Optical absorption molecular extinction

Figure 10 shows the optical absorption spectra of the single crystal of all-trans-/3-carotene measured upon irradiation with linearly polarized light parallel to the a- or fc-crystal axis. Large optical anisotropy for the molar extinction coefficient is seen in these spectra. This observation can be explained in terms of the molecular orientation of /3-carotene in the crystal (see Fig. 9). Since the molecular axis of /3-carotene is almost parallel to the fc-axis, the molar extinction coefficients along fc-axis should appear larger than those along a-axis. Based on the symmetry considerations, these spectra can be assigned to the transitions to the (//fc-axis) and B (//a-axis) molecular excitons, as illustrated in Fig. 10 (Chapman etal., 1967).

Equations (14)-(17) apply to isotropic media. In an orientationally ordered material the extinction coefficient becomes dependent on the angle between the alignment axis and the polarization direction of the incident light, and has the characteristics of a second rank tensor. At a microscopic level, the optical absorption depends on the angle between the molecular transition dipole moment iJ.j for the particular absorption band, and the electric field of the light wave. Restricting attention to uniaxial systems, an effective order parameter (5op) for optical absorption can be defined as ... 

According to the theory it can be explained by changes of extinction coefficient of optically active formation as the latter formations were taken into traps different by their nature. As the result of this, the change of the wave functions of the electron in main and excited states takes place. The first results involve the changing of the probability of its transformation from main to excited state as well as changes in the molecular absorption coefficient. The absorption spectrum became wider under these conditions and that is why its corresponding change can not be fixed. 

Several general characteristics of photosensitizers affect their efficacy as PDT agents photophysical, photochemical and pharmacological. The photophysical/ photochemical properties include the absorption (extinction spectrum) in vivo, the quantum efficiency for generating singlet oxygen (or other active photoproducts), the photobleaching rate and the quantum efficiency for fluorescence. The characteristics of particular photosensitizers and the relationship to their molecular structure are discussed in other chapters, as are the tissue uptake and clearance and microlocalization properties. Here, we will focus on the methods, primarily optical, that may be used to measure some of these characteristics in vivo. 

If a compound forms a derivative with a reagent which has a characteristic absorption band of high Intensify at a wavelength where the compound does not absorb, then the extinction coefficient of the derivative is usually the same as that of the reagent. Although the extinction coefficient, a, of the absorption band remains constant in all the derivatives, the opticed density, QD, is different for compounds of different molecular weights. The molecular weight, M, of the compound can then be readily calculated on the basis of its absorption data. 

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