Transition metal azahemiporphycenes as singlet oxygen sensitizers

Using free-base tris(p-X-phenyl)corrole (X = H, Me, OMe) as starting materials and 4-aminotriazole as the nitrogen source, we used a DDQ-mediated, room-temperature strategy for generating isocorroles to obtain the corresponding 6-azahemiporphycenes, H2[X-AHP], one of which was structurally characterized with single-crystal X-ray diffraction analysis. The free-base ligands were readily derivatized to their Pd(II), Pt(II), and Mn(II) complexes; of these, one Pd(II) complex was structurally characterized. Three of the complexes, M[Me-AHP] (M = Pd, Pt, Mn), were singled out for photophysical and singlet oxygen sensitization studies. Although none of the new complexes proved luminescent, Pd[Me-AHP] was found to be an excellent singlet oxygen sensitizer, with a singlet oxygen quantum yield of 84% in 9:1 v/v ethanol:tetrahydrofuran at room temperature. The corresponding value for Pt[Me-AHP] was found to be only 11% and that for Mn[Me-AHP] was essentially 0%. These singlet oxygen quantum yields were consonant with triplet lifetimes accessed via transition absorption spectroscopy. Combined with their strong absorption in the red, these findings suggest that palladium 6-azahemiporphycenes are worthy of exploration as a new class of triplet photosensitizers for photodynamic therapy of cancer and other diseases.