Screening Hypochromism

Abstract

A new model of hypochromism was suggested [68, 69] which is based on the assumption that in a molecular stack chromophores compete for the photon. This is displayed as a mutual “shielding” of chromophores from light and the effect therefore is termed “screening hypochromism”. The hypochromic cross section for identical dipoles was calculated from probabilities of absorption in a stack. The screening leads to a reduction in the average extinction coefficient and to a broadening of the spectrum. Screening was considered to be insignificant initially [58] on the basis that the absorbance in a macromolecule or in a molecular aggregate is too small. However, the Lambert-Beer absorption law is hardly correct for a single macromolecule or molecular aggregate when them size smaller than wavelength of light. This law is suitable when the object is thought as a lot of layers with thickness more than the wavelength. Moreover, this low is correct only when “the molecules distributed in a finite layer are so rare that they do not shield each other” [6]. In aggregates of chromophores and in macromolecules, where distances are much smaller than the wavelength, this condition is violated. In an aggregate or macromolecule, the probability of absorption of a photon by each subsequent chromophore depends on the probability of absorption by the previous one, since they compete for the photon [63, 68]. Moreover, the screening hypochromism could be additional increased due to strong molecular interactions at very short distances.