Abstract
In this work is presented a spectrophotometry 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. DOI: http://dx.doi.org/10.17807/orbital.v9i4.996
Highlights
Phenothiazine compounds as Methylene Blue (MB), Figure 1, and dyes from the same class, such as Azure A, Azure B and Thionine, have been extensively used for more than a century for various applications, especially in health [1]
In order to get more insight about this dye, in this study we investigate the role of the solvent on the spectroscopic properties of MB
It could be observed that the presence of water in organic solvents affects directly the MB aggregation process and, as consequence, changes the photophysical properties in the binary mixtures of solvent
Summary
Phenothiazine compounds as Methylene Blue (MB), Figure 1, and dyes from the same class, such as Azure A, Azure B and Thionine, have been extensively used for more than a century for various applications, especially in health [1]. MB presents a quantum yield of singlet oxygen (1O2) formation around 0.5 [4], with a low reduction potential, intense light absorption within the phototherapeutic window for Photodynamic Therapy [5]. It displays a high photodynamic efficiency causing apoptosis of cancer cells. When absorption spectra are measured in solvents of different polarity it is found that the position and the intensity and shape of the absorption band can vary, depending on the nature of the solvent [9,10,11] In this way, MB usually show strong polarity dependent Stokes shifts, large changes in dipole moments on excitation, and very high fluorescence quantum yields. It is important to investigate the processes of MB selfaggregation present in solvent mixtures and/or interaction with biomolecules [8]
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