Preparation of NIR absorbing axial substituted tin(<scp>iv</scp>) porphyrins and their photocytotoxic properties

Sn(iv) porphyrins ([Sn(iv)TTP(3PyO)₂] (<b>5</b>) and [Sn(iv)TPP(3PyO)₂] (<b>6</b>) [tetrathienylporphyrin (TTP), tetraphenylporphyrin (TPP), and pyridyloxy (PyO)]) were prepared and characterized and their photocytotoxicity upon irradiation with 625 nm light has been studied. The presence of the 3PyO axial ligands was found to limit the aggregation and enhance the solubility of <b>5</b> and <b>6</b> in DMF/H₂O (1 : 1). The photophysical properties and photodynamic therapy (PDT) activity of the <i>meso</i>-2-thienyl and <i>meso</i>-phenyl-substituted Sn(iv) porphyrins are compared. <b>5</b> and <b>6</b> were found to be photocytotoxic in MCF-7 cancer cells when irradiated with a Thorlabs M625L3 LED at 625 nm but remained nontoxic in the dark. The PDT activity of Sn(iv) <i>meso</i>-tetra-2-thienylporphyrin <b>5</b> was found to be significantly enhanced relative to its analogous tetraphenylporphyrin <b>6</b>. There is a marked red-shift of the Q₀₀ band of <b>5</b> into the therapeutic window due to the <i>meso</i>-2-thienyl rings, and <b>5</b> has an unusually high singlet oxygen quantum yield value of 0.83 in DMF. The results demonstrate that readily synthesized axially ligated Sn(iv) <i>meso</i>-arylporphyrins are potentially suitable for use as singlet oxygen photosensitizers in biomedical applications and merit further in depth investigation in this context.