Mechanistic Insight into the Thermal "Blueing" of Cyanine Dyes.

In recent work to develop cyanine dyes with especially large Stokes shifts, we encountered a "blueing" reaction, in which the heptamethine cyanine dye (IUPAC: 1,3,3-trimethyl-2-((1,3,5)-7-(()-1,3,3-trimethylindolin-2-ylidene)hepta-1,3,5-trien-1-yl)-3-indol-1-ium) undergoes shortening in two-carbon steps to form the pentamethine () and trimethine () analogs. Each step blue-shifts the resulting absorbance wavelength by ca. 100 nm. Though photochemical and oxidative chain-shortening reactions had been noted previously, it is simple heating alone or with amine bases that effects this unexpected net CH excision. Explicit acetylene loss would be too endothermic to merit consideration. Our mechanistic studies using H labeling, mass spectrometric and NMR spectroscopic analyses, and quantum chemical modeling point instead to electrocyclic closure and aromatization of the heptamethine chain in forming Fischer's base (1,3,3-trimethyl-2-methyleneindoline), a reactive carbon nucleophile that initiates chain shortening of the cyanine dyes by attack on their polymethine backbones. The byproduct is the cationic indolium species (IUPAC: 1,3,3 trimethyl-2-phenyl indolium).