The relationship between the number of conjugated double bonds of dyes and their color. The more conjugated double bonds in an organic compound molecule, the darker the color of the organic compound. This is because as the conjugated double bonds increase, their selective absorption shifts toward longer wavelengths, which results in a darker color effect.
The influence of polar groups on color in conjugated systems: nitro, nitroso, carbonyl, azo, etc. Chromophores are all polar groups, which have the effect of withdrawing electrons in the conjugated double bond system. They are called electron-withdrawing groups. Auxochromophores such as hydroxyl and amine groups are also polar groups and are often called electron-donating groups. When aromatic organic compounds have chromophores, the chromophores have color. When the chromophores contain auxochromophores, the polarization phenomenon is enhanced, which can increase the mobility of the π electrons of the conjugated double bonds in the molecule. Thereby reducing its excitation energy, the high-temperature-resistant metal complex dye shifts the selective absorption of the spectrum by the molecule to the direction of the longer wavelength, that is, a dark color effect occurs. At the same time, there are electron-absorbing substituents and electron-donating substituents. This kind of The effect is more significant.
The effect of ionization on color Molecules of organic compounds containing electron-withdrawing groups and electron-donating groups can be ionized Due to the different properties of the medium and substituents and the positions of the substituents, the depth of the color and the absorption intensity can change after ionization. For example, in acidic media, the originally neutral ones containing the electron-withdrawing group C=CNH Organic compound molecules become cationic, greatly increasing the electron-withdrawing ability of these groups, resulting in increased depth and intensity of color.
In alkaline medium, neutral organic compound molecules containing an electron-donating group – OH group become anions, and have significant electron-donating properties. Enhancement also increases the depth and intensity of color.
In acidic media, organic compound molecules containing an electron-donating group – NH2 group become cations, and the electron-donating properties of acidic dyes are significantly reduced, making The color becomes lighter and the absorption intensity also decreases.
For another example, phenolphthalein reagent is colorless in solutions below pH 9, but appears red in alkaline solutions. This is because the ionization conditions of phenolphthalein molecules in these two media are different, and the activation energy required for electrons in the molecules is different, so they have different colors.
In addition, due to the planar structure and spatial obstacles of molecules, organic compound molecules generate internal complexes. Different light sources will affect the dyed substances
Same and different humidity, etc., can affect the depth of the color.
