Octacationic and axially di-substituted silicon (IV) phthalocyanines for photodynamic inactivation of bacteria

The photodynamic inactivation of both Gram-positive and Gram-negative bacteria by polycationic silicon (IV) phthalocyanine photosensitizers bearing eight N-methylated pyridyloxy substituents in their periphery and different silyl ether moieties (OSi(iBu), OSi(nBu)3 and OSiPh3) at their axial positions has been evaluated. To this end, the influence of the nature of the axial and peripheral substituents on the spectroscopic, photochemical, photophysical and biological properties of the series of silicon (IV) phthalocyanines has been assessed. Specifically, this series of novel silicon phthalocyanines showed good fluorescence, singlet oxygen yields in both DMF and physiological media and no aggregation in aqueous media. Therefore, all the compounds were tested in Gram-positive and Gram-negative bacteria, such as Staphylococcus aureus (S. aureus), Methicillin Resistant Staphylococcus aureus (MRSA), and Escherichia coli (E. coli) obtaining the best results for the compound bearing axial OH groups. This fact, in combination with its strong absorption within the “phototherapeutic window” (650-850 nm) and its negligible dark cytotoxicity, yields the previous compound as a promising candidate for future studies.