Solvatochromism
Encyclopedia
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 increasing 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
.
- 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:
MLCT:
s and in molecular electronics
for construction of molecular switch
es.
Chemical substance
In chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
to change color
Color
Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
due to a change in solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
polarity. Negative solvatochromism corresponds to hypsochromic shift
Hypsochromic shift
Hypsochromic shift is a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a shorter wavelength...
(blue shift) with increasing solvent polarity and positive solvatochromism corresponds to bathochromic shift
Bathochromic shift
Bathochromic shift is a change of spectral band position in the absorption, reflectance, transmittance, or emission spectrum of a molecule to a longer wavelength ....
(red shift) with increasing solvent polarity. The sign of the solvatochromism depends on the difference in dipole moment between the ground and excited states of the chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
.
The Solvatochromic effect or solvatochromic shift refers to a strong dependence of absorption and emission spectra with the solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
polarity. Since polarities of the ground and excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
of a chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
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
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
and solvent molecules.
Due to the Franck-Condon principle
Franck-Condon principle
The Franck–Condon principle is a rule in spectroscopy and quantum chemistry that explains the intensity of vibronic transitions. Vibronic transitions are the simultaneous changes in electronic and vibrational energy levels of a molecule due to the absorption or emission of a photon of the...
(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
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
, red in slightly polar acetone
Acetone
Acetone is the organic compound with the formula 2CO, a colorless, mobile, flammable liquid, the simplest example of the ketones.Acetone is miscible with water and serves as an important solvent in its own right, typically as the solvent of choice for cleaning purposes in the laboratory...
, and red-violet in more polar methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
.
Examples of negative solvatochromism are 4-(4'-hydroxystyryl)-N-methyl-pyridinium iodide, which is red in 1-propanol, orange in methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, and yellow in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
.
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 stateOxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
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.
* orbitals of aromatic ligands
- diimines
- bipy, CN-, CO, NO
Uses
Solvatochromism can be used in environmental sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s and in molecular electronics
Molecular electronics
Molecular electronics, sometimes called moletronics, involves the study and application of molecular building blocks for the fabrication of electronic components...
for construction of molecular switch
Molecular switch
A molecular switch is a molecule that can be reversibly shifted between two or more stable states. The molecules may be shifted between the states in response to changes in e.g. pH, light, temperature, an electrical current, microenvironment, or the presence of a ligand. In some cases, a...
es.