The Methylene Blue Self-aggregation in Water/Organic Solvent Mixtures: Relationship Between Solvatochromic Properties and Singlet Oxygen Production

In this work is presented a spectrophotometric investigation focused on the solvatochromic effects upon Methylene Blue (MB).Measurements were carried out in four different water/organic solvent mixtures: low polarity protic solvent (ethanol), polar non-protic solvent (acetonitrile), highly polar protic solvent (glycerol), and non-polar solvent (dioxane).The results showed that the photophysical behavior of MB is highly affected by self-aggregates formation at 80% of water/organic solvent blends.Besides polarity, the protic character and the coordinating properties of the solvent molecules are the key parameters for its photophysical behavior, since the sulfur atom of the cationic structure can act as a coordination center due to its Lewis acid character.In fact, water and acetonitrile coordinating properties have proved to be important to determine MB interactions intensity and its spectroscopic properties as singlet oxygen emission.It was observed that an increase of the amount of the organic solvent resulted in an enhancement of the singlet oxygen emission intensity.The presence of the water increases the dielectric constant of the medium and favors the self-aggregation process.Besides, the water molecules can act as a quencher and it decreases the quantum yield of the fluorescence of MB.