Highly efficient near-IR cyclohexene cyanine photosensitizers for antibacterial photodynamic therapy

In spite of significant progress in the development of novel photosensitizers for photodynamic therapy (PDT) applications, the search for new approaches for designing organic dyes exhibiting high cytotoxicity upon biologically relevant near-IR light irradiation is still ongoing. Organic dyes such as porphyrins are widely used as photosensitizers, while cyanine dyes exhibiting much better absorption in the near-IR region have been only moderately explored. Herein, we report on a new series of cyclohexene-based cyanine dyes containing two and three indolenine, benzothiazole, and benzoselenazole terminal end-groups. These dyes are investigated for photodynamic inactivation of pathogenic S. aureus and E. coli bacteria. Both di-substituted benzothiazole and benzoselenazole cyanines were found to efficiently eradicate S. aureus and E. coli at nanomolar and micromolar dye concentrations, respectively, and at a low near-IR light dose, while exhibiting tolerable dark toxicity. The commercially available di-indolenine based dye IR786 that is used in many assays and imaging applications exhibits pronounced dark toxicity and phototoxicity to the cells which can substantially affect biomedical experiments utilizing this dye.