Chiro-optical

Another approach to CPL is the synthesis of conjugated polymers with intrinsic chiro-optical properties. A variety of polymers with CPPL have been synthesized so far. Most of them are based upon well-known conjugated polymers such as poly(thiophene)s [4,111], polyphenylene vinylene)s [123], poly(thienylene vinylene)s [124], ladder polymers [125], PPPs [126], polyphenylene ethynylene)s, [127] and poly(fluorenes) [128]. All of them have been modified with chiral side-chains, which induce the chiro-optical properties. 

A special mention in the field of enantioselective HPLC separations must be made of chiro-optical detection systems, such as circular dichroism (CD) and optical rotation (OR), which can be also used to circumvent the low UV detectability of chromophore-lacking samples [40, 61]. While sensitivity of chiro-optical detection is not always sufficient to perform enantiomeric trace analysis, the stereochemical information contained in the bisignate spectropolarimetric response is useful in establishing elution order for those compounds not available as single enantiomers of known configuration. An example of application of different online detection systems (UV and CD at 254 nm) in the enantioselective separation of a racemic sulfoxide on a commercially available TAG CSP is reported in Figure 2.12, under NP conditions. 

Although, the agreement with experiments did not show a significant improvement over the simpler COSMO model we are still confident that the combination of DRF with TDDFT is computationally an attractive solution for calculating chiro-optical properties of molecules in solution when the explicit solvent structure is of interest. 

When compared to developments associated with the structure and synthesis of chiral structures, less attention has been focused on the electronic and magnetic properties of chiral molecules. Circular dichroism and related optical probes of chirality have been developed mainly as analytical tools and, indeed, they are applied routinely. However, as demonstrated in this volume, the basic physical underpinnings that link structure and chiral properties, including chiro-optical properties, continue to emerge and require the development of physical models and improvement of electronic structure methods before they are fully elucidated. 

Lyotropic liquid crystalline cellulose derivatives exhibit unique optical properties because of their helicoidal supramolecular structure.The chiro-optical properties of the helicoidal structure can be described by a pitch p (or its inverse, the twist p ) p = 2o/fi, where is the reflection wavelength and h is the mean refractive index of a sheet, and the corresponding handedness of the twist right-handed helicoidal structure being assigned to a positive pitch p > 0) and left-handed helicoidal structures to a negative pitch p < The nematic mesophase can be... 

Table 1 Chiro-optical properties of lyotropic 50% AEC/ AA solutions with different degrees of acetylation...

The values of pitch for lyotropic AEC solutions in AA (50% w/w) are given in Table 1, where AEC-2 is a pure AEC with medium DA and AEC-3 is the mixture of EC and fully acetated AEC, which has the same average DA as AEC-2. However, in the liquid crystalline solutions, even at the same concentration, AEC-2 and AEC-3 have different pitch and handedness. This phenomenon was also observed in the lyotropic AEC/chloroform system. The difference in chiro-optical properties may come from the complex interactions of multiple chiral centers present in each repeating unit of the cellulose chain, not from simple racemic mixtures as in the PBG system. 

As shown in Fig. 9, the stress relaxation curves of all AEC/AA solutions collapse into one curve when the solutions were presheared with the same rate. Because the stress relaxation is at the molecular level and the chiro-optical properties reflect the suprastructural level, it is expected that the lyotropic solutions with different chiro-optical properties have the same stress relaxation behavior in both the tumbling and flow-align regions. 

Dai, Q.G. Gilbert, R.D. Khan, S.A. Kadla, J.F. Relaxation behaviors of lyotropic (acetyl)(ethyl) cellulose/acrylic acid solutions with different chiro-optical properties. Polym. Preprints (ACS, Div Polym. Chem.) 2004, 45 (1), 824-825. Shimamoto, S. Uraki, Y. Sano, Y. Optical properties and photopolymerization of liquid crystalline (acetyl) (ethyl) cellulose/acrylic acid system. Cellulose 2000, 7 (4), 347-358. 

The principle of this technique relies on the following the enantiomers possess the same free energy and all physico-chemical properties except chiro-optical ones. However, the free energy and, consequently, physico-chemical properties of the diastereomers are different. Thus, to be resolved using this technique, a mixture of enantiomers must be transformed in a mixture of diastereomeric compounds by stereoselective interaction with an optically pure reagent. 

The second category includes the response properties which describe the effects of any external applied field on a molecular system. This category includes the electronic and vibrational dipole (hyper)polarizabilities, both static and frequency dependent, magnetic and chiro-optic properties, etc.. Response properties are essential for a deeper understanding of molecular behaviors, and they represent the basis for an ever increasing number of technical applications. 

Since appropriate instruments have become commercially available, CD spectroscopy has developed into a routine method for the characterization of the chirality of newly synthesized polymers. As a typical example, the rather high chiro-optical activity of the ladder-type poly(p-phenylene) of the structure shown in Chart 1.11 was revealed CD spectroscopically molar ellipticity [6>] = 2.2x 10 rad cm mol (at 7max=461 nm), corresponding to an anisotropy factor of g = Ae/e = 0.003 [46]. 

Apart from NMR and die mefliods suitable for characterization of overall chain dimensions (which we will touch on below), fliere are not many simple experimental techniques that can be used to study sugar conformation and that are directly correlated to theoretical results based on calculated potential energy surfaces or force fields. An exception is given by chiro-optical techniques, which provide important (although empirical) structural information as optical rotation experiments have been shown to be very useful and informative in flic study of saccharides in solution. Literature gives die experimental evidence for the effects of the external conditions (solvent and temperature) on the optical rotation. The additivity mediods, proposed in the Fifties by Whiffen and Brewster, " were... 

Experimental Jc,h values for conformationally rigid carbohydrate derivatives allow to calculate the constant values of Aj and Bj in equations [12.2.1], The major practical use of these equations is their ability to estimate the glycosidic dihedral angles from experimental Jqh in combination with other complementary results, for example, from NOESY, X-ray and chiro-optical experiments. Since the experimental values are averaged over all the accessible conformational states in solution, they do not necessary reflect the property of only the most probable conformer but they nevertheless include contributions of all the conformers, each one taken with its proper statistical weight. Thus, the quantitative interpretation of experimental data in terms of accessible conformational states of flexible molecules requires the additional theoretical evaluation of the energy of the molecule as a function of internal coordinates. Since the dependence of the observed coupling constants on the conformation is non-linear, it derives that ... 

The measurement of chirality may be achieved in one of three ways—X-ray diffraction, NMR spectroscopy, and chiro-optical methods. 

All these observations suggest that tactic macromolecules of the above type are substantially, even sometime not formally, achiral and their symmetry properties are better represented by the infinite length chain. Thus in order to prepare macromolecules capable of displaying stable and appreciable chiro-optical properties in all conditions, the symmetry elements rendering the infinite chain model achiral must be eliminated. 

Optical properties of chitosan/myristic acid and chitosan/nicotinic acid derivatives show red shift. Chitosan/methoxycinnamaldehyde, the IV-substimted chitosan derivative showed fairly good photoluminescence (PL) properties, and introduces polymer conformations in organic and inorganic solvents (Fig. 10). The chiro-optical properties of chitosan-derivatives may draw the attention to biomedical apphcations [128, 129]. 

Chiro-optical properties. The optical properties specific of chiral molecules (optical rotation, circular dichroism, Raman optical activity (ROA)) may also be enhanced by the interaction with plasmons. This very interesting field is much less developed than others, although theoretical contributions have already been published (e.g. for surface-enhanced Raman optical activity, SEROA) [48,49]. 

The occurrence of transfer and amplihcation of chiral information could be particularly relevant for solid polymer hlms, since they could have, in principle, applications in chiro-optical devices and data storage systems. Recently, it has been reported that non-racemic SPS hlms, exhibiting the nanoporous 5 phase, are able to detect, amplify, and memorize the chirality of several volatile organic molecules [191,192]. 

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