Reflection, selective

The cholesteric phase maybe considered a modification of the nematic phase since its molecular stmcture is similar. The cholesteric phase is characterized by a continuous change in the direction of the long axes of the molecules in adjacent layers within the sample. This leads to a twist about an axis perpendicular to the long axes of the molecules. If the pitch of the heHcal stmcture is the same as a wavelength of visible light, selective reflection of monochromatic light can be observed in the form of iridescent colors. 

Connors and Jozwiakowski have used diffuse reflectance spectroscopy to study the adsorption of spiropyrans onto pharmaceutically relevant solids [12]. The particular adsorbants studied were interesting in that the spectral characteristics of the binary system depended strongly on the amount of material bound. As an example of this behavior, selected reflectance spectra obtained for the adsorption of indolinonaphthospiropyran onto silica gel are shown in Fig. 1. At low concentrations, the pyran sorbant exhibited its main absorption band around 550 nm. As the degree of coverage was increased the 550 nm band was still observed, but a much more intense absorption band at 470 nm became prominent. This secondary effect is most likely due to the presence of pyran-pyran interactions, which become more important as the concentration of sorbant is increased. 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Films of pure CNLCs have a unique transmission behavior as CP light with the same sense of circular polarization as the CNLC is filtered out by reflection, while CP light of the opposite handedness as the CNLC film is transmitted. This selective optical transmission characteristic is referred to as a one-dimensional photonic stop-band or a selective reflection band. The stop-band is centered at a certain wavelength Ac, which is dependent on the pitch length p and the average refractive index n of the CNLC ... 

Finally, in x-ray diffraction measurements the knowledge of the equation of state (EOS) of specific materials is used to calculate the pressure by the measurement of selected reflections. For this purpose, Au [266], NaCl [267], Re [268], Pt [269], and MgO [231] have been used in x-ray scattering measurements when no optical access was available. 

The conditions for the data collections are summarized in ref. The intensities of selected reflections [I > 2cr(I)] were corrected for Lorentz and polarisation factors but not for absorption. 

Selective hydroxylation of phenol with hydrogen peroxide was reported on acid zeolite catalysts [91-92]. Peroxonium ions, formed by H2O2 protonation, are the oxidizing species. When the reaction is carried out on a faujasite catalyst, a mixture of hydroxybenzenes and tars is obtained [91]. In the presence of H-ZSM-5 on the other hand, no tar formation was mentioned (which does not necessarily mean that it was absent) and p-selectivities close to 100% were reported for the hydroxylation [92]. These superior selectivities reflect the shape selective properties of ZSM type zeolites. 

The particular colour which we see from objects coloured with dyes and pigments arises from the absorption and selective reflection and transmission of light from the materials containing the colorants into our eyes, there to be received by the photoreceptors and interpreted by the brain. 

Selective reflection of light Thermochromic cholesteric devices, gel displays... 

Cholesteric LCs can act as hosts for dyes to produce coloured displays (see section 5.2.2.1 below) " their temperature dependent colour change has found applications in thermochromic inks, °" and as pigments and copy safe colours their selective reflecting capabilities have been applied in colours and filters for reflective displays and projection systems, " reflective polarisers and their electrical field induced switching in displays and smart reflectors, in colour patterning for full-colour recording. ... 

Brief treatment of 2,6-dibromo-2,6-dideoxy-D-mannono- (2) or -D-glucono-lactone (9) (Scheme 3) with potassium fluoride or potassium carbonate in acetone gave the 6-bromo-2,3-monoepoxymannonolactone (10), whereas after a more prolonged reaction time the diepoxylactone 11 was formed [19]. This selectivity reflects the difference in reactivity of the two bromine atoms, as also discussed for the selective reductions above. An interesting observation was that... 

Research Focus Cholesterol-containing liquid crystals that selectively reflect light of various wavelengths. 

A different situation arises if there are reflection or scattering effects which can contribute to the loss of transmission and to the form of the emission spectra. The selective reflections reduce the emission and occur in the vicinity of very strong absorptions 163), These effects tend to reduce the relative intensity of the stronger emissions with respect to the weaker emissions. 

The book is arranged in 14 chapters. After discussing general aspects, separation of hydrocarbons from natural sources and synthesis from Ci precursors with the most recent developments for possible future applications, each chapter deals with a specific type of transformation of hydrocarbons. Involved fundamental chemistry, including reactivity and selectivity, as well as stereochemical considerations and mechanistic aspects are discussed, as are practical applications. In view of the immense literature, the coverage cannot be comprehensive and is therefore selective, reflecting the authors own experience in the field. It was attempted nevertheless to cover all major aspects with references generally until the early 1994. 

The procedure consists of four steps 1) the computer surveying the intensities of previously measured reflections to choose about 20 high 20 reflections, 2) making highly accurate step-scans of the selected reflections, 3) calculating accurate 2-theta values from the scan data and 4) calculating unit-cell parameters from accurate 2-theta measurements. 

Useful applications have been found lor the varied effects of these crystal changes. One of the first came from the properly of selectively reflecting visible light because this is lempcraiurv-dependent. the property can be used as a temperature detector, and in gel lurm liquid crystals have been used lor the early detection of those cancers which cause hot spots in the body. Applications of the smectic modifications arise from their ferroelectric properties this phase can function as a fast-switching light-valve device with memory. This kind of application requires some... 

The selective reflection of circularly polarized light on radiation of normal light is also exhibited by the cholesteric polymers. Like the 1-l.c. systems, the Grandjean-texture is formed spontaneously, if the polymer is sandwiched between glass plates as shown in Fig. 23. The measurements of indicate no difference to low molar mass systems. In Fig. 24 VHT) is shown for the induced cholesteric polymers, whose birefringence was discussed above (refer to Table 8, copolymers No. 4, m = 6). The different curves refer to different mole fractions of the chiral comonomer. With... 

Among the most frequently used formylation methods, the Gattermann-Koch reaction shows the highest selectivity reflected both in the observed high toluene- benzene rate ratios as well as a high degree of para substitution (Table 5.30). 

An easily recognized difference of nDNA N with respect to longer DNA is that generally shorter pitch values are found, well within the visible range for some short sequences, so that distinctive colored textures are generated by selective reflection of incident light, as apparent in Fig. 11c [54],... 

Fig. 4 Selective reflection wavelength (optical pitch) as a function of temperature for three samples host material showing SmC and SmCA without dopant, with 1% and 3% dopant The chemical structures of the rod-shaped host and the bent-shaped dopant molecules are also shown. As shown in inset, inverse pitch increases almost linearly with dopant content [4]...

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