Hydrolytic breakdown of meso-diphenylamine substituted heptamethine dye IR140 and isomerism of the merocyanine breakdown product

Abstract

Nucleophilic attack by hydroxyl ions is found to occur preferentially at the C 2 position of the benzothiazolium heterocyclic moiety of the laser dye IR140, and not at the eneamine functionality that consists of the mesocarbon atom and the adjacent diphenylamine moiety, which is the other electrophilic site on the dye. The temperature-dependence of the absorption spectrum of the merocyanine breakdown product suggests that similar ground state isomerism is present in the breakdown product, as is found in the initial dye IR140. It is concluded that the non-fluorescent majority conformer of the merocyanine breakdown product is the one in which the meso-amine substituent is twisted so as to withdraw electron density by induction, and that the fluorescent minority conformer is that in which themeso-amine substituent is conjugated to act as an electron donor by resonance.