Hypsochrome effect

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The hypsochromic effect , also known as blue shift , is the shift in the absorption spectrum of a substance into the shorter-wave , higher-energy range of the electromagnetic spectrum .

If this happens when changing from a non-polar to a polar solvent , one speaks of solvatochromism . The hypsochromic effect in this case results from the stabilizing effect of the hydrogen bonds of the polar solvent on the free and non-bonding electron pairs (n orbitals) responsible for the absorption in the relevant wavelength range, as well as the antibonding π * orbitals (cf. molecular orbital theory ). The n orbitals are more strongly stabilized than the π * orbitals, which results in a larger energy difference, and this in turn causes absorption at shorter wavelengths according to the following law:

E = energy ; λ = wavelength; h = Planck's quantum of action ; c = speed of light

However, a hypsochromic effect can also be caused by a substitution or elimination reaction in a molecule , in which one chemical group of the molecule is replaced by another or completely removed. These substituents, the replacement or elimination of which produce such effects, are called auxochromes .

The reason for the shift in the absorption band in the direction of shorter wavelengths with such substitutions or eliminations is the reduction in size of the delocalized π-electron system - the more conjugated double bonds it extends, the greater the wavelengths of the light it can absorb, and vice versa. The fewer conjugated double bonds a π-electron system has, the shorter the wavelengths of light it can absorb. Reactions that are supposed to bring about such effects therefore play a major role in the synthesis of dyes.

Related terms

The opposite of the hypsochromic effect, i.e. the shift in the absorption spectrum of a substance towards longer wavelengths, is known as the bathochromic effect .

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