Highly conjugated streptocyanine-ESIPT dyes via Vilsmeier-Haack reagent

Polymethine and ESIPT dyes are known to act as fascinating compounds for several applications. However, a combination of both fluorophores as a single molecule is poorly investigated. In this work, we report for the first time the successful synthesis of streptocyanine-ESIPT derivatives through a simple three-step route. Our synthetic route involves the coupling between amino ESIPT dyes and an iminium salt formed through the reaction between Vilsmeier-Haack reagent and cyclohexanone. Remarkably, the presence of a polymethine chain between two ESIPT nuclei changed the photophysical properties of the fluorophores individually. The new streptocyanine-ESIPT dyes contains electron acceptors groups able to increase of the conjugation of the pi-system. In addition, the outstanding combination of these two different class of dye molecules increased significantly the Stokes shift of polymethine dyes, which enable the future application of these derivatives as efficient biological probes.