The photophysics of disulfonated metallophthalocyanines upon complexation with fluoride

The photophysics of the disulfonated metallophthalocyanines (MPcS(2)) AlPcS(2), GaPcS(2), and ZnPcS(2), upon complexation with the fluoride ion are characterised. The addition of fluoride to AlPcS(2) 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 AlPcS(2) molecule; an equilibrium constant of 4.2 x 10(3) dm(3) mol(-1) was determined for this complexation. Addition of excess fluoride to aqueous solutions of AlPcS(2) 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 GaPcS(2) and ZnPcS(2) 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 AlPcS(2) in the absence of fluoride ion have been found to be different from previously reported values. The measured lower triplet quantum yields for AlPcS(2) in buffered solutions of H2O and D2O were 0.17 and 0.22 respectively. These differences are attributed to the triplet extinction coefficient (A epsilon(T)) used to calculate literature values. Using the singlet depletion technique we have shown that A epsilon(T) = 36 000 dm(3) 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-Delta of 0.77.