Solvatochromism

Solvatochromism is the ability of a chemical substance to change color due to a change in solvent polarity. Negative solvatochromism corresponds to hypsochromic shift (blue shift) with increasing solvent polarity and positive solvatochromism corresponds to bathochromic shift (red shift) with decreasing solvent polarity. 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 differential 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-methyl-pyridinium iodide, which is red in 1-propanol, orange in methanol, and yellow in water.

Charge transfer bands

Charge transfer bands (CT) in electronic spectra - Migration of an electron from an orbital that is predominantly metal in character to an orbital that is predominantly ligand in character (MLCT) or vice versa (LMCT). -It is sensitive to solvent polarities -CT character often identified with solvatochromism - change in transition frequency with change in solvent permittivity

LMCT:

Observed in the visible region of a spectrum when the metal is in a high oxidation state and the ligands contain non-bonding electrons (p-orbitals), eg. MnO₄^-

MLCT:

Observed when the metal is in a low oxidation state and the ligands have low-lying acceptor orbitals e.g. $\pi$* orbitals of aromatic ligands

• diimines
• bipy, CN-, CO, NO

Uses

Solvatochromism can be used in environmental sensors and in molecular electronics for construction of molecular switches.

See also

• Bathochromic shift
• Hypsochromic shift
• Brooker's merocyanine

External links

• Negative solvatochromism experiment
• Positive solvatochromism experiment
• What is Solvatochromism? A quanto-mechanical revisitation