Aggregate anisotropy

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

Fluorescence is a well-observed phenomenon characteristic of many materials and the different forms of their aggregation. Meantime the vast majority of studies on fluorescence have been on small organic molecules in liquid solutions. Parameters of their emission (intensity, lifetime, anisotropy, and positions of excitation and emission spectra) were found to be extremely sensitive to intermolecular interactions [1], which justifies their extensive application in various sensing technologies... 

The long lifetime of phosphorescence allows it to be used for processes which are slow—on the millisecond to microsecond time scale. Among these processes are the turnover time of enzymes and diffusion of large aggregates or smaller proteins in a restricted environment, such as, for example, proteins in membranes. Phosphorescence anisotropy is one method to study these processes, giving information on rotational diffusion. Quenching by external molecules is another potentially powerful method in this case it can lead to information on tryptophan location and the structural dynamics of the protein. 

Ferrofluid NMR studies can also be used in order to determine geometrical and physical properties of the super-paramagnetic crystals, like their specific magnetization or radius. They also give valuable information on the aggregation level and on anisotropy. 

Note A pronounced anisotropy in the shapes and interactions of molecules, or molecular aggregates is necessary for the formation of liquid crystals. 

The ultrafast excitation-energy transfer of the J-aggregates on the octahedral AgBr is studied by the time-resolved fluorescence-anisotropy decay (r(t)) measurements [9]. They are biphasic with two time constants of -0.15 ps and 2-7 ps as shown in Fig. 6. Each phase should reflect some difference in the orientation of the dye molecules of the J-aggregates. 

Ion-pairing between the paramagnetic [Co(tdt)2] monoanion and nine different cations was examined by Tsao and Lim.105 The cations belong to either of two classes, quaternary ammonium or substituted V-octylpyridinium ions. By recording the H NMR spectra as a function of concentration (nitrobenzene, 307 K), the concentration association constants (Kas) were obtained. Substituent effects were found to influence the ion-pair geometry, as deduced from the isotropic shifts of the cationic protons and their shift ratios. In low dielectric constant solvents, speculation consistent with the magnetic anisotropy and the relation between the cationic proton shifts and concentration was tendered for cylindrically shaped aggregates. 

When micellar aggregates are formed in solutions and their aggregation numbers are not very large, they are randomly dispersed, owing to thermal motion. Weak indications of anisotropy are found at very high concentrations only. 

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