Three benzothiazole-derived heptamethine cyanine dyes were prepared via nucleophilic substitution at meso-position along the polymethine backbone. The synthetic route to the title polymethines includes a total of four steps starting from inexpensive commercially available materials. Structural elucidation of the newly synthesized chromophores was achieved using 1H-NMR, 13C-NMR and high-resolution mass spectrometry (HRMS) in ESI mode. Current functionalized amino-derived NIR dyes exhibit significant Stokes shift (Δν up to 1835 cm−1, 108 nm) compared to the parent unsubstituted chromophore (Δν = 303 cm−1, 23 nm). The probability of solvatochromism was determined from the correlation between the Stokes shift against the Dimroth-Reichardt ET (30) and the Lippert–Mataga function (ΔfLM) of orientation polarizability. Based on the experimental findings, this large Stokes shift presumably reflects a solvatochromic intramolecular charge transfer (ICT) across the molecule, which is induced by the introduced secondary amine.
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