Solvatochromism is defined as the ability of a chemical substance to change its color by changing the solvent polarity. Reichardt’s dye is the most known substance with such ability, and is used on this video. The substance is green in acetone, blue in isopropanol, violet in ethanol and red in methanol.
We can explain the changes in solution colors by considering the structure of Reichardt’s dye as well as by understanding why some substances are colored. The structure of Reichardt’s dye is shown on the picture bellow:
In the structure of Reichardt’s dye positive and negative charges are separated, and such neutral/dipolar molecules are called betaines or zwitterions. The molecule has a dipolar structure, but is neutral as a whole. This is a ground state of the molecule. When the betaine dye absorbs light, an electron is transferred from the HOMO (Highest Occupied Molecular Orbital) to the LUMO (Lowest Unoccupied Molecular Orbital) molecular orbital. HOMO and LUMO are over the phenoxide and pyridinium moieties respectively, so there is only a charge delocalization induced by light. This electronic transfer (or charge transfer) from the negative end of the molecule (from oxygen atom) to the positive end (nitrogen atom) is shown on the following equation:
It can be rationalized that polar solvents would better stabilize polar ground state, while nonpolar or less polar solvents would be better for stabilization of the excited nonpolar state. Consequently, it will be easier to excite the dye in nonpolar solvents, i.e. less amount of energy is needed for the excitation in nonpolar solvent. According to the equations
light of a higher wavelength (lambda) has a lower energy, which belongs to red part of the visible spectrum. That means that in nonpolar solvent energy of red light would be enough to excite the molecule.
White color consists of all colors of the spectrum. When one specific color from the spectrum is absorbed, what we see is so called complementary color. For example yellow color is observed when violet color is excluded from the spectrum; red color is observed when green color is excluded. This can be shown on the color wheel:
When red light is absorbed, the solution has green color. This is the case when relatively nonpolar acetone (e = 20.7 D) is used as a solvent. Methanol is much more polar (e = 32.6 D) and better stabilize polar ground state. Out of this reason, light of a lower wavelength (higher energy; green light in this case) is needed for the excitation and the solution is red in color. The same theory can be applied for isopropanol or ethanol, and all data are shown in the table: