The addition reaction of hydroxide or ethoxide ion with benzindolium heptamethine cyanine dyes

This paper pertains to a nucleophilic addition reaction at the C2 atom of a benz[c]indolium or 3,3-dimethyl-1H-benz[e]indolium subunit of the corresponding near-infrared heptamethine cyanine that contains a chlorine atom at the central meso position of the chromophore. An important finding is that the efficient SRN1 replacement of the chloro substituent in such dyes is completely suppressed in the reactions (i) of hydroxide and ethoxide ions, both of which are poor single electron donors and (ii) conducted in aqueous alcohol, a medium that does not promote single electron transfer. The adducts produced were isolated and characterized by elemental analysis, 1H NMR, and 13C NMR. The NIR-absorbing parent dye is regenerated quantitatively upon treatment of the corresponding adduct with a weak acid, including silica gel.