Synthesis and spectral properties of near-infrared cyanine dyes showing enhanced Stokes shift: A paradigm of ICT dipolar state polymethine chromophoric systems

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 E_T (30) and the Lippert–Mataga function (Δf_LM) 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.