Solvatochromy

Under Solvatochromism is generally understood to influence the color of a dye by solvent ( solvent ). The recognizable color of the solution is based on the interactions of the dye on the one hand with the solvent molecules and on the other hand on the mutual interactions of the solvent molecules. A distinction is made between negative solvatochromism, which causes a hypsochromic (color lightening) shift, and positive solvatochromism, which has a bathochromic (color deepening) effect.

Some dyes interact with their environment in solution and act as an indicator for intermolecular interactions. Solutions of these solvatochromic dyes change color with temperature, that is, they are also thermochromic .

Relationship between excitation light (above) and observed color (below)

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Negative solvatochromia

Solutions of the solvatochromic dye betaine 30 in different solvents show different colors.

In negative solvatochromism, the ground state of the color molecule is dominated by a polar mesomeric state (limit formulas with partial charges). Therefore, the more polar the solvent, the lower the energy level of the substance. The level of the excited state of the molecule remains approximately the same.

Thus the wavelengths of the absorption maximum become smaller (and therefore more energetic) the more polar the solvent is. So more energy is required for absorption , a hypsochromic shift takes place. The color of the dye changes from green to blue and red to yellow in more strongly interacting solvents.

Substances with negative solvatochromism also dissolve better in strongly interacting solvents (water and alcohols) than in hydrocarbons, which are considered to be weakly interacting.

Positive solvatochromy

In positive solvatochromism, on the other hand, a non-polar boundary structure predominates in the ground state of the color molecule. The ground state is destabilized by increasingly polar solvents, the energy level is increased and the wavelength of the absorption maximum becomes longer and longer. Less energy is required for absorption, a bathochromic shift takes place, the color of the dye shifts from the red to the blue to the dark green part of the visible spectrum.

Characterization of the solvent polarity

Reichardts Betaine Dye No. 30

The color of a dissolved substance is particularly dependent on polar interactions with the solvent. The “polarity” of a solvent is often used as a reason for how strongly it influences the color of a dissolved substance. However, this view is by no means unproblematic, as various polarity scales (in addition to material constants such as dielectric constant , refractive index , acid and base constants, as well as empirical values such as Taft's π * polarity scale, E _T (30) values) have been discovered and developed over time which sometimes provide contradicting statements and all of which are only applicable to a limited extent.

If the interactions between the solvent molecules are significant, then they will also have significant effects on the solute molecules. There are different indicators for the intermolecular interactions in liquids and gases: With comparable substances, the interactions increase with increasing molar mass and this increases the boiling temperature. The "surface" of the particles increases with the molar mass, and the intermolecular interactions attack the surface. In addition to electromagnetic forces, “non-polar” masses are also effective. In the case of isomers, measured values for density, viscosity and surface tension give indications of structure-dependent interactions of varying degrees. The existing interactions between solvent molecules decrease with increasing temperature. This also reduces the interactions with dissolved particles.

Web links

Christian Reichardt - Demonstration and Explanation of Solvatochromy ( Video )
Christian Reichardt - Applied Solvatochromy ( Video )

Individual evidence

↑ Mortimer J. Kamlet, Jose Luis Abboud, RW Taft: The solvatochromic comparison method. 6. The π * scale of solvent polarities. In: J. Am. Chem. Soc. 99 (18), 1977, pp. 6027-6038, doi: 10.1021 / ja00460a031 .
↑ Karl Dimroth , Christian Reichardt , Theodor Siepmann, Ferdinand Bohlmann: About pyridinium-N-phenol-betaine and their use to characterize the polarity of solvents. In: Justus Liebig's Annals of Chemistry. 661, 1963, pp. 1-37, doi : 10.1002 / jlac.19636610102 .
↑ Ralf Lemke: Intermolecular interactions. In: Research into causes in the practical science of chemistry in schools. 54/6, 2005, pp. 39-43.

[1] Mortimer J. Kamlet, Jose Luis Abboud, RW Taft: The solvatochromic comparison method. 6. The π * scale of solvent polarities. In: J. Am. Chem. Soc. 99 (18), 1977, pp. 6027-6038, doi: 10.1021 / ja00460a031 .

[2] Karl Dimroth , Christian Reichardt , Theodor Siepmann, Ferdinand Bohlmann: About pyridinium-N-phenol-betaine and their use to characterize the polarity of solvents. In: Justus Liebig's Annals of Chemistry. 661, 1963, pp. 1-37, doi : 10.1002 / jlac.19636610102 .

[3] Ralf Lemke: Intermolecular interactions. In: Research into causes in the practical science of chemistry in schools. 54/6, 2005, pp. 39-43.