Time-Resolved Spectroscopy of Indigo and of a Maya Blue Simulant

Abstract

Maya Blue is a puzzling pigment found in objects produced by the ancient Maya civilization. It is a combination of indigo and palygorskite, and it is well-known for the high chemical and photochemical stability of the dye promoted by the clay confined environment. This pigment has survived over 1500 years, and it was first thought to be purely inorganic. The reasons for such stability have been investigated over the past years, and it may involve hydrogen bonds, complexation, and oxidation to dehydroindigo. However, these theories are not completely understood, and more evidence about indigo/palygorskite interactions must be obtained. In this study indigo and a Maya Blue simulant pigment were, for the first time, studied by transient absorption (TA) and time-resolved infrared (TRIR) spectroscopy. From such analysis it was possible to investigate the electronic excited states of indigo and the photochemistry behavior of the dye when interacting with the palygorskite. Concerning the TRIR measurements, the shifts of the C O and N−H vibrations indicate that hydrogen bonds are formed involving the dye and the coordinated water molecules present in the clay. Furthermore, a red shift is observed in the absorption of electronic ground state (50 nm) and also in the electronic excited state (27 nm) of Maya Blue simulant, suggesting that the excited states are stabilized by the clay. Indigo in DMSO solution presents a lifetime of ca. 120 ps while in the clay it becomes much shorter, ca. 3 ps. The shorter lifetime and also the red shift observed in the TA results suggest a stabilization of the first electronic excited state, which promotes a more efficient energy relaxation through conical intersection and, as a consequence, a faster excited state decay. Such factors can be important for the Maya Blue photostability, as they are also believed to be responsible for the high photostability of DNA and melanin.

Excited-State Proton Transfer in Indigo

“…In general, no further spectral evolution was observed after 350 ps. The fs-TA data for Ind and NHxInd show a positive broad TA band in the 440−700 nm range, in agreement with previous reports, 12 corresponding to the excited singlet state absorptions, ESA, of these two compounds. In the 700−800 nm range, the stimulated emission, SE, is visible and in the case of indigo in 2MeTHF, bleaching of the ground-state absorption, GSA, is observed in the 565−600 nm region (Figure 4).…”

A Comparative Study on Color Stability of Anthocyanin Hybrid Pigments Derived from 1D and 2D Clay Minerals

“…It also could be clearly seen that the color of ACN/Sep was much closer to the red area than other hybrid pigments (Figure 4a). This might be attributed to the encapsulation of ACN molecules in nanochannels and grooves of Sep, as previously reported [28].…”

“…For example, Pal protected indigo from the damage of external environments by enclosing the natural dyes in its nanochannels, and thus hybrid pigments exhibited excellent sunlight resistance and chemical stability [24,25]. In the case of Sep, the involved interactions between clay minerals and dyes mainly included encapsulation of nanochannels, H-bond formation, complexing action, and the presence of dehydroindigo [26,27,28]. Furthermore, Bernardino et al proposed that indigo molecules were constrained by the micro-channels of Pal to avoid molecular geometry rearrangement and fast energy relaxation, thus preventing from the inactivation of Maya blue [29].…”

Evidence for competing proton-transfer and hydrogen-transfer reactions in the S1 state of indigo

“…Time resolved spectroscopy of an indigo analogue that only allowed a single proton transfer revealed a fluorescence spectrum with a biexponential decay, hinting at fluorescence from two different states, possibly the diketo (KK) and keto-enol (KE) states [11]. Transient absorption spectroscopy of indigo in DMSO revealed a lifetime on the order of 120 ps, but showed no evidence of proton transfer (PT) [12]. Iwakura et.…”