Quantum liquid crystals

Because STM measures a quantum-mechanical tunneling current, the tip must be within a few A of a conducting surface. Therefore any surface oxide or other contaminant will complicate operation under ambient conditions. Nevertheless, a great deal of work has been done in air, liquid, or at low temperatures on inert surfaces. Studies of adsorbed molecules on these surfaces (for example, liquid crystals on highly oriented, pyrolytic graphite ) have shown that STM is capable of even atomic resolution on organic materials. 

The maintenance of a connection to experiment is essential in that reliability is only measurable against experimental results. However, in practice, the computational cost of the most reliable conventional quantum chemical methods has tended to preclude their application to the large, low-symmetry molecules which form liquid crystals. There have however, been several recent steps forward in this area and here we will review some of these newest developments in predictive computer simulation of intramolecular properties of liquid crystals. In the next section we begin with a brief overview of important molecular properties which are the focus of much current computational effort and highlight some specific examples of cases where the molecular electronic origin of macroscopic properties is well established. 

Static quadrupole effects in NMR are observed in solids (9) and also in anisotropic liquid crystals (10, 11, 12). For nuclei with spin quantum numbers, I, greater than V2, the distribution of positive charge over the nucleus can be nonspherical and the situation can be described in terms of a nuclear electric quadrupole moment. The interaction between the quadrupole moment, eQ and electric field gradients, eq, shifts the energy levels of the nuclear spin states. 

Quantum dots used in liquid crystal systems (ranging in size from a few to tens of nanometers) are usually from the II-VI group, and include CdS, CdSe, and CdTe capped with trioctylphosphine/trioctylphosphine oxide (TOP/TOPO) [101-103], CdS as well as CdTe stabilized with thiols [104], and CdSe protected with amines [105]. 

A major concern for the use of these quantum dots in liquid crystal applications is their stability. Weller et al. using NMR spectroscopy, UV-vis spectrophotometry, and analytical ultracentrifugation (AUC) showed that thiol-stabilized cadmium chalcogenide quantum dots are unstable at low particle concentrations (<10 pM in DMF), i.e., even the covalently bound thiols desorb from the quantum dot surface... 

Beneficial electro-optic effects have also been reported for semiconductor quantum dots doped into nematic liquid crystals. Khoo and Mallouck et al. published one of the earlier reports on suspensions of quantum dots in nematic liquid crystals [331], This work, however, focused on CdSe nanorods and will be discussed in a later section on two-dimensional nanomaterials in liquid crystals. 

Kitzerow et al. recently demonstrated that temperature-induced phase transitions (Iso-N) and electric field-induced reorientation of a nematic liquid crystal (5CB in this case) can be used to tune photonic modes of a microdisc resonator, in which embedded InAs quantum dots serve as emitters feeding the optical modes of the GaAs-based photonic cavity [332],... 

Fig. 23 A graphical summary of liquid crystal nanoscience summarized in this review. This chart shows the link between nanoparticles (their size, shape, and coating) and liquid crystals, effects induced in liquid crystal phases, as well as nanoparticle mesomorphism (NP nanoparticle, LC liquid crystal, QD quantum dot)...

The model of a dipole in a spherical cavity can only provide qualitative insights into the behaviour of real molecules moreover, it cannot explain the effect of electrostatic interactions in the case of apolar molecules. More accurate predictions require a more detailed representation of the molecular charge distribution and of the cavity shape this is enabled by the theoretical and computational tools nowadays available. In the following, the application of these tools to anisotropic liquids will be presented. First, the theoretical background will be briefly recalled, stressing those issues which are peculiar to anisotropic fluids. Since most of the developments for liquid crystals have been worked out in the classical context, explicit reference to classical methods will be made however, translation into the quantum mechanical framework can easily be performed. Then, the main results obtained for nematics will be summarized, with some illustrative... 

B. Mennucci and R. Cammi, Ab initio model to predict NMR shielding tensors for solutes in liquid crystals, Int. J. Quantum Chem., 93 (2003) 121-130. 

M. Pavanello, B. Mennucci and A. Ferrarini, Quantum-mechanical studies of NMR properties of solutes in liquid crystals A new strategy to determine orientational order parameters, J. Chem. Phys., 122 (2005) 064906. 

Although the physical thickness of the liquid crystal is only about 15 Angstrom, it is quantum-mechanically resonant at the wavelength of the incident photons due to its slow-wave electronic structure. The spatial profile of the anisotropic absorption spectrum is shown without dimensions. It is dependent on the absorption cross section of the liquid crystalline film. The array factor for this array cannot be determined easily using conventional antenna theory because of its sub-wavelength dimensions and other currently unknown parameters. 

However, the whole of chemistry is much more than just small, noninteracting gas-phase molecules, and much remains to be done before quantum theory can accurately describe complex chemical reactions and the properties of condensed matter such as hydrogen-bonded liquids, superconductors, liquid crystals, and glasses. Thus the experimental skills you will hone in the experiments in this book can be expected to serve you well in the foreseeable future. 

Three of the experiments are completely new, and all make use of optical measurements. One involves a temperature study of the birefringence in a liquid crystal to determine the evolution of nematic order as one approaches the transition to an isotropic phase. The second uses dynamic laser light scattering from an aqueous dispersion of polystyrene spheres to determine the autocorrelation function that characterizes the size of these particles. The third is a study of the absorption and fluorescence spectra of CdSe nanocrystals (quantum dots) and involves modeling of these in terms of simple quantum mechanical concepts. 

The excitation and detection of multiple quantum transitions in systems of coupled spins offers, among other advantages, an increase in resolution over single quantum n.m.r. since the number of lines decreases as the order of the transition increases. This paper reviews the motivation for detecting multiple quantum transitions by a Fourier transform experiment and describes an experimental approach to high resolution multiple quantum spectra in dipolar systems along with results on some protonated liquid crystal systems. A simple operator formalism for the essential features of the time development is presented and some applications in progress are discussed. 

The spectrum of benzene dissolved in a liquid crystal served as a prototype in the development of the single quantum n.m.r. of complex spin systems in ordered phases, The multiple quantum spectrum of ordered benzene is shown in fig. 2. The resolution is limited by magnetic homogeneity and the inhomogeneous linewidth is proportional to 1 1. 

Applications in progress include the configurational analysis of both ring and chain regions of liquid crystals, the study of relaxation effects in multiple quantum spectra... 

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