What is "solvatochromism"

n. (context chemistry English) A change in colour (due to changes in position and/or intensity of absorption or emission bands) that accompanies a change in the polarity of a solvent

Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. Negative solvatochromism corresponds to hypsochromic shift (or blue shift) with increasing solvent polarity. The corresponding bathochromic shift (or red) is termed positive solvatochromism. The sign of the solvatochromism depends on the difference in dipole moment between the ground and excited states of the chromophore.

The Solvatochromic effect or solvatochromic shift refers to a strong dependence of absorption and emission spectra with the solvent polarity. Since polarities of the ground and excited state of a chromophore are different, a change in the solvent polarity will lead to different stabilization of the ground and excited states, and thus, a change in the energy gap between these electronic states. Consequently, variations in the position, intensity, and shape of the absorption spectra can be direct measures of the specific interactions between the solute and solvent molecules.

Due to the Franck–Condon principle (atoms do not change position during light absorption), the excited state solvent shell is not in equilibrium with the excited state molecule ("solute"). In fact, charge-transfer transitions of ground state ion-pairs give the largest changes in absorption spectra, and are thus, the most useful for measuring solvent polarity.

An example of positive solvatochromism is the 4,4'-bis(dimethylamino)fuchsone, which is orange in nonpolar toluene, red in slightly polar acetone, and red-violet in more polar methanol.

Examples of negative solvatochromism are 4-(4'-hydroxystyryl)-N-methylpyridinium iodide, which is red in 1-propanol, orange in methanol, and yellow in water.