Dichroic effects

The growth of fiberlike aggregates of optically active molecules [48], like those encountered with some optically active organogelators, is known to be accompanied by important CD modifications. CD data, free of linear dichroic effects, so as to avoid the contribution of macroscopic anisotropy, reveal chirality effects typical of helical structures. The electronic absorption spectra of the gelators indicate the wavelengths at which the electronic transitions occur and where dichroic effects have to be sought (unless selective reflection components... 

In the experimental studies that have identilied synergistic two-photon processes, little attention has yet been paid to effects of laser polarization. However, judicious control of polarization should enable more detailed information to be derived from the recorded spectra, particularly in the case of two-beam excitation where more polarization parameters are variable. This section provides a detailed theory of the polarization dependence of synergistic two-photon absorption in fluid media, including an analysis of some unusual dichroic effects. 

Fig. 2.2 Geometrical arrangements being necessary in photoemission experiments using dichroic effects. Left CDAD. Middle MCDAD. Right MLDAD. q denotes the photon propagation direction, Oph the angle of the incoming photon beam, the photoelectron momentum, n the surface normal, and M the magnetization direction...

A phthalocyanine may also have a chiral structure if the metallic centre coordinated by the nitrogen atoms is out of the plane defined by the Pc ligand. A subphthalocyanine (6.41) with C3 symmetry was resolved by HPLC. The CD showed dichroic effects at 560 and 570 nm (Qoo- band) and 280 and 300 nm (Soret band), being either positive or negative according to the enantiomer. Unfortunately, the authors were not able to assign configurations to the enantiomers. 

Bradbury et al. (1961) have described in detail the experimental techniques used in a study of films of DNA polymers and have recorded polarized infrared spectra of oriented sheets of the sodium and lithium salts of DNA, in both undeuterated and deuterated states, over a range of humidities between 0 and 94% relative humidity (r.h.). They found two forms of sodium DNA, one at humidities greater than 90% r.h. where the bases are perpendicular to the helix axis (B-form) and the other at humidities between 70 and 80% r.h. where the bases have tilted by an angle not less than 13° to the normal to the helix axis (A-form). Lithium DNA exists in the B-form for humidities greater than 66 % r.h., and in a second form, different from the A-form of sodium DNA, between 44 and 56% r.h. where the bases are tilted by about 4° from the perpendicular (C-form). These workers have discussed the orientation of the phosphate groups, which they inferred from dichroic effects. 

MOLECULAR ORIGIN OF DICHROIC EFFECTS IN DIFFERENT SPECTRAL REGIONS... 

To illustrate the dichroic effects the polarization >ectra taken during a loadingunloading cycle of polyester urethane (c) at 300 K in the 3500-2700 cm wavenumber region are shown in a stacked plot in Fig. 49. In Fig. 50 the corresponding orientation functions of the v(NH) and vfCH ) absorption bands have been plotted for the... 

Chiral discrimination also extends to the formation of the helical polymer 37 in water when the 4-tert-butoxyaminocinnamoylamino substituent at the C3 carbon of the modified a-CD monomer is complexed by a second monomer and so on to form a polymer composed of at least 15 such units in water as shown in Figure 12. Negative and positive Cotton circular dichroic effects at 327 and 288 nm, respectively, are consistent with the polymer assuming a left-handed anticonfiguration and a slanted complexation of the substituents in the a-CD atmnh. 

The subject of orientation measurements by visible dichroism also received attention in this period (48-50). In 1950, a fundamental analysis of visible dichroism was published (51), showing that the dichroic effect had the same second-order form as refractive index ... 

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