Dehydroindigo, the Forgotten Indigo and Its Contribution to the Color of Maya Blue Raquel Ronda ˜o, † J. Se ´rgio Seixas de Melo,* ,† Vasco D. B. Bonifa ´cio, ‡ and Maria J. Melo ‡,§ Department of Chemistry, UniVersity of Coimbra, 3004-535 Coimbra, Portugal, REQUIMTE-CQFB and Faculty of Sciences and Technology of the New UniVersity of Lisbon, Campus da Caparica, Portugal, and Department of ConserVation and Restoration, New UniVersity of Lisbon, Campus da Caparica, Portugal ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: October 19, 2009 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 (φ T ) of 70-80%, with negligible fluorescence (e0.01%) are observed in these solvents. In methanol the φ T value decreases to ∼15%, with an increase of the fluorescence quantum yield to 2%, which makes these processes competitive with the S 1 ' S 0 internal conversion deactivation process. The data are experimentally compatible with the existence of a lowest lying singlet excited state of n,π* origin in toluene and of π,π* 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 f π* 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°, 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. Introduction Indigo, a molecule which has influenced mankind’s history for several millennia and is a chemical icon, still harbors mysteries. Indigo was the blue color used by almost all the ancient civilizations and one of the first natural molecules to be synthesized, thus paving the way to the modern chemical industry. 1 The blue indigo is a mythical molecule not only due to its magnificent color but also because of its remarkable stability as a dye and paint. Its presence (even until our days) in denim jeans preserves the identity and importance of this molecule. Relevant aspects of indigo’s chemistry are related to the stability of this molecule as a dye. Its photodegradation was recently investigated, and it was shown that its reduced form, the leuco form, is significantly more prone to degradation than its neutral keto form. 2 Dehydroindigo (DHI), the oxidized (and third) form of the blue indigo, has been the forgotten form of indigo. This may be related to its more limited stability and the fact that the more common leuco is the water-soluble form used in the dyeing processes, which when exposed to oxygen leads to the colored neutral keto form. Nevertheless, although not normally isolated, DHI has gained recent interest since it was identified in the procedure leading to Maya Blue, 3-6 the source of blue of this ancient civilization, considered as the first fabricated organic (indigo)-inorganic (clay) hybrid. 7-9 Its chemical structure, although puzzling until recently, seems to involve the incorporation of indigo into palygorskite or sepiolite clays. 10-12 Recent works suggest that indigo reveals a strong attachment to this clays and that, together with DHI, it penetrates more deeply (and is consequently more protected) into the channels of palygorskite. 13 This incorporation protects the organic dye, leading to outstanding stability which has made possible the preservation of the color for centuries in paintings subjected to severe environment conditions, in par- ticular those involving light (photodegradation). However, the mechanism leading to indigo’s incorporation and its localization within the clay, i.e., if it is distributed in the surface or if it goes into the clay channels, or a distribution of these two, is still under debate. 13-16 It appears improbable that the blue indigo with its rigid structure can penetrate in its normal form into the palygorskite clay. Dehydroindigo and leuco-indigo seem more * To whom correspondence should be addressed: e-mail, sseixas@ ci.uc.pt; fax, 00351 239 827703. † Department of Chemistry, University of Coimbra. ‡ REQUIMTE-CQFB and Faculty of Sciences and Technology of the New University of Lisbon. § Department of Conservation and Restoration, New University of Lisbon. J. Phys. Chem. A 2010, 114, 1699–1708 1699 10.1021/jp907718k  2010 American Chemical Society Published on Web 01/07/2010