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'''Solvatochromism''' is the ability of a [[chemical substance]] to change [[color]] due to a change in [[solvent]] [[polar molecule|polarity]]. Negative solvatochromism corresponds to [[hypsochromic shift]] (or blue shift) with increasing solvent polarity. The corresponding bathochromic (or red) shift is termed positive solvatochromism.<ref>{{cite book|last=Reichardt|first=Christian|title=Solvents and solvent effects in organic chemistry.|year=2010|publisher=Wiley-VCH|location=Weinheim, Germany|isbn=9783527324736|pages=360|edition=4th, updated and enl. ed.|coauthors=Welton, Thomas}}</ref> The sign of the solvatochromism depends on the difference in [[Molecular dipole moment|dipole moment]] between the ground and excited states of the [[chromophore]].

The ''Solvatochromic effect'' or ''solvatochromic shift'' refers to a strong dependence of [[UV/VIS spectroscopy|absorption]] and emission spectra with the [[solvent]] [[polar molecule|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 [[Solution|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 [[Brooker's merocyanine|4-(4'-hydroxystyryl)-''N''-methylpyridinium iodide]], which is red in [[1-propanol]], orange in [[methanol]], and yellow in [[water]].

==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 observed in the visible region of a spectrum for many high [[oxidation state]] metal complexes. One example is MnO4- MLCT bands are observed when the metal is in a low oxidation state and the ligands have low-lying acceptor orbitals e.g. <math>\pi</math>* orbitals of aromatic ligands such as diimines (e.g., [[bipy]]) and CN-, CO, and NO..

==Uses==
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 [[sensor]]s and in [[molecular electronics]] for construction of [[molecular switch]]es.

==See also==
* [[Bathochromic shift]]
* [[Hypsochromic shift]]
* [[Brooker's merocyanine]]

==References==
{{Reflist}}

==External links==
* [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/cassy_sol_neg-e.htm Negative solvatochromism experiment]
* [http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-pos_sol-e.htm Positive solvatochromism experiment]
* [http://pubs.acs.org/doi/abs/10.1021/jp1097487 What is Solvatochromism? A quanto-mechanical revisitation]
{{chromism}}

[[Category:Chromism]]
[[Category:Spectroscopy]]

Stock dilution - Revision history

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