Effects of UV-decolouring of aromatic dyes with different chemical structures

In this study, the photodecomposition of various aromatic dyes with assorted chemical structures such as chromophores, physical and chemical properties were explored at 253.7 nm of ultraviolet (UV) light. The solubility of dye molecule was found to be the primary factor to determine the efficiency of photodecomposition. That is, the higher the solubility of aromatic dyes in water, the higher the efficiency of the dye being decolourized under UV-irradiation. We also found that, compared with mono-azo dyes, dyes With multi-azo groups (di-, tri-, or poly-) are easier decolourized. Surprisingly, the irradiation of anthraquinone dyes could enhance the colour content of dye solution because of the formation of intermediates that carry higher molar extinction coefficients (ε) during the photodecay process. These compounds absorbed more visible light at the detecting wavelengths (λmax) than their original molecules, and therefore the degree of colour was increased. However, evidences also showed that most of these intermediates can be further decomposed under extended UV-irradiation.