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, positive solvatochromism corresponds to bathochromic shift with increasing solvent polarity. The sign of the solvatochromism depends on the difference in dipole moment of the molecule of the dye between its ground state and excited state.

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 stablization 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 2-(4'-hydroxystyryl)-N-methyl-quinolinium betaine, which is ink-blue in nonpolar chloroform and blood-red in polar water, and 4-(4'-hydroxystyryl)-N-methyl-pyridinium iodide, which is violet in n-butanol, red in 1-propanol, orange in methanol, and yellow in water.

Charge transfer bands
Charge transfer bands (CT) in electronic spectra - Migration of electron from orbital that is predominantly metal in character to orbital that is predominantly ligand in character (MLCT) of 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 visible region of spectrum when metal is in high oxidation state and ligands  contain non-bonding electrons (p-orbitals), eg. MnO4-

MLCT:
 * Observed when metal is in low oxidation state and ligands have low-lying acceptor orbitals eg. $$\pi$$* 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.