Monobromoindigos:: a new general synthesis, the characterization of all four isomers and an investigation into the purple colour of 6,6′-dibromoindigo

A new general synthesis of all four isomers of monobromoindigo is outlined, the synthesis of 4-, 6- and 7-bromoindigo being described for the first time. All four monobromoindigos as well as indigo and the historically important 6,6'-dibromoindigo (Tyrian purple) have been characterized by H-1 NMR spectroscopy of their N,N'-bis(trifluoroacetyl) derivatives, by FT-IR and FT Raman spectroscopy, and by their visible absorption spectra in solution and as dyes on wool. The absorption spectra of the dyes have been calculated by molecular mechanics (MM +) and semiempirical MO (PM3) methods. The purple colour of wool dyed with 6,6'-dibromoindigo is thought to be due to the interaction of the pi systems of two parallel molecules that are brought together by van der Waals attraction between the bromine atoms.