Applications of Linear Dichroism

The orientation of molecules in thin solid films, liquid crystals, and fibers can be studied by linear dichroism spectrometry. If the spectrum of an oriented film is measured with polarized radiation, its dipole moment derivative, (9p/9(2,) (see Section 1.2) must have a component that is nonzero in the direction of the polarization if that band is to be seen in the infrared spectrum. As an example, let us consider the case of a film of poly(ethylene terephthalate) (PET) that has been drawn (stretched during the manufacturing process) such that the polymer chains 

In this case, the C=0 groups are predominantly oriented in a direction that is perpendicular to the draw direction. If we define p- and -polarizations as being 

Absorbance spectra measured with p- and 5-polarization are denoted as Ap(v) and A5(v), respectively. Ap(y) — A5(v) is known as the dichroic difference, and Ap v)/As v) is the dichroic ratio. Quantitative treatments of molecular orientation often require knowledge of the dichroic ratio. However, in regions where the sample absorption is very weak, calculation of the dichroic ratio involves taking the ratio of noise on the baseline of Ap(v) and A5(v) when both parameters are approximately zero. Thus, if qualitative or semiquantitative information is all that is required, the dichroic difference spectrum is more useful than the dichroic ratio. 

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A classic application of linear dichroism to study molecular orientations and motions in a complex biological system was R. Cone s study of induced dichroism in retinal rod outer segements [14]. Rhodopsin, the light-sensitive pigment-protein complex of the retina, contains 11-cw-retinal attached covalently to a protin (opsin) by a Schiff base linkage (Fig. 4.13A, B). Its transition dipole is oriented... 

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