Synthesis of pH-sensitive benzothiazole cyanine dye derivatives containing a pyridine moiety at the meso position

Abstract

Four benzothiazole-derived pentamethine cyanine dyes were synthesized with a pyridine moiety at the meso-position of the methine chain with a reaction yield of 89–93%. These dyes were prepared with high purity in a three-step synthesis as prospective pH-responsive probes. Each compound absorbed and fluoresced between 630 and 670 nm within the far-red range of the visible spectrum with high molar absorptivity values calculated between 134,910 L mol −1 cm−1 and 163,816 L mol −1 cm−1 and quantum yields between 9% and 17%. When a 20 μM solution of dye derivatives are exposed to hydrochloric acid in the range of pH 5–6 in ethanol, all the dyes experienced a blueshift in absorbance and emission as well as a decrease in spectral intensity that enables them to operate as pH probes. These properties were further validated via computational studies showing pyridine's influence on each molecules' spectral properties, and pathways for electron delocalization in the presence and absence of acid were proposed. © 2021 Elsevier Science. All rights reserved.