Two phenothiazine dyes as photosensitizers for the production of singlet oxygen. Photophysics, photochemistry and effects of aggregation

Abstract

The aim of this work was to investigate the aggregation effect of Azure B and monobrominated Azure B on their singlet oxygen generation. The monomeric and dimeric forms of these dyes were evaluated to select the species with better properties as photosensitizers. The results demonstrate that the monomeric forms are better producers of singlet oxygen than their aggregated species. However, the dimeric species of monobrominated Azure B is also a good producer of singlet oxygen. The bromination of this phenothiazine dye significantly increases the singlet oxygen quantum yields of the monomeric and dimeric species, enhancing its properties like photosensitizer and its application in Photodynamic Therapy. In addition, significant differences were observed in the singlet oxygen production of the monomeric forms of both photosensitizers determined by phosphorescence decay of singlet oxygen and photooxidation of 9,10-dimethylanthracene (DMA). The physicochemical interaction between the excited species of the phenothiazine monomers and the molecules of DMA could be responsible for the lower values of singlet oxygen production because the disaggregated forms did not present any activity as physical or chemical attenuators. The dimeric form of phenothiazine dyes did not show this interaction. In conclusion, there is a need for developing vehiculization systems that stabilize the monomers of phenothiazine dyes because they have better properties than their aggregates in terms of singlet oxygen production.