Theoretical Analysis of the Light Fastness of Reactive Dyes on Cellulose

Abstract

Five selected reactive dyes were used to perform theoretical calculations of photochemical reactivity indicators in electro-(S^E) and nucleophilic reactions (S^N). The study utilised reactive dyes in which the reactive group is cyanuric chloride, but which differ in their chromophore structure. They belong to the groups of monoazo, disazo and anthraquinone dyes. The influence of changes in these indicators after the formation of the dye-cellulose covalent bond on their light fastness was analyzed. Using the PM3 limit molecular orbital method (MO), the electron density distribution was calculated for dyes in the highest occupied orbital (HOMO) and the lowest unoccupied (LUMO) in the singlet state. These values reflect the tendency towards an electrophilic reaction with a singlet oxygen atom ¹O₂ or a nucleophilic reaction with the superoxide radical anion ${\text{O}}_{\text{2}}^{·-}$ on atoms in the dye molecule. Reactivity indicators as super delocalization (S^E, S^N) and electron density distribution in the ground and excited states were calculated. The values of the super delocalization coefficients indicate the activity of sites in the molecules in the oxidation reaction, the resistance of these dyes to photo-oxidation and their influence on the durability of chemical bonds with the cellulose. It was found that the formation of bonds with the cellulose slightly affects the resistance in the electrophilic oxidation reaction, but this effect is significant in the nucleophilic reaction.