LD spectroscopy

Linear dichroism is defined as the differential absorption of linearly polarized light (Eq. 6). 

A / is the absorbance of the sample when the light is polarized parallel to a reference axis, and Aj is the absorbance of light which is polarized perpendicular to this axis. The strength of the absorption depends on the orientation of the electric field vector of the light and the transition moment of the chromophore - parallel orientation results in maximum absorption whereas perpendicular orientation leads to zero absorption. By dividing the LD value by the absorbance of the unoriented sample under isotropic conditions (Aiso), the reduced linear dichroism (LDr), i.e. the wavelength-dependent LD, is obtained (Eq. 7) [36]. 

The LDr correlates with the orientation of the transition moment of the dye relative to the reference axis, as quantified by the angle a. LDr is also proportional to an orientation factor S (S = 1 denotes perfect alignment of the dye, S = 0 random orientation). For an isolated, non-overlapping transition, Eq. (7) establishes the correlation between LDr, a and S. These definitions lead to the qualitative rule that with an angle a 55°, a negative LD signal is observed, whereas with a 55°, a positive signal appears in the spectrum. Thus, with an appropriate set-up the orientation of a chromophore relative to a reference axis can be determined. 

From Eq. (7) it is obvious that in an isotropic medium a LD signal cannot be detected because of the statistical orientation of the transition moments under these conditions. Thus, methods are required to promote preferential orientation of molecules. For LD experiments two general approaches have been established  

In summary, it has been demonstrated that complex formation between dyes and DNA may be conveniently monitored by absorption and emission spectroscopy and that these methods provide useful data for discussion of the binding strength and binding mode. 

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The molecular structure of naphthazarin (44) has been a subject of considerable interest, particularly because of the rapid intramolecular proton transfer effects observed for this species . Andersen recently investigated naphthazarin and its 2,3-dichloro derivative (68) by means of IR LD spectroscopy on samples aligned in stretched polyethylene. Unfortunately, no useful LD was observed for naphthazarin (partly because of low solubility), but the dichloro derivative was readily dissolved and aligned in stretched polyethylene . The observed wavenumbers, IR intensities and polarization directions were well reproduced by the results of B3LYP/6-31G calculations. Two strong, differently in-plane polarized bands at 1230 and 1204 cm were assigned to transitions with... 

Fig. 17 Top. Self-assembled zinc porphyrin leading to supramolecular nanofibres. Bottom Designed nanofibres can be acoustically aligned in solution with audible sound emitted fi om a loudspeaker located 20 mm above the cuvette. Linearly polarised light was used to record LD spectroscopy. Reproduced with permission from [89]. Copyright 2010 Macmillan Publishers Ltd...

LD spectroscopy is a technique based on the interaction between linearly polarized radiation and chemical species [3]. Such a technique has no relation with molecular chirality and circular dichroism. It relies on the fact that the probability of inducing an electronic transition with electromagnetic radiation is proportional to the cosine square of the angle between the transition dipole moment and the polarization direction of the radiation (which coincides with the direction of oscillation of the electric field). Therefore, if a linearly polarized radiation is employed, the probability of a transition to occur is maximum when the corresponding dipole moment is parallel to the polarization direction, while the probability is zero when the transition dipole moment is oriented perpendicularly to the polarization direction. The dependence of the absorption of a sample on the polarization direction of the light is called linear dichroism. 

Moments of transition that are colinearly oriented with respect to the director n, to which bands of the same dichroic ratio and the same response toward the polarization measnrement correspond, are generated by vibrations of the same type of symmetry. This is why the area of successful application of the IR-LD spectroscopy for structural characterization can be significantly expanded by preliminary analysis of the mode of molecular vibrations and their grouping, according to identical types of symmetry. From the practical point of view, the efficiency of the method depends also on the resnlting orientation of a sample as well as the mode of processing the IR-LD spectral data obtained. 

This analysis illustrates a particular case of one fundamental IR spectroscopy rule The dipole moment of transition is characterized by the same symmetry mode as the normal vibration, which generates it [57]. This means that colinear moments of transition and bands of equal dichroic ratio correspond to vibrations belonging to the same symmetry class. This conclusion is of principal importance for the application of IR-LD spectroscopy for the identification of the IR bands. The statement is supported by the example given in Figure 1.8. 

Because of the successful application of polarized IR-LD spectroscopy for analysis of the tautomerism in the different substituted pyridines, this method is a unique experimental tool for assignment of IR bands in the solid state and for detailed analysis of the effects in crystals. For this reason, we have studied a series of hydrogensquarate crystals, where all possible effects of corresponding IR spectra are observed. The following model systems and their explanation will illustrate the applicability of the tool. 

For these reasons we have performed investigations by means of linear-dichroic infrared (IR-LD) spectroscopy analysis of oriented solids on both monoclinic and orthorhombic polymorphs of Paracetamol by the orientation technique as a liquid crystal (LC) suspension. The conventional IR spectral analysis of Paracetamol in solution has been previously danonstraled [351-355]. However, here for the presented CS-NLC orientation technique, IR-LD analysis leads to (a) detailed vibrational assignment of characteristic bands of both polymorphs and (b) the supramo-lecular solid-state structural characterization at room temperature and atmospheric pressure. It also avoids the phase transition and guarantees the study of the different forms. A quantitative approach to the yield determination of monoclinic form I in mixtures with the orthorhombic modification was also illustrated. 

We performed polarized IR-LD investigations on forms I and II and will now illustrate how IR-LD spectroscopy can relatively easily provide information on the polymorphism of the pharmaceutical products. 

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