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 — except cromophores), 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.
Charge transfer bands
Charge transfer bands (CT) in electronic spectra involve promotion of an electron from an orbital that is predominantly metal in character to an orbital that is predominantly ligand in character (MLCT) or the reverse (LMCT). The energies of such bands are characteristically sensitive to solvent polarities. In fact, CT character is often identified with solvatochromism - change in transition frequency with change in solvent dielectric constant. LMCT bands are often observed in the visible region of a spectrum for many high oxidation state metal complexes. One example is MnO4− which has no d-electrons. Visible MLCT bands are often observed when the metal is in a low oxidation state and the ligands have low-lying acceptor orbitals e.g. * orbitals of ligands such as bipy or CN−, CO, and NO.
The main value of the concept of solvatochromism is the context it provides to predict colors of solutions. Solvatochromism can in principle be used in sensors and in molecular electronics for construction of molecular switches.
Solvatochromic dyes are used to measure solvent parameters, which can be used to explain solubility phenomena and predict suitable solvents for particular uses.
Solvatochromism is also the predominant process in detecting explosives in a new technique using carbon nanotubes; the light frequency emitted of the nanotubes changes when exposed to explosive particles, allowing for sensitive and easy detection.