Dichroism, optical

Campbell DM, Farago PS (1987) Electron optic dichroism in camphor. J Phys B 20 5133... 

Mayer S, Kessler J (1995) Experimental verification of electron optic dichroism. Phys Rev Lett 74 4803... 

Thus, P oo for either p —> oo, which is the limit of an infinitely thin prolate ellipsoid, or p — 0, which is the limit of an infinitely flat oblate one. In these two limits, the particle rotates until a long axis is parallel to the flow direction, and then rotation slows to a halt. For large, but finite, aspect ratios, the particle rotates slowly when its long axis is nearly parallel to the flow direction, and rapidly otherwise. The Jeffery orbits of rod-like and disklike particles have been observed directly (Anczurowski and Mason 1967a, 1967b) and indirectly by optical dichroism (Frattini and Fuller 1986). 

P. Jones, W. J. Jones, and G. Williams, Transient optical dichroism in photochromic polymer glass and its relationship to molecular reorientation, J. Chem. Soc., Faraday Trans. 86, 1013-1014 (1990). 

I. Jonas, K. Fontell, G. Lindblom, and B. Norden, Linear Dichroism Spectroscopy, Proceedings of the Nobel Workshop on Molecular Optical Dichroism and Chemical Applications of Polarization Spectroscopy 1976, ed. B. Norden, Lund University Press, Lund, Sweden, 1977, p. 217. 

Molecular properties, whose variations can be attributed to variations in molecular sh, include odor, taste, optical dichroism (the octant rule ), chirality, and drug-receptor interaction. In 1920 RuiiCka forwarded a theory that the character of an odoriferous substance is determined by its molecular shape, while the variations of this character depend on the osmophoric groups in a molecule. This presented but one illustration of Emil Fisher s lock and key model for interaction of drugs and enzymes. ... 

The properties listed above illustrate interactions between two partners, and are not manifested by a single isolated object. Thus, in structure-activity studies, which include odor and taste, the shape of a small molecule has to match the cavity in the large molecule (receptor of a protein). In the case of optical dichroism the other component is circularly polarized light (electromagnetic radiation). Finally, chirality manifests itself only in the presence of a medium that can differentiate the enantiomers. 

There are numerous polarizing optical components available. Their operation is based on birefringence, polarization at mirrors at the Brewster angle, optical dichroism, etc. Precision achieved with sheet polarizers is inadequate, thus devices like Glan-Thompson, Glan-Foucault or Rochon prisms have to be used. 

In recent years, optical dichroism and birefringence based on photo-induced trans-cis-trans isomerization of azobenzene groups has been observed with preoriented liquid-crystalline polymers [31-35] at temperatures above the glass transition temperature, and also with various amorphous polymers at temperatures well below the glass transition temperature. In the case of a polyimide (see Chart 5.7), a quasi-permanent orientation can be induced [36-38]. Here, the azobenzene groups are rather rigidly attached to the backbone and photoisomerization occurs at room temperature, i.e. 325 °C below the glass transition temperature, Tg = 350°C. This behavior is in accordance with the fact that the isomerization quantum yields of azobenzene compounds are very similar in solution and in polymer matrices 0 trans cis) 0,1 and 0(cis trans) 0.5. 

Parameter S2 can be found from the anisotropy of magnetic susceptibility, optical dichroism and birefringence, NMR, etc. The determination of higher order parameters requires for more sophisticated techniques. For instance, S4 can be found from Raman light scattering [15], luminescence or other two-wave interaction optical experiments. Data on 5e, Sg are not available at present. In some cases, the X-ray scattering can even provide/(1 ) as a whole but with limited accuracy. 

W. Gotschy, K. Vonmetz, A. Leimer and F. R. Aussenegg, Optical dichroism of lithographically designed... 

Gotschy W, Vonmetz, K., Leitner, A., and Aussenegg, F. R. (1996) Optical dichroism of lithographically designed silver nanoparticle films. Opt. Lett., 21,1099-1101,... 

The most attractive feature of a-6T and a-8T crystals is undoubtedly the access that they provide to optical dichroism (absorption and luminescence) and charge transport anisotropy (carrier mobility) so that comparison can be made with what is currently observed in polycrystalline thin films and disordered polythiophenes. Beside a-6T and a-8T, a few studies have also been done on doped Q -4T(Q -Me)2 single crystals. 

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