Insights into the photophysics of zinc phthalocyanine and photogenerated singlet oxygen in DMSO-water mixture

This research was focused on the effects of DMSO-water ratio on the photophysics of the sensitizer Zn phthalocyanine (ZnPc) and on the photogenerated singlet oxygen. To reach this objective optical absorption, flash photolysis and time-resolved phosphorescence were used. Potential dye accumulation at the surface of the sample was studied by surface-tension measurements and at the chosen concentration no such effects were found. It was established that the sharp drop of dye absorbance at 40% water content is produced by dye dimerization, probably induced by a re-clustering of molecules in the solvent composition. When water runs over 0-80% range the dye triplet lifetime follows a sigmoid evolution with an inceptive plateau at 120 mu s and a sharp decrease in the 25-35% water range. Complementary experiments in neat DMSO established that while the ground dye molecules strongly quench the dye triplet states they do not significantly quench singlet oxygen. Experiments also showed that previous reported behavior of the lifetime of photosensitized singlet oxygen, exhibiting a prominent peak in the range 20-40% of water content is a common feature for both water soluble and insoluble solvates. The work outlines the hypothesis of a DMSO first solvation shell around the solvated dye molecules for a water content less than 25% and of a molecular re-organisation of the solvent taking place beyond this point, which induces important variations of photophysical quantities. (C) 2016 Elsevier B.V. All rights reserved.