Thio-Mayan-like Compounds: Excited State Characterization of Indigo Sulfur Derivatives in Solution and Incorporated in Palygorskite and Sepiolite Clays

Following what may mirror the recipe used by the ancient Maya civilization, consisting of a mixture of indigo and palygorskite, leading to the organic/inorganic pigment known as Maya Blue, we have switched indigo with thioindigo (TI) and a derivative (Ciba Brilliant Pink, CBP) and synthesized what was Christianized as “Maya Pink”. The spectral and photophysical behavior of TI and CBP was investigated in solution and solid state (in powder and incorporated in palygorskite clays). In solution, TI was investigated in different organic solvents (benzene, toluene, and dioxane) at room and low temperatures and further compared with indigo. TI displays a different spectral and photophysical behavior when compared to indigo: fluorescence dominating the deactivation of the first excited singlet state (50–70% depending on the solvent), with the S1〰→S0 internal conversion and the S1〰→T1 intersystem crossing deactivation channels representing the remaining quanta loss. Moreover, whereas in the case of indigo the proton transfer process in the excited state (between the N–H and CO groups), involving the formation of two excited species, leads to an efficient internal conversion radiationless process, with TI this process is precluded leading to a single exponential fluorescence decay. Incorporation of TI and CBP in palygorskite and sepiolite clays has showed different photophysical properties with decrease of the fluorescence quantum yields, a single exponential decay for TI and a multiexponential decay for CBP in palygorskite. Based on molecular fluorescence data, potential locations of TI and CBP in the clay are equated.