Colour and constitution of conjugate bases of benzodifurantrione, its ring-opened derivatives and benzodifuranone dye analogues

The observation of ready deprotonation of the phenylogous enol of benzodifurantrione (BDT) to give a bright violet conjugate base has led to two follow up explorations. Extension of BDT enol by insertion of a p-phenylene unit into the enol C-O bond gives the known bright red 4-hydroxylated benzodifuranone dyes. Their deprotonation results in previously unreported near infrared-absorbing conjugate bases. These appear to aggregate in solution, the more so in less polar solvents. Ring-opened derivatives of BDT containing α-dicarbonyl substituents also give coloured conjugate bases, but α-keto-ester and -anilide derivatives differ substantially (ester yellow; anilide intense red). Investigation of this nonintuitive difference leads to the conclusion that while the anilide is essentially planar the ester is nonplanar. The contrast in conformation impacts on the auxochromic effects of the otherwise closely related α-dicarbonyl substituents and thus the variation in colour. The latter observation has potential across colour chemistry in general. In contrast to the readily observed BDT enol, no evidence has been adduced for enol tautomers amongst the ring-opened analogues.