Dehydroindigo, the Forgotten Indigo and Its Contribution to the Color of Maya Blue

A comprehensive investigation of the electronic spectral and photophysical properties of the oxidized form of indigo, dehydroindigo (DHI), has been carried out in solution at 293 K. It is shown that dehydroindigo readily converts into its neutral keto form, the blue indigo, in a process which depends on the solvent and water content of the medium. DHI was investigated in toluene, in benzene, and in methanol and it was found that both the oxidized and the keto indigo forms are present in solution. In marked contrast to what has been found for keto-indigo, where the internal conversion channel dominates >99% of the excited state deactivation, or with the fully reduced leuco-indigo, where fluorescence, internal conversion, and singlet-to-triplet intersystem crossing coexist, in the case of DHI in toluene and benzene, the dominant excited state deactivation channel involves the triplet state. Triplet state yields (phi(T)) of 70-80%, with negligible fluorescence ( S(0) internal conversion deactivation process. The data are experimentally compatible with the existence of a lowest lying singlet excited state of n,pi* origin in toluene and of pi,pi* origin in methanol. A time-resolved investigation in the picosecond time domain suggests that the emission of DHI involves three interconnected species (involving rotational isomerism), with relative contributions depending on the emission wavelength. DFT calculations (B3LYP 6-31G** level) were performed in order to characterize the electronic ground (S(0)) and excited singlet (S(1)) and triplet (T(1)) states of DHI. The HOMO-LUMO transition was found to accompany an n --> pi* transition of the oxygen nonbonding orbitals to the central CC and adjacent C-N bonds. Calculations also revealed that in S(0) the two indole-like moieties deviate from planarity from ca. 20 degrees, whereas in S(1) and T(1) the predicted structure is basically planar; a gradual decrease of the carbon-carbon central bond distance is seen in the order S(0), S(1), T(1). An additional study on the blue pigment Maya Blue was made, and the comparison between the solid-state spectra of indigo, DHI, and Maya Blue suggests that, in line with recent investigations, DHI is present together with indigo in Maya Blue. These results are relevant to the discussion of the involvement of dehydroindigo in the palette of colors of the ancient Maya Blue pigment.