Synthesis, solvatochromic studies, and water sensing applications of methylene blue derivatives

Solvatochromic dyes constitute a large group of molecules that change their color in response to their environment and have been used in many areas of research and industry. One of the widely investigated dyes is methylene blue. In these studies, two compounds, including methylthioninium bis(trifluoromethanesulfonyl)imide ([MB][TFSI]) and methylthioninium oleate ([MB][OLE]), were prepared from commercially available methylthioninium chloride ([MB][Cl]) by using ion-exchange reactions. These compounds were characterized using FTIR spectroscopy, NMR spectrometry (¹H, ¹³C, and ¹⁹F), UV–vis absorption measurements, elemental analysis, and high-resolution mass spectrometric analysis. Solvatochromic properties of these two compounds, as well as the starting compound, were evaluated in seven different organic solvents with similar and different polarities: methanol, ethanol, acetone, acetonitrile, dichloromethane, dimethyl sulfoxide, and N,N-dimethylformamide. The color change exhibited by [MB][Cl] and [MB][TFSI] in different solvents was minimal, while [MB][OLE] exhibited enhanced color change with approximately five different colors, indicating the potential for discrimination of organic liquids of very similar polarity. Visible absorption spectra of [MB][OLE] showed an approximately 15–17 nm blue shift with enhanced absorption in high energy region compared to the spectra of [MB][Cl] and [MB][TFSI]. Then, [MB][OLE] was applied to detect water in dimethyl sulfoxide and acetone. The key novel finding of this work is the demonstration that oleate ion significantly enhances the solvatochromic properties of methylthioninium ion. These results have significant implications in developing ionic materials with enhanced photophysical properties by pairing different cationic dyes with bulky fatty acid anions for diverse applications.