Dichroic absorption

Circular Dichrograph. An instrument similar to a specliopolaiimeter. Instead of a change in angle of optical rotation versus wavelength, the instrument records the difference in dichroic absorption versus wavelength,... 

Optical measurements The circular dichroism (CD) measurements were carried out on a Jasco-500A spectropolarimeter at 24C, under a stream of high purity, dry N2. The coefficient of dichroic absorption, Ae, was calculated from the molar ellipticity [0] by the following equation molar ellipticity [0] = 3300 Ae. Both the molar ellipticity [q] and Ae are expressed in degree x cm x dmol 1(9). 

Figure 5. Spectropolarimetry of the ice-feature in the BN object in Orion [38], The continuous curve is the excess polarization and the broken curve the excess optical depth, relative to the continuum. Note that the peak polarization occurs at a slightly longer wavelength than the overall extinction, indicative of polarization by dichroic absorption.

Summary of grain properties obtained by dichroic absorption and... 

As discussed in the earlier review of the chiroptical properties of the amino chromophore, it was recognized very early on the basis of optical rotatory dispersion (ORD) measurements that for some chiral amines. Cotton effects are associated with the electronic transitions below 250 nm, and the ECD spectra are characterized by two oppositely signed dichroic absorption bands in the region 190-240 nm (Figure 6) corresponding to the electronic absorption maxima of trimethylamine in the gas phase at 199 and 227 nm. The disappearance of the ECD bands on protonation of the nitrogen lone pair of electrons supports the conclusion that this dichroic absorption is associated with electronic transitions of the lone-pair electrons on the nitrogen atom. ... 

The first experiments demonstrating the effect were conducted on cholesteryl nonanoate in which was dissolved small quantities of PAA or n-p-methoxybenzylidene-/>-phenylazoaniline.< The temperature of the system was adjusted so that the reflexion band overlapped with the strongly linearly dichroic absorption band of the solute molecule. Under these circumstances, the circular dichroism exhibits the features predicted by theory (fig. 4.1.12). Similar measurements were reported subsequently by Aronishidze et (fig. 4.1.13). 

In the spectral region in which an optically active medium has an absorption band, the two circularly polarized components of plane-polarized light will also be differentially absorbed. As a result, the light will be el-liptically polarized and a CD spectrum can be obtained. A differential dichroic absorption. 

The general features are as follows The crystals are highly dichroic, absorption setting in with the light polarized parallel at about 500 mp. In perpendicular polarization the crystals are optically clear much further out towards the blue, and the observed absorption bands are both sharper and more welldefined than are the parallel ones. The overall spectrum is as follows Broad parallel bands are seen at 23000 cm and at 27000 cm and a sharp perpendicular band is seen at 27000 cm" The molar extinction coefficients of these bands are below 100. These numbers differ somewhat from complex to complex, but the general pattern seems to be the same. 

Optical circular dichroism spectra are usually depicted as plots of molar ellipticity or circular dichroic absorption — Sr or Aa) vs. wavelength. The reason for the use of the latter term is that, in practice, a series of equations have been described which lead to equation 8 ... 

Dissymmetric systems shows exciton splitting of dichroic absorptions and optical activity and photochromic materials having NLO, photoresponsiveness and photorefractivity properties, which occurred due to the presence of azoaromatic and chiral functionalities in the polymers [62]. Finally, novel optically active multifunctional methacrylic copolymers were synthesized, which contained side-chains chiral moieties and linked to a photoconductive carbazolic and to a photochromic azoaromatic chromophores... 

For polymer XXVIb, an enhancement of rotatory strength and redshifted bands were observed with respect to the model. The reported strong optical rotation as well as the high dichroic absorption are remarkable for a polymeric compound but an ordered structure is not demonstrated. 

It should be mentioned that there is still no general quantitative theory for these problems, and therefore we can usually only apply empirical knowledge and analogies. A few typical examples will serve to illustrate various types of interaction (e.g. salt formation, inclusion compounds and associates), and their influence on optical rotation and dichroic absorption will be discussed together with the conclusions drawn from this. 

The amplitudes depend on pH. At pH 4, although the solution is deep blue, the circular dichroism is zero between 600 and 200 nm, as with pure amylose. Measurement of the dichroic absorption at pH 7 and 515 nm as a function of the concentration and the... 

Finally, reference is made to the dynamic dichroic absorption difference, which is related to an absorption change induced by an external time-dependent perturbation. Dynamic absorption spectroscopy is performed with linearly polarized IR light and serves for example to detect intermolecular hydrogen bonds in polyamides of the structure... 

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