HYDROLYTIC BREAKDOWN OF MESO-DIPHENYLAMINE SUBSTITUTED HEPTAMETHINE DYE IR140 AND ISOMERISM OF THE MEROCYANINE BREAKDOWN PRODUCT

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 o f the meso-carbon atom and the adjacent diphenylamine moiety, which is the other electrophilic site on the dye. The temperature-dependence o f the absorption spectrum of the merocyanine breakdown product suggest s that similar ground state isomerism is present in the breakdown prod uct, as is found in the initial dye IR140. It is concluded that the no n-fluorescent majority conformer of the merocyanine breakdown product is the one in which the meso-amine substituent is twisted so as to wit hdraw 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. (C) 1998 Elsevier Science Ltd. All rights reserved.