With polarized light

Guttler F, Sepiol J, Plakhotnik T, Mitterdorfer A, Renn A and Wild U P 1993 Single molecule spectroscopy fluorescence excitation spectra with polarized light J. Lumin. 56 29-38... 

Any factor that affects the size or shape of a molecule, the hindered movement of a fluorophore within a molecule, or the energy transfer within the molecule will affect the measured depolarization of its fluorescence emission. Therefore, the conformation of humic fractions in solution can be studied as a function of pH, ionic strength, temperature, and other factors by depolarization measurements. The principle of the method is that excitation of fluorescent samples with polarized light stimulates... 

In a typical ellipsometry experiment a sample is irradiated with polarized light, which subsequently is reflected from the sample surface and detected after passing an analyzer. The ratio p of complex reflectances for perpendicularly (s) and parallelly (p) polarized light usually is represented as follows ... 

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 (—). 

Fig. 16. Spectra of the Si B—H stretching mode under stress taken with polarized light near 15 K. F is the applied force and E the electric vector of the light. [Reprinted with permission from The American Physical Society, Bergman, K., Stavola, M., Pearton, S.J., Hayes, T. (1988). Phys. Rev. B 38, 9643 ]...

One quantum effect that chemists cannot ignore consistently is molecular chirality and the interaction of chiral molecules with polarized light. Although a detailed understanding of this issue will, in the final analysis, be beyond the scope of this preliminary discussion, it provides an easy introduction and a useful guide. While the discussion of molecular chirality only becomes possible at a much later stage, a phenomenological discussion of polarimetry is a common topic even for discussion at the elementary level. 

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. 

Scheme 19 shows that arylation of spz G-H bonds with aryl iodides can be applied to the synthesis of light-emitting and liquid crystalline molecules.164 The Pd-catalyzed tandem G-H coupling reactions of thiazole with aryl iodides give the differently substituted 2,5-diarylthiazoles, which would be a highly potential single-layer EL method with polarized light emission. 

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 absorbance of the PPV LB film at peak wavelength measured with polarized light parallel to the dipping direction is about twice large that measured with polarized light perpendicular to the dipping direction. The anisotropy of absorbance... 

Fig.24. Absorption spectra of a PPV LB films (1,2) and a PPV cast film (3). Spectra of 1 and 2 were measured with polarized light parallel and perpendicular to the dipping direction.

A PC interfaced with a microscope photometer provides a powerful analytical tool working down to cellular and subcellular levels (with polarized light). With the relatively low cost of modem PCs and the wide availability of surplus components for light microscopy, the methodology is available to anyone who can combine microscopy with computer... 

Bennett HS. Methods applicable to the study of both fresh and fixed material. The microscopical investigation of biological materials with polarized light, in McClung s Handbook of Microscopical Technique, 3rd ed., (McClung Jones R, ed.), Paul Hoeber, New York, 1950, pp. 591-677. 

A rigorous and complete mathematical treatment of the polarization of light and the interaction of light with oriented matter is outside the scope of this chapter. These subjects have been thoroughly dealt with before and can be found in a number of comprehensive texts [29-32] the reader is referred to the excellent book by Michl and Thulstrup [3] for a more detailed treatment of optical spectroscopy with polarized light. Here, a conventional, qualitative representation is given to establish the nomenclature and conventions to be used and to facilitate the understanding of the concepts presented. 

J Michl and EW Thulstrup, Spectroscopy with Polarized Light, 1st ed., VCH Publishers, New York, 1986. 

The concept of transition moment is of major importance for all experiments carried out with polarized light (in particular for fluorescence polarization experiments, see Chapter 5). In most cases, the transition moment can be drawn as a vector in the coordinate system defined by the location of the nuclei of the atoms4 therefore, the molecules whose absorption transition moments are parallel to the electric vector of a linearly polarized incident light are preferentially excited. The probability of excitation is proportional to the square of the scalar product of the transition moment and the electric vector. This probability is thus maximum when the two vectors are parallel and zero when they are perpendicular. 

Figure 3.34. Photochromism of azo dyes under irradiation with polarized light. In an amorphous matrix, trans-cis isomerizations are coupled to rotational diffusion. After many isomerization cycles, the molecules are trapped in an orientation with the transition moment M perpendicular to the polarization direction of the light P.

Chiral molecules interact to form complexes that are related as enantiomers or as diastereomers. Enantiomers are perfect chemical models for each other except in their interactions with polarized light or with other chiral molecules, and this provides the basis for an absolute method for demonstrating subtle differences in physical properties that might otherwise be confused with the effects of impurities. 

Fluorescence anisotropy decay measurements, which are based on the excitation of probes with polarized light and subsequent polarized fluorescence emission, can... 

When polymer solids are produced from their melts, the most common structures are these spherulites that can be seen by the naked eye and viewed as Maltese crosslike structures with polarized light and crossed Nicol prisms under a microscope. 

Bronk, B. V., Druger, S., Czege, J., and Van de Merwe, W. (1995). Measuring diameters of rodshaped bacteria in vivo with polarized light scattering. Biophys. J. 69,1170-1177. 

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