Singlet oxygen quantum yields of potential porphyrin-based photosensitisers for photodynamic therapy

The singlet oxygen formation quantum yield (Φ_Δ) for solutions of the di-cation, free-base and metallated forms of hematoporphyrin derivative (HpD), hematoporphyrin IX (Hp9) and a boronated protoporphyrin (BOPP) are reported using the method of direct detection of the characteristic phosphorescence following polychromatic excitation. Values of Φ_Δ for the free-base form of all the porphyrins and the di-cation forms of Hp9 and HpD are in the range of 0.44 to 0.85 in the solvents investigated. Incorporation of zinc ions into the macrocycle reduces Φ_Δ for all three porphyrins. BOPP facilitates the coordination of certain transition metals (Mn, Co and Cu) compared to Hp9 and HpD and results in a dramatic decrease in Φ_Δ. The experimental data suggest the introduction of low energy charge transfer states associated with the disruption of the planarity of the macrocyclic ring provides alternative non-radiative deactivation pathways. In BOPP, this non-planarity is augmented by the large closo-carborane peripheral substituent groups.