Abstract
A solvent-free, efficient, and green approach for the synthesis of monomethine cyanine dyes was carried out via grinding at room temperature in the presence of a minimum amount of catalyst. This green methodology aims to overcome the limitations and drawbacks of previously reported methods such as heating at high temperatures, use of large amounts of catalysts, use of solvents, modest yields, and long reaction times. Moreover, the advantages of the process include mild conditions with excellent conversion, and inexpensive procedure. The new synthetic method was demonstrated for eight reported dyes, as representatives, and showed high applicability for their analogues in the same way. The green context of the new methodology was noticeable, compared with the non-green components of the conventional one. The structure and purity of the resulting compounds were compared with the previously reported data and confirmed using thin layer chromatography, absorption spectra, elemental analysis, mass spectrometry, H-1-nuclear magnetic resonance (NMR) and C-13-NMR spectroscopy.