Absorption and emission of dyes in organic colorants

摘要

A physically measurable property of any chemical entity is influenced by the environment in which the chemical entity is situated. It has long been known that absorption spectra of chemical compounds may be influenced by the surrounding medium. Particularly in colorants solvents can bring about a change in the position, intensity and shape of the absorption bands. It may be said in simple terms that color of a solution of colorant depends on the nature of the solvent as well as the nature of the colorant. This phenomenon was later referred to as Solvatochromism by Hantsch. Today, however, the term solvatochromism has wider connotation. Solvatochromism in colorants can thus be defined as the pronounced ability of a chemical substance to change its color due to a change in the polarity of the medium. The change in color is due to change in position and sometimes intensity of an electronic absorption or emission band. Positive solvatochromism corresponds to bathochromic shift with increasing solvent polarity, and negative solvatochromism corresponds to hypsochromic shift with increasing solvent polarity. The sign of the solvatochromism depends on the difference in dipole moment of the molecule of the colorant between its ground state and excited state.