Solvatochromism

Generally understood as the solvatochromism influencing the color of a dye by solvent ( solvent). The apparent color of the solution is based on interactions of the dye with the solvent molecules on the one hand and on the mutual interactions of the solvent molecules on the other. A distinction between the negative solvatochromism, which causes a hypsochromic ( farbaufhellende ) shift, and the positive solvatochromism, a bathochromic ( bathochromic ) draws effect, however. Most ( habitually ), one speaks of the " polarity " of a liquid, which is believed to know what is polarity. If the interactions of the solvent molecules are considerably among themselves, then they will also exert a significant impact on the solute molecules. There are several indicators of the intermolecular interactions in liquids: For comparable substances interactions with increasing molar mass are larger and increases the boiling point. With the molecular weight, the "surface" of the particles increases, and at the surface access to the intermolecular interactions. In addition to electromagnetic forces also " non-polar " compounds are effective. In isomers give readings for density, viscosity and surface tension of evidence varying degrees of structure-dependent interactions. With increasing temperature, remove the existing interactions between solvent molecules. This reduces the interactions with solutes. Some dyes in solution interact with their environment and act as an indicator of intermolecular interactions. Solutions of these solvatochromic dyes change color with temperature, that is, they are also thermochromic.

Negative solvatochromism

In the negative solvatochromism in the ground state of the color molecule is dominated by a polar mesomeric state ( resonance structures with partial charges ). Therefore, the energy level of the substance is reduced more strongly, the more polar that solvent. The level of the excited state of the molecule remains approximately the same. Thus, the wavelengths of the absorption maximum, the smaller (and therefore energy ), the more polar that solvent. So there is more energy for the absorption necessary to a hypsochromic shift takes place. The absorption maximum of the dissolved dye shifts with increasing interaction of the solvent used by the red region of the visible spectrum towards the violet part, for example, red → orange → yellow → green → blue → purple - therefore there are solutions in less change effective solvents for example green; while the color can range in strongly interacting solvents to turquoise, blue, purple, red and orange to yellow. Substances with negative solvatochromism dissolve better in strongly interacting effective solvent (water and alcohols) as in alkanes, which are regarded as weakly interactive.

Positive solvatochromism

The positive solvatochromism in the ground state of the color molecule outweighs the other hand, a non-polar resonance structure. By increasingly polar solvent, the ground state is destabilized, the energy level increases and the wavelength of the absorption maximum longer. Less energy is required for the absorption, a bathochromic shift takes place (the color of the dye shifts from violet part of the visible spectrum towards the red part, for example, red orange ← ← ← yellow green blue ← ← purple).