The absorption wavelength of the dye solution changes with the polarity of the solvent. When the dye is dissolved in a polar solvent, the polarity of the dye increases as the polarity of the solvent increases, so that the excitation energy is reduced, the absorption wavelength moves to the long-wave direction, and the color of the dye solution deepens.
Many solvents can also form hydrogen bonds and solvates with dyes, which will change the color of the dye solution.
The color of the dye on the fiber (except for optical factors, such as scattering and refraction of different fibers) will be different due to different fiber polarity. Generally speaking, the color appears darker on fibers with high polarity, and lighter on fibers with lower polarity.
For example, the color of disperse dyes on acetate fibers is lighter than that of polyamide fibers; the color of cationic dyes on polyester fibers is lighter than that of polyacrylonitrile fibers.
When the dye concentration is very small, the dye exists as a single molecule in the solution, but if the dye concentration increases, the dye molecules will aggregate into dimers or multimers. The excitation energy of π electrons of aggregate molecules is higher than that of single molecules, so the dye absorption spectrum shifts to the shortwave direction and the color becomes lighter.
The change of temperature will affect the aggregation tendency of the dye, and then promote the change of the color of the dye. When the temperature rises, the degree of aggregation of the dye decreases, and the absorption wavelength moves to the long wave direction, which has a dark color effect. The color of some organic compounds and dyes will change reversibly with temperature. This phenomenon is called thermochromism.
Some azo, thioindigo, and cyanine dyes exist in stable trans structure at normal temperature. Under light irradiation, the trans structure of the dye will become cis structure. When the light source leaves, the cis structure will return to Trans structure. The absorption spectra of dyes of trans and cis structures are different, and the colors displayed are also different. This phenomenon is called photochromism.