Polarized light induced dichroism

Choi S-W, Kawauchi S, Tanaka S, Watanabe J, Takezoe H (2007) Vibrational circular dichroism spectroscopic study on circularly polarized light-induced chiral domains in the B4 phase of a bent mesogen. Chem Lett 36 1018-1019... 

Circular birefringence will induce a differential retardation in the phase of the orthogonal states of circularly polarized light. Circular dichroism, on the other hand, results in anisotropic attenuation of left- and right-circularly polarized light. The Jones matrix of circularly dichroic materials is normally written as ... 

Ever since Pasteur s work with enantiomers of sodium ammonium tartrate, the interaction of polarized light has provided a powerful, physical probe of molecular chirality [18]. What we may consider to be conventional circular dichroism (CD) arises from the different absorption of left- and right-circularly polarized light by target molecules of a specific handedness [19, 20]. However, absorption measurements made with randomly oriented samples provide a dichroism difference signal that is typically rather small. The chirally induced asymmetry or dichroism can be expressed as a Kuhn g-factor [21] defined as ... 

Irradiation with unpolarized blue light does not erase the induced dichroism, because only the cis isomer has significant absorption in the blue region around 450 nm consequently, the trans molecules cannot be excited and reoriented. The in-plane isotropy in both the trans and the cis molecular distributions can be restored only after successive unpolarized UV and blue light irradiations. The initial spectra of a freshly prepared sample prior to irradiation is not restored by this procedure, however, because a net out-of-plane orientation of the azomolecules remains. It is noteworthy that spectra recorded with different analysis light polarizations do not intersect at isosbestic points because of the anisotropy that exists in the sample. Heating the azo-polyglutamate sample at 80°C for 30 minutes and 14.5 hours failed... 

Absj is shown), demonstrating that the monolayer was in-plane optically isotropic at that time. The UV-induced dichroism could be erased on irradiation with unpolarized blue light upon which the initial spectra for both Abs/v and Abs were recovered. Further irradiation with linearly polarized UV light restores the initial dichroism, and successive cycles of UV (linearly... 

In an external magnetic field all matter becomes optically active. This observation was first made by Faraday in 1845 the magnetically induced rotation of the plane of polarized light is therefore referred to as the Faraday effect. In recent years, the common mode of study of this phenomenon has been the measurement of magnetic circular dichroism (MCD). Similarly to natural circular dichroism, magnetic circular dichroism is defined as the difference Af = fi, - of the extinction coefficients for left-handed and right-... 

Because circular dichroism is a difference in absorption for left and right circularly polarized light, its theoretical description includes subtraction of the transition probabilities induced by left and right circularly polarized radiation. The interaction Hamiltonian that determines transition probability includes electric, , and magnetic, B, fields of electromagnetic circularly polarized radiation, and the electric, /i, and magnetic, m, dipole moments of the molecule. 

In this liquid crystal phase, the molecules have non-symmetrical carbon atoms and thus lose mirror symmetry. Otherwise optically active molecules are doped into host nematogenic molecules to induce the chiral liquid crystals. The liquid crystals consisting of such molecules show a helical structure. The most important chiral liquid crystal is the cholesteric liquid crystals. As discussed in Section 1.2, the cholesteric liquid crystal was the first discovered liquid crystal and is an important member of the liquid crystal family. In some of the literature, it is denoted as the N phase, the chiral nematic liquid crystal. As a convention, the asterisk is used in the nomenclature of liquid crystals to mean the chiral phase. Cholesteric liquid crystals have beautiful and interesting optical properties, e.g., the selective reflection of circularly polarized light, significant optical rotation, circular dichroism, etc. 

We have also measured the UV-induced dichroism for different UV irradiation times for both cis and trans molecular distributions. This was performed by irradiating the sample with linearly polarized UV light for a defined period of time, recording the absorption spectrum, and subsequently calculating the anisotropy AA and the order parameter S ... 

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