Liquid optical dichroism

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. 

Constantine S, Zhou Y, Morals J and Ziegler L D 1997 Dispersed optical heterodyne birefringence and dichroism of transparent liquids J. Phys. Chem. A 101 5456-62... 

Optical Spectroscopy General principles and overview, 246, 13 absorption and circular dichroism spectroscopy of nucleic acid duplexes and triplexes, 246, 19 circular dichroism, 246, 34 bioinorganic spectroscopy, 246, 71 magnetic circular dichroism, 246, 110 low-temperature spectroscopy, 246, 131 rapid-scanning ultraviolet/visible spectroscopy applied in stopped-flow studies, 246, 168 transient absorption spectroscopy in the study of processes and dynamics in biology, 246, 201 hole burning spectroscopy and physics of proteins, 246, 226 ultraviolet/visible spectroelectrochemistry of redox proteins, 246, 701 diode array detection in liquid chromatography, 246, 749. 

Over the past decade two forms of vibrational optical activity have become established. One is called vibrational circular dichroism (VCD), the extension of electronic circular dichroism into the infrared vibrational region of the spec-tram. The first measurements of VCD were reported by George Holzwarth and co-workers at the University of Chicago in 1973 for crystals (3) and 1974 for neat liquids (4). In VCD one measures the small difference in the absorption of a sample for left versus right circularly polarized incident infrared radiation. The early stages of the development of VCD have been reviewed from several perspectives (5-8). 

Optical chirality of molecules is a characteristic measure of stereo-chemical property of biological, pharmaceutical, and metal coordination compounds. Choral structures of amino acids, proteins, DNAs, and various drugs in solutions have been determined from the measurement of circular dichroism (CD). However, small amount of molecules at the liquid-liquid interfaces has never been measured before CLM/CD method [19] and SHG/CD method have been reported [20],... 

The anisotropy itself may be linear or circular, or a combination of both. In linear anisotropy the refractive index depends on the direction of polarised light. It is found in solid polymers under tension and in viscous polymeric liquids during flow (shear and elongation). The refractive index can also depend on the chirality of polarised light in this case one speaks of circular or elliptic anisotropy. Thus the so-called "optical activity" is circular birefringence its extinction analogue is circular dichroism. 

The cholesteric mesophase formed by cholesteryl p-nitrobenzoate at 200 °C has been used as the solvent to effect an asymmetric synthesis lrans-but-2-enyl p-tolyl ether gave the product of an ortho-Claisen rearrangement, 2-(but-1 -en-3 -yl)-4-methylphenol. This material exhibited circular dichroism, although neither the optical yield nor the configuration of the product is yet known.262 Decarboxylation of ethylphenylmalonic acid in cholesteryl benzoate at 160 °C (cholesteric liquid-crystalline phase) also proceeded with asymmetric induction to give (R)-(—)-2-phenylbutyric acid, with 18% optical yield.263 Electric dipole moments are reported for some esters of 5a-cholest-8(14)-en-3j8-ol there is some slight correlation with melting points.264... 

LFnder the influence of a static electric field liquid crystalline solutions of PBLG become uniaxially oriented and show both linear and circular birefringence and linear and circular dichroism. Accordingly, the measured CD shows a dependence of the microscopic angle, a, that the fast optical axis of the oriented solution makes with respect to the plane of polarization of the polarizer in the CD instrument. When the film is reasonably thin (0.01-0.1 mm thick) or/and only partially oriented (less than 5% orientation), the apparent CD of oriented film of deoxyribonucleic acid is expressed by the equation (50) ... 

The measurement of vibrational optical activity (VOA) lacks some of the severe disadvantages mentioned. Vibrational spectral bands are less likely to overlap and can be measured using two complementary techniques namely infrared and Raman spectroscopy. They can be measured as well in the crystalline as in the liquid or gaseous state, and the techniques are applicable to solutions while nearly reaching (complemented with the appropriate theoretical models) the accurateness of the X-ray method. VOA has drawbacks too the effects are quite small and tend to be obscured by artifacts. They are about 10 times weaker than the optical rotatory dispersion (ORD) and the circular dichroism (CD) in the UV-VIS range. However, this apparent disadvantage is more and more relieved by instrumental advances. 

In Fig. 7 the optical rotatory dispersion (ORD) as well as the circular dichroism (CD) is shown for the right-handed cholesteric liquid crystal. A right-handed helical structure reflects right circularly polarized light and it shows positive optical rotation on the short wavelength side of the reflection band. 

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

Three-dimensional Cholesteric liquid crystal Multilayer Single crystal Optical notary pourcf Circular dichroism... 

Raman optical activity has only been measured so far in pure liquids and strong solutions. Crystals and powders are harder to study crystals must be polished and oriented carefully to eliminate artefacts, whereas multiple scattering in powders depolarizes the incident light. It would be of great interest to measure pure rotational, and rotational-vibrational, ROA in gases, but insufficient scattered intensity has so far prevented this. An additional complication in resonance scattering is that circular dichroism of the incident beam can contribute to the measured circular intensity difference. 

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