The structure and the absorption spectra of the tautomers of phenylazo-1,3-indandione were studied by quantum chemical methods (MNDO and PPP). In contrast to the interpretation of the experimental results, the enolazo tautomer is more stable and absorbs at significantly longer wavelength than the hydrazone form. The five-membered rings within the tautomers have a small anti-aromatic character, which is responsible for the longest wavelength absorption. A pronounced anti-aromatic character is characteristic for the enolate ion of phenylazo-1,3-indandione, which should absorb at about 700 nm. © 1992.
Dietz, F., Rommel, K., & Tyutyulkov, N. (1992). The chromophore of phenylazo-1,3-indandiones: the azo group or the cyclopentadienyl cation fragment? Dyes and Pigments, 18(4), 251–257. https://doi.org/10.1016/0143-7208(92)80015-F