The photophysics of disulfonated metallophthalocyanines upon complexation with fluoride

Abstract

The photophysics of the disulfonated metallophthalocyanines (MPcS₂) AlPcS₂, GaPcS₂ and ZnPcS₂ upon complexation with the fluoride ion are characterised. The addition of fluoride to AlPcS₂ produces a decrease in the fluorescence lifetime and yield, whereas the triplet lifetime, triplet yield and singlet-oxygen yield increased. The measured enhancement for the triplet yield in aqueous and methanol solvents was 47 and 37% respectively, with a concomitant increase in the singlet-oxygen yield observed in both cases. The photophysical changes are due solely to the intersystem crossing process which is seen to increase on addition of fluoride. The fluoride ion is believed to act as an axial ligand to the AlPcS₂ molecule; an equilibrium constant of 4.2 × 10³ dm³ mol^–1 was determined for this complexation. Addition of excess fluoride to aqueous solutions of AlPcS₂ at low and high pH gave a common absorption spectrum. This suggests the H₂O axial ligand of the aluminium complex is substituted by fluoride rather than OH^–. The photophysics of methanol solutions of GaPcS₂ and ZnPcS₂ are unchanged upon addition of fluoride, although a blue shift in the absorption spectrum for the gallium species is indicative of complex formation. The consequences of these results for the effect of added fluoride upon the efficacy of these agents as PDT sensitisers is discussed. Also, several properties of triplet-state AlPcS₂ in the absence of fluoride ion have been found to be different from previously reported values. The measured lower triplet quantum yields for AlPcS₂ in buffered solutions of H₂O and D₂O were 0.17 and 0.22 respectively. These differences are attributed to the triplet extinction coefficient (Δε_T) used to calculate literature values. Using the singlet depletion technique we have shown that Δε_T= 36 000 dm³ mol^–1 cm^–1 at the triplet absorption maximum of 490 nm, in all solvents. Similarly, the singlet-oxygen yield, determined in D₂O buffered solution was found to be 0.17, giving an S_Δ of 0.77.