancy, Alfred Saupe and L.J. Yu investigated disodium cromoglycate using deuteron magnetic resonance spectroscopy. Among their findings were that the quadrupolar splitting of the D2O spectrum increased with decreasing temperature and that the addition of salt to the solution increased the temperature stability of the nematic phase. A good deal of research on chromonic liquid crystals has been reported since that time, and therefore a lot more is known. However, it is also interesting that some of the questions from that time remain unanswered.

Field-dependent tilt and birefringence of electroclinic liquid crystals: Theory and experiment

An unresolved issue in the theory of liquid crystals is the molecular basis of the electroclinic effect in the smectic-A phase. Recent x-ray scattering experiments suggest that, in a class of siloxane-containing liquid crystals, an electric field changes a state of disordered molecular tilt in random directions into a state of ordered tilt in one direction. To investigate this issue, we measure the optical tilt and birefringence of these liquid crystals as functions of field and temperature, and we develop a theory for the distribution of molecular orientations under a field. A comparison of theory and experiment confirms that these materials have a disordered distribution of molecular tilt directions that is aligned by an electric field, giving a large electroclinic effect. It also shows that the effective dipole moment, a key parameter in the theory, scales as a power law near the smectic-A--smectic-C transition.

Lattice melting at the clearing point in frustrated systems

Abstract This communication was stimulated by a meeting of the Sonderforschungsbereich 335, ‘Anisotrope Fluide’ at the Technische Universitat Berlin, in December 1994. At this conference we realized that there are some very strong similarities between the melting processes of blue phases, twist grain boundary phases, and cubic D phases to the isotropic, amorphous liquid. In all three cases the melting appears to be a two stage process with a sharp melting enthalpy followed by a broad diffuse endotherm as the disorganized liquid is formed. In this article we wish to draw attention to similarities in the nature of these melting processes, to suggest a general descriptive model for the breakdown in molecular ordering, and to stimulate a theoretical debate in order to gain an understanding of the role of frustration phenomena in melting processes.