Novel halochromic and antimicrobial azopyrimidine dyes: synthesis, colorimetric studies and biological assays

Abstract

Halochromism is the occurrence of a color change induced by a pH alteration. The main commercial classes of halochromic dyes are phthalides, triarylmethanes and fluorans, but other minor classes exist including styryl, merocyanines, indophenols and azobenzenes.1a The success of azobenzene dyes is due to their structural diversity, high molar extinction coefficient, and fastness properties in relation to light and washing fastness.1b Replacing one or both aryls with a heteroaryl offers broader structural diversity.1c In addition, the presence of basic sites, as well as H-bonding interactions, further affects the azo chromophore, leading to very different spectral properties. The synthesis of azopyrimidines and azopurines has long been known, and similarity to DNA nucleobases makes them promising candidates for applications in photopharmacology and biocompatible real-time information transmission. In a recent work, a novel method for the synthesis of a new class of azo-pyrimidines with both halochromic and antimicrobial properties has been developed to obtain compounds with an unusual pattern of substituents in the heteroaryl unity. The new synthetic approach starts from imidazole derivatives that are easily obtained from accessible commercial reagents.2 Preliminary results had suggested that these imidazole precursors easily evolve in the presence of oxygen leading to the in situ formation of highly colored products. These results motivated us to study the reactions of imidazole-based precursors in the presence of oxidants such as silver and copper salts. A very fast reaction occurred leading to azo imidazole intermediates that promptly reacted with secondary amines through a novel and unexpected rearrangement. Deep colored products were obtained, which were identified as azo-pyrimidines on the basis of NMR spectroscopy (including 1H RMN, D2O shake, 13C RMN, HMQC, HMBC and NOE techniques), mass spectroscopy, ATR-FTIR, melting point and elemental analysis data. Colorimetric studies have been performed by UV-Vis spectroscopy at variable pH values and in different solvents, revealing interesting halochromic properties. The antimicrobial activity of these new azopyrimidine dyes has been also evaluated and highly promising results were obtained. All the results will be presented and discussed.