Observations with polarized-light

Solvent Conditions (Temp. °C) Observation with Normal Light Observation with Polarized Light... 

The depolymerization of crystalline cellulose in wood has been observed with polarized light microscopy (IT). A loss of birefringence was associated with early stages of brown rot. The attack on cellulose is rapid and difiuse throughout the wood. The cells appear to maintain their usual form, but will shatter and collapse because they lack the strength that cellulose provides the woody cell wall (Figure 6b). 

Most of the physical properties (e.g., boiling and melting point, density, refractive index, etc.) of two enantiomers are identical. Importantly, however, the two enantiomers interact differently with polarized light. When plane polarized light interacts with a sample of chiral molecules, there is a measurable net rotation of the plane of polarization. Such molecules are said to be optically active. If the chiral compound causes the plane of polarization to rotate in a clockwise (positive) direction as viewed by an observer facing the beam, the compound is said to be dextrorotatory. An anticlockwise (negative) rotation is caused by a levorotatory compound. Dextrorotatory chiral compounds are often given the label d or ( + ) while levorotatory compounds are denoted by l or (—). 

Morphology. Observations with the light microscope, under polarized light, showed that the end blocks in the case of both types of polymers crystallized in the form of the usual spheru-lites, but not as well as the analogous homopolymer, H2-l,4-polybutadiene. The formation of the spherulites was improved with increasing end-block content and/or higher molecular weight of the end blocks. 

If a sample is excited with polarized light and emission is measured through a second set of polarizers parallel and perpendicular to the first polarizer, the ratio of the two emission signals reflects the rotatory freedom of the chromophore. In practice, binding of a second molecule to the labeled one can be detected if the size of the chromophore complex increases considerably. The advantage of this method is that no changes in quantum yield are necessary for the observation of the binding reaction. 

The starting system is achiral (plates at 90° with isotropic fluid between), but leads to the formation of a chiral TN structure when the fluid becomes nematic. In this case, enantiomeric domains must be formed with equal likelihood and this is precisely what happens. The size of these domains is determined by the geometry and physics of the system, but they are macroscopic. Though the output polarization is identical for a pair of heterochiral domains, domain walls between them can be easily observed by polarized light microscopy. This system represents a type of spontaneous reflection symmetry breaking, leading to formation of a conglomerate of chiral domains. 

To analyze the second-order susceptibility, the symmetry of the films was first analyzed by measuring the intensities of the second-harmonic light. The sample was irradiated with polarized light from a Nd YAG laser incident at 45°, and the second-harmonic light emanating from the sample was detected while the sample was rotated around its surface normal. No variation in the second-harmonic intensity was observed as the sample was rotated, indicating... 

The observed rotation depends on the number of chiral molecules that interact with polarized light. This in turn depends on the concentration of the sample and the length of the sample tube. To standardize optical rotation data, the quantity specific rotation ([a]) is defined using a specific sample tube length (usually 1 dm), concentration, temperature (25 °C), and wavelength (589 nm, the D line emitted by a sodium lamp). 

As Fig. 2.6 reveals, optical microscopy with polarized light makes it possible to distinguish various intermediate products formed from a precursor of graphitic carbon at different stages of pyrolysis. The observed morphology is the so-called optical texture, which enables carbon sohds to be classified according to their degree of anisotropy. Besides optical microscopy, other techniques... 

Figure37. (a) Crystalline and right-handed helical fibersmadeofGlc-NC(12)CN-Glc (116, n= 12) observed using polarized light microscopy (at 25 C in water). Periodical structures of the fibers are denoted by arrows, (b) Polarized light micrographs of representative dehydrated and right-handed fibers from Glc-NC(12)CN-Glc (116, rt = 12), (top) photographed trough cross-polarized filters and (bottom) through plane-polarized filters. Reproduced from ref. 338 (Shimizu and Masuda, J. Am. Chem. Soc. 1997, 119,28)2) with permission of the American Chemical Society.

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