Bathochromic shift

The bathochromic effect, also called red shift, describes a deepening of the color, ie a shift of the absorption spectrum in the longer wavelength, lower-energy region of the electromagnetic spectrum.

Basics

For organic dyes, the bathochromic groups -OH,- NH2, - NR2 and -OCH3 cause as Auxochromes a shift of the absorption spectrum of violet, blue, cyan, green and yellow to red, with the result that the " perceived color " to the corresponding modified complementary color of the absorption spectrum.

The strength of the bathochromic effect is influenced by the structure of the molecule, and certain substituents. These influences lead to greater delocalization of the π - electrons. The stronger the π - electrons of a molecule are delocalized, the less energy is required for their excitation. The light required for excitation is absorbed and the material appears in the corresponding complementary color.

Cyclic structures have a weaker bathochromic effect as linear. Proceeding have chromophores and especially conjugated systems of chromophores a stronger bathochromic effect.

Substituents with a bathochromic effect ( Bathochromic ) participate in the resonance of the chromophore. This are the π - electrons present in delokalisierterer form. Auxochromes as the - NH2 group usually cause a deepening of the color. If the H atoms substituted on the - NH2 group by alkyl or aryl radicals, intensified the color deepening as the lone pair can strongly participate in the resonance. Whether an auxochrome batho or hypsochromically (color enhancing ) effect depends on the position of the chromophore. Particularly intense is the bathochromic effect when complement to one and the same dye molecule a auxochrome ( M) and antiauxochromes (-M) functional group in their effect, ie a functional group, the electron density of the π -electron system increases ( M effect) and the other to reduce simultaneously examined ( - M effect ), or the resonance is generally reinforced by a push-pull system.

When salts of certain dyes color deepens occurs ( halochromism ). If a hydroxyl group of a phenol deprotonated, so the resulting free electron pairs of the phenolate group more involved in the resonance and the absorption maximum is shifted.

Application

Generally, a dye having a different color is formed by the insertion bathochromic groups. The chrome halo effect is used in indicators, since the color change takes place at a certain pH value of the solution.

Related terms

Results in the bathochromic shift from a negatively charged ( salt-like ) group, this is called a halo -chromic effect.

Carry substituents or functional groups to the displacement of the absorption spectrum of a substance in the direction of shorter wavelengths, this is in contrast to the effect of a hypsochromic bathochromic effect.