meso-Thiophenium Porphyrins and Their Zn(II) Complexes: A New Category of Cationic Photosensitizers.

ABSTRACT

A new category of cationic meso-thiophenium porphyrins are introduced as possible alternatives to the popular meso-pyridinium porphyrins. Combinations of cationic porphyrins bearing meso-2-methylthiophenium and meso-4-hydroxyphenyl moieties T2(OH)2M (A2B2 type) and T(OH)3M (AB3 type) along with their zinc(II) complexes T2(OH)2MZn and T(OH)3MZn, are reported. The increase in the number of thienyl groups attached to the meso-positions of the porphyrin derivatives (A2B2 frame) has been shown to impart longer fluorescence lifetimes and stronger photocytotoxicity toward A549 lung cancer cells, as evident with T2(OH)2M and its corresponding diamagnetic metal complex T2(OH)2MZn. The photoactivated T2(OH)2MZn imparts an early stage reactive oxygen species (ROS) upregulation and antioxidant depletion in A549 cells and contributes to the strongest oxidative stress-induced cell death mechanism in the series. The DFT calculations of the singlet-triplet energy gap (ΔE) of all the four hydrophilic thiophenium porphyrin derivatives establish the potential applicability of these cationic photosensitizers as PDT agents.