Light-induced effects in sulfonated aluminum phthalocyanines — potential photosensitizers in the photodynamic therapy: Spectroscopic and kinetic study

Sodium salts of sulfonated hydroxyaluminum phthalocyanines [Al(OH)Pc(SO3Na)i] containing various numbers of sulfonate groups (i = 1–4), were studied by spectroscopic techniques. The lifetimes of the singlet states were found to range between 5 and 8 ns, experimentally determined fluorescence quantum yields ranging between 0.45 and 0.74. Both parameters depend on the number of sulfonate groups, the singlet lifetime depending additionally on concentration. The triplet lifetime was indirectly estimated for aqueous solutions of Al(OH)Pc(SO3Na)3 to amount to ca. 0.6 μs. The decay time of singlet oxygen in the same system was also found to be close to 0.6 μs. The molecules were found to dimerize in acidic and neutral solutions, the second order rate constants of the dimerization being of the order of 10 mol−1 dm3 s−1, and the equilibrium constants of the order of 104. The experiments were supplemented with quantum chemical calculations allowing us to assign electronic transitions in monomers and dimers.