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Solvatochromism describes the ability of a substance to change color due to a shift in the polarity of the solvent. This can be observed as either a negative transition or positive transition depending on the relative change of polarity in either direction. This change can then be quantified by measuring the transition dipole moment. Positive and negative solvatochromism depend on the difference between the dipole moment of the ground and excited states of the substance. This phenomenon is explained by the solvatochromic effect, which relates the absorption and emission spectra with the polarity of the solvent. This can then be seen by the human eye due to the presence of chromophores. Chromophores are known to absorb a precise wavelength of light, which can be seen when it falls within the range of the visible spectrum. On a basic level, compounds that exhibit this effect can be used as indicators to compare the polarities of various solvents. It has also been used in the characterization and understanding of solvents, mixed solvent systems, dry solid surfaces, and ‘wetted’ or ‘solvated’ solid surfaces. Currently there is research being done to determine if this effect can be applied towards the development and creation of LEDs, environmental sensors, and molecular switches.

Compound dissolved in solvent with various polarities.

Positive and Negative Solvatochromism

Visible spectrum.

Positive Solvatochromism

Positive Solvatochromism describes the observed change in color when there is a relative decrease in solvent polarity. This is known as a bathochromic shift (red shift). A bathochromic shift is caused when the transmission, absorption, or emission spectra of a molecule transitions to a longer wavelength. This is also known as a red shift because within the visible spectrum, red has the longest relative wavelength. For example, 4,4'-Bis-dimethylamino-fuchsone (BDF) absorbs at a wavelength of 485 nm when dissolved in toluene (non-polar), which is visible as orange. The absorbance of BDF then shifts to 500 nm when dissolved in acetone (more polar) seen as red, and then to 555 nm in methanol (most polar) seen as purple. The resulting increase in wavelength is the predicted bathochromic shift (red shift).

Negative Solvatochromism

Negative Solvatochromism describes the observed change in color when there is a relative increase in solvent polarity. This is known as a hypsochromic shift (blue shift). A hypsochromic shift is caused when the transmission, absorption, or emission spectra of a molecule transitions to a shorter wavelength. This is also known as a blue shift because within the visible spectrum, blue has the smallest relative wavelength. For example, acetone absorbs at a wavelength of 279 nm when dissolved in hexane (non-polar). The absorbance of Acetone then shifts to 264.5 nm when dissolved in water (polar). The resulting decrease in wavelength is the aforementioned hypsochromic shift (blue shift).

Components of Solvatochromism

Solvatochromic Effect

Solvatochromic Effect – also called solvatochromic shift, refers to the correlation of absorption and emission spectra with the polarity of the solvent. This is caused by the presence of differing polarities of the ground and excited state of a chromophore. Therefore, a change in the solvent polarity will lead to a change in stabilization energy for each of the ground and excited states. This will in turn change the energy gap between these two states. This can be greatly influenced by various polar interactions including: hydrogen bonding, dipole-dipole, cation-dipole, etc.

This is further supported by the Franck-Condon principle, which considers the electronic transitions of chromophores dissolved in liquids. The molecules are dissolved and subsequently surrounded by the solvent, which forms a stabilizing interaction known as solvation. This is particularly true in the case of polar chromophores that are dissolved in polar solvents, as the energy of the interactions is significantly higher. The solvation sphere will constantly change and evolve until it has minimized the total energy of the system. These interactions include the same ones mentioned previously, including: hydrogen bonding, dipole-dipole, van der Waals, etc. This is significant to solvatochromism when the solvation interactions are different between the ground state and the excited state. The change in dipole moment after the energy transition can change the energetic drive of solvation. This can be further complicated when considering bulk forces such as viscosity. With small molecules such as methanol, the rearrangement of solvation can occur much faster than the life of the chromophores in an excited state. With bulkier and more viscous solvents, the speed of the solvent rearrangement might become slower than the lifetime of the excitation state, causing the system never to fully reach a resting state. Either way, it is important to consider these effects when analyzing the solvation system as a whole.

A representative example of this effect is 1-methyl-4-[(oxocyclohexadienylidene)ethylidene]-1,4-dihydropyridine, also known as MOED or Brooker’s Merocyanine. This is a compound that has been well studied due to its solvatochromic properties. As shown, it can exist in two different resonance forms depending on the solvent conditions. The zwitterion form is predominated by polar solvents such as water and acetone, while the neutral form is dominant when dissolved in less polar solvents including chloroform or toluene. This illustrates how a change in solvent polarity can quickly lead to visible change in both structure and color. In both cases, the solvent has a noticeable effect on stabilizing that given form. This will in turn cause a change in the relative energy levels of the ground and excited states, allowing for different colorimetric observations.

Resonance forms of Brooker’s Merocyanine.

Resonance forms of Brooker’s Merocyanine.

Chromophores

Electronic absorptions of carbonyls, alkenes, and nitro groups.

Chromophores are groups within a molecule with characterisitic optical absorptions. The word chromophore comes from the Greek, meaning ‘color bringer’. The presence of certain chromophores often accounts for the colors of various substances. Chromophores absorb particular wavelengths of visible light with energy corresponding to the energy difference between two molecular orbitals of the chromophore. An example of a chromophore is a carbonyl group (C=O), which has a typical electronic absorption around 290 nm, depending on the nature of the groups attached or surrounding the carbonyl moiety. Examples of chromophores and typical absorbances are given in the table below. In the case of absorption by C=C double bonds, the absorbed photon excites an electron from a π orbital to a π* orbital. The chromophore activity is therefore due to the π*←π transition. When a C=C double bond is within a conjugated π-system, the orbital energies lie closer together and and the π*←π transition moves to longer wavelengths, often into the visible region. In the case of Brooker’s Merocyanine, the conjugation of the π-system with the carbonyl and amino groups bring the π*←π transitions into the visible region. The change in resonance structures from the alkoxide to the ketone forms reflects a change in chromophores and thus a change in color.

Transition Dipole Moments

The transition dipole moment is regarded as a measure of charge redistribution that accompanies a transition. The intensity of a transition is directly proportional to the square of the transition dipole moment. This can be derived from the time-dependent perturbation theory, which states that for a molecule to interact with an electromagnetic field and absorb or emit a photon with a given frequency, it must possess a dipole oscillating at that same frequency. Spectroscopic transition rules can also be derived from the transition dipole moment, where a forbidden transition has a zero dipole moment, but an allowed transition has a non-zero dipole. The value and magnitude of the transition dipole moment are reflective of the strength with which a given molecule or complex couples with the electromagnetic field. Solvatochromic shifts are an indication of a large shift in electron density as a result of the transition, thus consistent with a large transition dipole moment.

General MO diagram of charge transfer bands in a metal complex.

Charge Transfer Bands

Charge transfer (CT) bands exist in metal complexes when electrons flow between orbitals that are predominantly metal in character and orbitals that are predominantly ligand in character. Charge transfer transitions are often identified by their high intensities. If the electron migration is from the metal to the ligand, the transition is classified as a metal-to-ligand charge transfer (MLCT). Often, metals in low oxidation states with ligands having low-energy acceptor orbitals, such as those capable of π-backbonding, exhibit MLCT bands, for example tris(2,2’-bipyridyl)ruthenium(II). Alternatively, if the electron migration is from the ligand to the metal, the transition is classified as a ligand-to-metal charge transfer (LMCT). Typically, metals in high oxidation states with ligands containing non-bonding electrons exhibit LMCT bands, for example: MnO₄^-, TcO₄^-, ReO₄^-, CrO₄^2- and WO₄^2-. The CT character is often distinguished from π*←π transitions on ligands by demonstrating solvatochromic shifts. The variation of transition frequency with respect to changing solvent permittivity is more consistent with metal-ligand transitions than ligand-ligand or metal-metal transitions because of the required shift in electron density necessary to provoke solvatochromism.

ET 30 Scale

The ET 30 scale developed by Christian Reichardt is based on the spectroscopic behavior of the betaine indicator dye, 4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide, which is also referred to as pyridinium N-phenolate betaine dye no. 30 or ET 30. The dye works because it has exceptionally large negative solvatochromism. This molecule changes color depending on the solvent polarity, solution temperature, external pressure, and the nature and concentration of added salts. The ET 30 molecule shows pronounced solvent-dependent spectral shifts, i.e., it is pink in methanol (λmax = 515 nm), green in acetone (λmax = 677 nm), and blue in acetonitrile (λmax = 620 nm)3. The scale derived from this has been shown to help indicate of both dipolarity and hydrogen bonding donating acidity of the solvent4. The ET 30 solvent however is not soluble in fully not polar solvents, eg. TMS, therefore a different dye is used to standardize those solvents.

4-(2,4,6-triphenylpyridinium)-2,6-diphenylphenoxide (ET-30).

Current Research

Recent studies of trinuclear copper complexes (Cu₃L₃ type clusters) have shown that such species can be used for molecular LED devices, due to the ability to tune the luminescence by several factors, including solvent. Solvatochromatic effects have also been described in tetrathiafulvalene systems where strong intramolecular charge transfer are responsible for the phenomenon. There is also research being conducted on fluorescent protein species using solvatochromsim. The use of solvatochromic flourophores in general is quite common in research lately.

Applications

Solvatochromic dyes can be used to characterize solvents as well as mixed solvent systems, dry solid surfaces such as silica, silicalite and zeolites, and alumina, and also ‘wetted’ or ‘solvated’ solid surfaces, such as liquid chromatographic stationary bonded phases. The solvatochromatic effect can also be exploited to generate environmental sensors and molecular switches. A patent that came out in 2010 that uses Reichardt dyes in a temporary tattoo to detect a specific microbial contaminant. The tattoo can be applied, and then will change color if it comes in contact with the specified analyte. These dyes are also being used in optical devices. Coupled with an anisotropic material a multi-color image can be displayed in high quality and in a reproducible fashion.