Bridging the gap between Molecular Dynamics and EPR spectroscopy: Application to Liquid Crystal systems

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Liquid crystals (LCs) are an intermediate state of matter that combines both long range order, as found in crystals, and disorder, a characteristic of fluids. This combination of properties gives rise to unique optical and electrical properties that allow LCs to be switched rapidly by electric fields. These properties underlay their widespread application, for example, in displays as seen in flat TV screens and digital watches. The ordering of molecules in LC phases can be, for example, as parallel rods (termed nematic) or as stacks of discs (termed columnar discotics). The latter can conduct charge up and down the columns. This offers potential applications as organic electronic charge transport materials in devices such as light emitting diodes (LEDs), one-dimensional conductors, to provide new types of photoconductors, and photovoltaic solar cells. The underlying molecular organisation must be engineered to produce electron-hole recombination for luminescence or separation for solar cells. Molecular organisation can be by vacuum deposition or the formation of films. The growing demand for LCs with new properties is leading to the development of new mixtures of liquid crystals, new ways to align LC layers for holographic video projection, and the production of new polymeric liquid crystals. The exploitation of self-organising LCs is an important avenue of current research. In order to design novel materials with desired properties it is necessary to describe and predict their behaviour from the molecular level. Recently, major advances in both theoretical and experimental areas have emerged which promise progress in the studies of complex partially ordered molecular systems such as LCs. Firstly, spin labels, specially designed chemical agents that carry a stable unpaired electron, can be introduced within complex molecular systems in order to report on the order and dynamics of surrounding molecules. Because an electron has a magnetic moment it can interact with an external magnetic field. Electron Paramagnetic Resonance (EPR) measures this interaction in the form of spectral line shape. The orientation of the spin label to the magnetic field has a dramatic effect on this line shape and therefore molecular mobility, dynamics and distribution can be studied. EPR is a technique that acts as a snapshot of very fast molecular motions and can resolve molecular re-orientational dynamics of the introduced spin probe over times shorter than a billionth of a second. Recent advances in EPR instrumentation, using different frequencies with spin labels and probes has become an important method for studying structure and dynamics of complex phases such as LCs, of proteins and their complexes, DNA/RNA, polymers, lipids and nanostructures. Secondly, the huge increase in computer power over the last decade has led to an increase in the use of molecular dynamics (MD) simulations as a tool to understand complex chemical systems with the potential to predict various properties of complex self-organising systems such as LCs, LC mixtures and composite systems. Yet there is no general methodology and user-friendly computational tools which are able to link directly state-of-the art MD simulations of complex molecular systems with the simulation and analysis of EPR spectra.The aim of this proposal is to bridge the gap between MD simulations and EPR spectroscopy and to develop methodology, which would enable one to obtain a detailed description and reach unambiguous conclusions about molecular arrangement and interactions within complex molecular systems. In the proposed work, this methodology will be applied to different types of LCs, mixtures and hybrid systems. The output of this proposal will be available to the international scientific community in the form of user-friendly software.