Orientation dichroism measurements

The classical scheme for dichroism measurements implies measuring absorbances (optical densities) for light electric vector parallel and perpendicular to the orientation of director of a planarly oriented nematic or smectic sample. This approach requires high quality polarizers and planarly oriented samples. The alternative technique [50, 53] utilizes a comparison of the absorbance in the isotropic phase (Dj) with that of a homeotropically oriented smectic phase (Dh). In this case, the apparent order parameter for each vibrational oscillator of interest S (related to a certain molecular fragment) may be calculated as S = l-(Dh/Di) (l/f), where / is the thermal correction factor. The angles of orientation of vibrational oscillators (0) with respect to the normal to the smectic layers may be determined according to the equation... 

Of course, it is possible to contemplate experiments that examine photoionization of oriented chiral molecules. An expression has been given for the angle integrated (total) ionization cross-section in such circumstances [48] and CDAD-type measurements have been reported on adsorbed chiral molecules [49, 50], but the interplay of natural and geometric chirality in angle-resolved dichroism measurements remains very much a topic for future investigation. 

The linear dichroism measurement demonstrated that MOPPV chains formed the planar orientation chains in the LB films dichroic ratio Ap/As... 

Well organized Langmuir-Blodgett (LB) films have been obtained from mixtures of a push-pull carotenoid and co-tricosenoic acid as shown in 7. These mixed films exhibit a very good cohesion, with an area of about 25 A2 per carotenoid molecule. They can easily be transferred onto solid substrates. Examination by UV-visible linear dichroism measurements confirms that the carotenoid chains are oriented perpendicularly to the surface of the substrate in card-packed aggregates, in which the polyenic chains interact via excitonic coupling, as indicated by the large hypsochromic shift of the tc-tc transition (20). 

Evidently, Raman scattered light contains information about both the second and the fourth moments of the orientation distribution function. This is in contrast to birefringence and dichroism measurements, which respond only to anisotropies in the second moments. 

Infrared linear dichroism measurements may be performed on unidirectionally oriented samples. In the case of DNA, the favored axis of orientation is the axis of the double helix. Films are prepared by unidirectional stroking while the sample is gently dried. Two spectra are recorded with the electric field of the incident light oriented parallelly... 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

On the other hand, stretching the polymer matrix causes molecular orientation of the photochrome as shown by dichroism measurements it wfll also restrict their isomerization possibilities Thus, stretching of a film of poIy(bisphenol-A-pimelate) (M 32000)... 

Alegria and Dutton used the Langmuir-Blodgett (LB) technique to deposit multiple (mono) layers of either the Rp. viridis reaction-center complex or of the corresponding chromatophores, in either case dispersed in a phospholipid matrix. The absorption spectmm of the LB-film was almost identical to that of the reaction center complex in solution, as shown in Fig. 10 (A), indicating that the in vitro properties of the photosynthetic apparatus are stable and essentially retained in the LB-films. The authors used a combination of redox potentiometry and absorption spectroscopy to identify the four Rp. viridis hemes and their redox potentials, and also determined their orientations by linear-dichroism measurements. 

Note that the ring planes of the chlorophyll molecules in Fig. 5 (A) were shown oriented mostly perpendicular to the membrane plane. This orientation appears to be similar to that in the PS-II chlorophyll a/b protein (LHC II), as determined by Kiihlbrandt and Wang and predicted by Haworth, Tapie, Amtzen and Breton on the basis of circular-dichroism measurements. The center-to-center distances between the porphyrin planes of the CA-Chl a molecules in the PS-I reaction center range from 8 to 15 A, which are comparable to those in PS tf and in the BChl-a protein complex as measured by Mathews, Fenna, Bolognesi and Olson (see Chapter 8, Section V). Some of the closely spaced chlorophyll-a molecules appear to form chains mnning between the stroma and lumen sides of photosystem I. Between 12 and 16 P-carotene molecules are thought to be present in the PS-I reaction center, but they have not been identified by X-ray crystallographic measurements. 

The measurement of orientation by sonic techniques has received relatively little attention. This method along with that of birefringence and dichroism measures only the second moment of the orientation distribution function. It does offer, however, some advantages, probably the most important being that it can be easily used for measuring the average orientation in fibers. 

The detailed information about molecular orientation in oriented film specimens can be obtained from IR dichroism measurements (Hu et al., 1985 Wu et al., 1986). Light is absorbed when the direction of oscillation of its electrical vector has a component along the direction of oscillation of the absorbing group. The intensity of the absorption band of an oriented polymer thus depends on the direction of the electrical vector of the incident ray... 

The B-DNA structure given by Amott and Hukins [171] is taken as initial conformation for the DNA fragment as this structure is close to the native DNA at physiological conditions. According to electric linear dichroism measurements [154], there are several possibilities for the mutual geometrical arrangement of the components of the physical complex. Hence, different initial orientations and conformations of PAH metabolites were considered with the aim to find the most stable structure of the intercalated complex. 

Rheo-optical techniques (46 8) afford information on the strain dependence not only of stress but also of optical quantities associated directly with the structure or molecular morphology. The techniques were developed extensively for crystalline polymers to investigate the molecular deformation mechanism underlying the tensile elongation. In this part, the chain orientation behavior is characterized by infrared dichroism measured simultaneously with tensile deformation at a constant rate of elongation. 

Because of their inability to obtain fiber patterns. X-ray studies have not provided any conclusive evidence as to the nature of the conformations of syndiotactic poly(methacrylic acid) (PMMA). IR dichroism measurements cannot be applied because the oriented films cannot be prepared however, it was possible to obtain Raman depolarization data in aqueous solutions (i 7). Figure 5 shows Raman spectra in the solid state and in aqueous solution. Because of striking similarity of the spectra, it was concluded that the local conformations in both phases are similar. The frequency shift of two bands at 1452 and 1685 cm" was attributed to the differences in the extent of hydrogen bonding. On the basis of the IR and Raman activity of polarized and depolarized spectra, it was proposed that both solid and aqueous PMMA exist as helices that contain more than six monomer repeating units per two... 

With optically anisotropic samples (crystals and oriented films and fibers), dichroism measurements indicate the direction of the transition dipole moment. Since the... 

The orientation of the crystals was also followed by infra-red dichroism using the 1220 cm crystal band. The absolute orientation could not be independently established from these measurements because of the uncertainty of the angle that the transition moment makes with respect to the crystal axes. This angle may, however, be determined by calibrating the infra-red dichroism measurements against X-ray diffraction measurements of crystal orientation. These measurements by Samuels S established that the transition moment for this band makes an angle of 12° with respect to the crystal c-axis. 

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