Axial ligand modified high valent tin(iv) porphyrins: synthesis, structure, photophysical studies and photodynamic antimicrobial activities on Candida albicans

Abstract

Herein, we report the synthesis, structure, photophysical properties and photodynamic antimicrobial activities on Candida albicans of axial ligand modified high valent tin(IV) porphyrins, namely Sn^IV(OH)₂T(4-CMP)P (1), Sn^IV(OH)₂T(4-CP)P (2) and Sn^IV(O-NO₂Ph)₂T(4-CMP)P (3). The newly synthesized porphyrins were characterized by various spectroscopic methods and single crystal X-ray diffraction analysis. The crystal structures of the precursor porphyrin, Sn^IV(Cl)₂T(4-CMP)P and 3 are well stabilized by various intermolecular interactions and porphyrin 3 shows complementary bonding interactions between the nitro groups (N⋯O) forming a one-dimensional array. The fluorescence lifetime of 3 is lower compared to other porphyrins which indicates that there is a considerable interaction between the tin(IV) porphyrin core and nitrophenyl system and this could be the reason for its significant phototoxicity. This is further evident from X-ray crystallography and DFT calculations. In the presence of light, tin(IV) porphyrins significantly inhibited the growth of C. albicans in liquid as well as in the solid agar medium and the growth inhibitory effects were much less under dark conditions. The porphyrin internalization as well as localization within the Candida cells were examined by fluorescence microscopic analysis. Our results suggest that the mechanism behind the photodynamic inactivation of C. albicans could be through the generation of singlet oxygen species within the cells.