A comparison between the photophysical and photosensitising properties of tetraphenyl porphycenes and porphyrins

The photophysical properties of 2,7,12,17-tetraphenylporphycene (TPPo) and its palladium(II) (PdTPPo) and copper(II) (CuTPPo) complexes as well as comparisons with those of their porphyrin counterparts are reported. All porphycenes absorb in the red part of the spectrum, but only TPPo shows fluorescence (Φ_F = 0.15). This compound presents good quantum yields of triplet (Φ_T = 0.33) and singlet oxygen (Φ_Δ = 0.23) formation. In the case of PdTPPo, fluorescence is inhibited by the internal heavy-atom effect and the triplet and singlet oxygen quantum yields are enhanced (Φ_T = Φ_Δ = 0.78). The presence of the paramagnetic ion GU(II) in CuTPPo further enhances the non-radiative transitions leading to an internal conversion quantum yield Φ_ic = 0.65 and to a triplet quantum yield Φ_T = 0.35. With a triplet lifetime of 700 ns, CuTPPo is nevertheless able to sensitise singlet oxygen with a quantum yield that strongly depends on the oxygen concentration in the environment. These photophysical properties, together with their ability to kill several cancer cell lines, place these sensitisers in a good position to be used in photodynamic therapy (PDT).