The molecular structure of dyestuffs has a very close relationship with the application properties of dyestuffs on fibers, such as light resistance, wet treatment resistance (washing, soaping), shrinkage resistance, level dyeing performance and dyeing rate in the dyeing process, etc. Therefore, some laws between the molecular structure of dyestuffs and their application properties have been paid attention by dyestuff workers. Therefore, some laws between the molecular structure of dyestuffs and their application properties have been emphasized by dyestuff workers.

 The dyeing performance of acid dyestuffs and the fastness of dyestuffs on fibers have some relationship with the type and nature of dyed fibers, but the most important factor is the structure of the dyestuff molecule itself.

Relationship between the molecular structure of dyestuffs and light fastness
 
The light fastness of dyestuffs on fibers is related to many factors, such as the type and nature of dyed fibers, the characteristics of the light source, temperature, humidity, and the depth of dyeing, and the structural characteristics of the dyestuff molecules.
Another effective way to improve the light fastness of acid dyes is to introduce some special groups into the dye molecules to improve the stability of the dye molecule structure or to influence the physical state of the dye molecules in the dyed fibers, thus improving their light fastness. Typical substituents are different sulfonamides, such as SO2NR2, NHSO2Ph, SO2NHCH3, etc., which can be introduced into acid dyes as well as metal complex dyes. 

In practice, more aliphatic carbon chains are introduced into acid dye molecules, which not only improves the light fastness of the dyes, but also improves the wet-processing performance. Measurement results show that as the length of the introduced aliphatic carbon chain increases, the light fastness increases, usually C4~C3 is better, if longer aliphatic chains are introduced, such as C16, the light fastness decreases. 

The effect of the length of the aliphatic carbon chain in the molecule on the light fastness of dyed fabrics can be considered to be related to the physical state of the dye molecule in the fiber. Dyes with medium-length carbon chains are more likely to form aggregates or micelles, which can easily eliminate or decompose the energy of the excited state of the dye molecules before their photochemical decomposition, or reduce the duration of the excited state of the dye molecules, or reduce the area attacked by the reactive plasmas such as O2, free radicals, H2O2, and so forth, and thus increase the fastness of the dyes to light. The longer aliphatic carbon chains can increase the surface activity of these dye molecules, reduce the stability of the dye aggregates, and make the dyes exist as single molecules inside the fiber. Therefore, it is necessary to choose the appropriate length of aliphatic chain according to the application requirements. 

Acidic and weakly acidic dyestuffs derived from amino anthraquinone sulfonic acid have good light fastness to wool dyeing, which is determined by the structure of the anthraquinone matrix. Similar to the amino-anthraquinone disperse dyes, the introduction of a sulfonic acid group, methyl group or other substituents in the neighboring position of the amino group reduces the reactivity of the amino group in the 1-position of the anthraquinone to photo-oxidation and improves its lightfastness stability. For example, the following dyes have good light fastness.

Meanwhile, if an aliphatic carbon chain is introduced into the anthraquinone acid dye molecule, C8~C12 is preferred, otherwise the light fastness will also be reduced. 

Aromatic methane dyes have the advantages of high strength and bright color, but usually on natural fibers, such as wool, silk or cotton fibers, the light fastness can be 1 level to 4~5 levels, the average is 2 levels. 

Effective way to improve the triarylmethane acid dyes, in addition to the introduction of the appropriate number of sulfonic acid group in the molecule, but also in the triarylmethane molecular structure of the center of the carbon atom in the neighboring position to introduce some specific substituent groups, such as a C1, a CH3 and a SO3H, due to the presence of these groups, resulting in the spatial resistance effect, so that the three benzene ring is not in the same plane, reducing the reaction of the central carbon atom Due to the presence of these groups, there is a spatial barrier effect so that the three benzene rings are not in the same plane, which reduces the reactivity of the central carbon atom, increases the photochemical stability of the dye molecule and improves the light fastness of the dye on the fiber. Typical varieties, such as acid violet 4BNS, have a light fastness of grade 1 only, while the light fastness of acid brilliant green B, which has introduced substituents, can be increased to grade 2~3.