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C: Physical Processes in Nanomaterials and Nanostructures

Probing the Indigo Molecule in Maya Blue Simulants with Resonance Raman Spectroscopy Nathalia D'Elboux Bernardino, Vera Regina Leopoldo Constantino, and Dalva Lucia Araujo de Faria J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01406 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry

Probing the Indigo Molecule in Maya Blue Simulants with Resonance Raman Spectroscopy

N. D. BERNARDINO†, V. R. L. CONSTANTINOǂ AND D. L. A. DE FARIA†*

†

Molecular Spectroscopy Laboratory, Department of Fundamental Chemistry, Institute

of Chemistry, University of São Paulo C.P. 26077, 05513-970, São Paulo (SP), Brazil ǂ

Lamellar Solids Laboratory, Department of Fundamental Chemistry, Institute

of Chemistry, University of São Paulo C.P. 26077, 05513-970, São Paulo (SP), Brazil

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Maya blue (MB) simulants were prepared from mixtures of indigo and palygorskite (indpaly) and investigated by resonance Raman and UV-VIS absorption spectroscopies together with thermogravimetric analysis, aiming at to enlarge the understanding of the dye-clay interaction, the relationship between color and chemical stability, and the alleged formation of dehydroindigo (DHI); for comparative purposes, other simulants were prepared using sepiolite, laponite and montmorillonite. The results here obtained suggest that the greenish hue that develops when the ind-paly is heated seems to be linked with a decrease in the indigo molecular symmetry, which causes an increase in the oscillator strength of an absorption band at 500 nm (forbidden under the C2h symmetry) and a bathochromic shift and narrowing of the intense electronic transition at 657 nm (indigo). The DHI characteristic features are not observed in the resonance Raman spectrum (457.9 nm) of the ind-paly system, contrarily to what happen with laponite and montmorillonite which, however, do not present the chemical stability observed for MB. Resonance Raman, UV-VIS absorption spectroscopy and thermal analysis provided firm evidences that in ind-paly the dye is inside the micropores, interacting through hydrogen bonding with water molecules coordinated with the metal ions of the clay framework.

Keywords: indigo, Maya blue, resonance Raman, UV-VIS


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1. INTRODUCTION The resonance Raman effect was first observed, although not recognized as a special effect, in the early days of Raman spectroscopy1,2. With the advent of lasers as excitation sources, a wealth of vibronic information was obtained from a wide range of samples, therefore permitting the improvement of the theoretical models3,4 and supporting the proposed analytical use of the technique5. Resonance Raman spectroscopy is currently a tool tailored for the investigation of excited states in complex molecules, driving such information from the intensities of bands associated with the chromophoric group vibrations, which present dependence with the excitation wavelength (λ0). Taking into account such selectivity and an enhancement factor typically of 3 to 5 orders of magnitude, resonance Raman spectroscopy is considered a sensitive and selective tool in the materials characterization6. The applications of the technique now go from explosives7 to biological systems8, elucidating aspects related to electronic and molecular structures9. It thus seems to be the technique of choice in the investigation of some yet intriguing aspects of an important archaeological pigment: the Maya blue. Maya blue (MB) was produced in Central America, in the area which is currently Mexico and Guatemala, between the VI and XV centuries10. It consists of a mixture of indigo (a natural blue dye extracted from plants of the Indigofera and Isatis genus) with a clay mineral, specifically palygorskite and sepiolite, which were locally available11. When a previously ground mechanical mixture of both dye and clay is heated at moderate temperatures (typically 90 – 200 °C) a turquoise pigment with outstanding chemical and photochemical stability is obtained, which has been investigated for decades aiming at the understanding of such an exceptional stability12.



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Raman spectroscopy has been used in the last 15 years in investigations involving MB, and most of the studies were carried out with excitation at 785 nm13-17 where the luminescence background does not affect significantly the spectrum of indigo but, as a compromise, the resonance enhancement factor, if present as pre-resonance effect, is much smaller. Excitation at 514.5 nm16,18-20, 325 nm21,22, 488 nm20,23, 532 nm24 and 1064 nm25 were also used to address the nature of the indigo and clay interaction as well as the reasons for the pigment stability. Furthermore, although the genuine pigment was investigated in some works13,14,18,21, most of the published studies were carried out with MB simulants, prepared with palygorskite13,15,17,21,23-25 or sepiolite16,22. Regarding the 632.8 nm excitation it is likely that the luminescence background at this wavelength have discouraged its use in MB investigations. As far as the authors are aware, the first investigation on MB using Raman spectroscopy was reported in 200318 in which the spectral changes were related to a deviation from the planarity of the indigo molecule upon interaction with the clay, causing a lowering in its molecular symmetry. However, the proposed band assignment lacked of consistency and completeness because their conclusion is not supported by infrared absorption spectroscopy; moreover, the authors did not make clear why just a few Raman forbidden vibrational modes of indigo are observed in the Raman spectrum of Maya Blue. Such explanation was used in another study19 but theoretical and experimental evidences reported in the literature24 indicated that factors other than planarity should be taken into account to explain the MB spectral features. Currently, the most accepted model for indigo-clay interaction considers that the organic molecule penetrates the palygorskite microchannels after removal of zeolitic water21 forming hydrogen bonds with the water molecules coordinated to the metal cations (namely Mg2+ and Al3+)26. The formation of H-bonds was investigated using mainly spectroscopic


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techniques and thermal analysis12,21,23. Not all the the authors, however, agree with such an interpretation; instead, they suggest that the dye interacts with the clay at the channel edges17. The oxidation of indigo to dehydroindigo (DHI) at the clay surface27 was proposed as an explanation for the spectral behavior of MB28 and the first Raman study that considered such possibility used excitation at 1064 nm25 and, later, visible excitation (785 nm)15 however, the formation of DHI has been questioned by other investigations23,29. At this point it is important to emphasize that the resonance Raman effect, strictly speaking, was never used to clarify the contradictory points cited above, although some investigations on MB simulants using excitation at 488 nm23 and 325 nm21,22 clearly provided a significantly altered spectral pattern for MB when compared to other wavelengths indicating the potentiality of the technique in probing the electronic excited states of indigo in different environments. It seems, therefore, that several issues are yet awaiting a definitive answer: What is the role played by the water molecules coordinated to metals in the clay channels? A direct interaction (coordination) between indigo and metals ions of the clay mineral framework contributes to the pigment stability? Does dehydroindigo participate in the Maya blue simulants color and/or stability? What is the role played by molecular distortion regarding Maya blue simulants chemical and photochemical stability? What is the main reason for MB stability? In the investigation here reported, resonance Raman spectroscopy was used as the main tool to address such issues.



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2. EXPERIMENTAL 2.1. Materials Maya blue simulants were synthetized from indigo (Sigma-Aldrich, 95 %) and palygorskite clay (Source

Clay Repository of Clay Minerals Society, PFL-1).

Montmorillonite (Swy-2) and sepiolite were also acquired from Clays Minerals and a synthetic hectorite, Laponite RD, was provided by Southern Clay Products, Rockwood Specialties. The solvents used in the MB simulants extraction were N,N-dimethylformamide (DMF, Sigma-Aldrich, 99.8%), dimethyl sulfoxide (DMSO, Nuclear, P.A.) and methanol (Merck, 99.8%). DMSO from Sigma-Aldrich (≥99.9%) was used to obtain the Raman and UV-VIS absorption spectra of indigo. The reagents used in the DHI synthesis (anhydrous calcium chloride, synthetic indigo and glacial acetic acid) were obtained from Sigma-Aldrich, except PbO2 (Vetec, Química Fina Ltda) and toluene (F. Maia Ind. Com. Ltda). Maya blue simulants (ind-paly) were prepared by grinding a mixture of indigo (5 mg) and palygorskite (0.5 g) in a mortar and thence heating the mixture at different temperatures (90, 130, 170, 200, 220 and 250 °C) for 2 hours. Except when otherwise stated, the indigo excess was extracted by shaking vigorously the dispersion with DMF; the solid was separated by centrifugation at 6297 xg for 20 min (Eppendorf 5430) and dried in a desiccator. The same procedure was adopted for mixtures of DHI (2 mg) with palygorskite (0.2 g), here designated DHI-paly; the excess of dye was also removed by DMF extraction. Duplicates of the samples were analyzed. Sepiolite (sepi), Montmorillonite (mont) and Laponite RD (lap) were also used in an attempt to produce MB simulants with indigo (1 % weight mass) by heating the mixture at 130 °C for 2 hours.


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Dehydroindigo (DHI) was synthetized following a modified Kalb’s procedure30, using PbO2 as oxidizer, and was characterized by UV-VIS absorption spectroscopy (solid state and solution), Raman and infrared absorption spectroscopy. 2.2. Methods UV-VIS absorption spectroscopic analyses were conducted with a Shimadzu UV3101PC, using a diffuse reflectance Shimadzu ISR-3100 accessory. The solid samples (except DHI) were analyzed using barium sulfate as diluent; the UV-VIS absorption spectra of indigo were obtained in DMF and DMSO solutions. The UV-VIS reflectance spectrum of DHI was obtained from the dye trace on white paper whereas its solution was studied using toluene, chloroform and DMF. Raman spectra were obtained using diode lasers (532 nm and 785 nm, both from Renishaw) and an air refrigerated He-Ne laser (632.8 nm, Renishaw), with a Renishaw inVia Reflex Raman Microscope, fitted with a Peltier cooled CCD detector (Renishaw, 600x400 pixels) and a Leica optical microscope (DM2500 M); a Leica objective (x50, N.A. 0.75) was used in all the measurements. The spectra excited at 457.9 and 488 nm (mixed Ar+/Kr+ laser, Coherent Innova 70C) were obtained in a Jobin Yvon T64000, fitted with a liquid N2 cooled CCD detector (1024x256 pixels) coupled to an Olympus microscope (BX41); the spectra were obtained with a x100 (N.A. 0.90) objective. For the Raman microscopy measurements, laser power was kept below 0.5 mW. FT-Raman spectra (1064 nm from a Nd3+/ YAG laser) were obtained with a Bruker RFS 100/S, fitted with a liquid nitrogen cooled Ge detector; laser power was below 70 mW at the sample and spectral resolution was 4 cm-1. A high luminescent background was observed for the spectra excited at 488 nm; excitation at 785 nm and 1064 nm were used for ordinary (non-resonant) spectra. All the ind-clay spectra were obtained directly from the solid samples.



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FTIR studies were carried out in a Bruker (Alpha) fitted with KBr optics and DTGS detector, both in absorption and ATR (Attenuated Total Reflection) modes. For the ATR spectra, a single bounce accessory (Bruker, Platinum) with diamond crystal was used. Spectral resolution was 4 cm-1 and 128 scans were co-added. All the spectra were obtained directly from the solid samples. Both FTIR and Raman data were analyzed using the Grams AI package (Thermo Inc.) and the Cary WinUV Color Software v. 3.1 (Agilent Tech.) was used to obtain the L*a*b* parameters according to the color space defined by the Commission internationale de l'éclairage (CIELAB) from the UV-VIS spectra of the ind-paly samples. DFT (Spartan’14 for Windows) was used to obtain the vibrational frequencies, using B3LYP algorithm and 631G* basis set.

3. RESULTS AND DISCUSSION 3.1. Indigo and palygorskite compounds – the Maya blue simulants It is long recognized that heating is an essential step to obtain MB simulants with enhanced chemical and photochemical stability31; the physical mixture of dye and clay mineral is not enough to produce a pigment whose color resists to solvent extraction and nitric acid oxidizing action. The heating effect is clearly observed as the appearance of a greenish hue, which increases with the temperature (or heating time), as extensively reported in the literature20,32. The UV-VIS spectra of ind-paly heated at different temperatures are shown in Figure 1.


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Figure 1. UV-VIS spectra of (a) indigo and ind-paly mixtures (b) unheated and (c) heated at 90 °C, (d) 130 °C, (e) 170 °C, (f) 200 °C, (g) 220 °C and (h) 250 °C.

According to Figure 1, the temperature exerts two main effects in the indigo spectrum: the strong absorption band at 657 nm (pure solid) is red-shifted and a band at 500 nm progressively shows up as the temperature is raised; furthermore, a change in the 657 nm band shape is also observed, as demonstrated by the disappearance of the 560 nm shoulder ascribed as one component of the 657 nm band vibronic structure33. Reinen et al. had also reported the sharpened of the band at 657 nm, resulting from a decrease in the higher-energy side of this absorption band34. Regarding the absorption band at 657 nm, it emerges from Figure 1 that, apart from the red shift and change in profile, it also loses intensity as the sample is heated, making more evident the presence of a band at 500 nm, which was not observed in the unheated ind-paly sample because it was partially swamped by the indigo bands or because it was not an allowed electronic transition. A hypochromic effect is also observed when the UV-VIS absorption spectrum of indigo is recorded from solution35 and probably is related to the rupture of strong intermolecular interactions that exist when the dye



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is in the solid state36. On the other hand, the 500 nm band is not observed (or is barely seen) in solution35,37 but theoretical calculation (CASPT2) has found a forbidden transition at 2.64 eV (470 nm)38, therefore, if a lowering in the indigo symmetry occurs through a specific interaction (i.e. strong hydrogen bond) with the clay surfaces this band could gain in intensity and its intensity would depend on the number of interacting indigo molecules which is known to increase with temperature. The intensity of the 500 nm and ca. 660 nm (the exact position varies with temperature) were obtained as band height and the I500/I660 ratio was plotted against temperature (from room temperature to 250 °C), making clear that the change in relative intensity of the 500 nm band reaches a maximum at ca. 220 °C, as shown in Figure 2.

Figure 2. Intensity ratio of the 500 nm and ca. 660 nm bands measured as band height plotted as a function of heating temperature.

Another procedure (CIELAB color space)39,40 was used in an attempt to quantify the change in color promoted by heating the indigo and clay mixtures. Using such color space, a* is associated with the green color (red/green coordinate) whereas b* values represents the blue one (yellow/blue coordinate). The a* and b* values were obtained from the UV-VIS absorption spectra shown in Figure 1 using the Color software and the a*/b* ratio was plotted 10 ACS Paragon Plus Environment

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as a function of temperature (Figure 3). The results reported in Figure 3 qualitatively agree with a previously reported study on genuine MB samples that also used the CIELAB color space to describe the behavior of the UV-VIS absorption spectra20.

(I)

(II)

Figure 3. (I) COLOR graph of ind-paly heated at different temperatures before () and after () DMF extraction and (II) a*/b* (after DMF extraction) plotted as a function of temperature. The a*/b* reaches a maximum at ca. 180-200 °C.

Considering that the behavior of the I500/I600 and a*/b* ratios with temperature (Figure 2 and Figure 3(II), respectively) both show a maximum in the 180-200 °C temperature range, it can be concluded that the pigment color is largely affected by the absorption band around 500 nm. Two possible explanations initially considered for the change in color of the MB simulants with heating were (i) a red shift and change in the spectral profile (narrowing of the ππ* transition at 657 nm) that uncovered the band at 500 nm and (ii) a forbidden electronic transition whose oscillator strength was increased by a reduction in the indigo molecular symmetry caused by the interaction with the clay). The results reported in Figures 2 and 3(II) are incompatible with the first hypotheses, because narrowing and red shift of the intense 11 ACS Paragon Plus Environment


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transition at 657 nm would only cause a decrease in the absorption intensity at 500 nm. Another explanation given by some authors32,41,42 is that the band at 500 nm is a DHI absorption. In fact, a band centered at ca. 530 nm in the absorption spectrum of a MB simulant (sepiolite) was found by spectral subtraction42, but unfortunately it was an artefact generated by an incorrect procedure (subtraction of the MB spectrum from the indigo one (ind - MB), when the correct would be the other way around, i.e., MB – ind spectrum, please refer to Figure 9 of reference 42). When Raman spectroscopy (785 nm) was used to investigate MB simulants, the presence of DHI was supported by indirect assumptions, such as the assignment of a band at 1635 cm-1 to a stretching vibration of the DHI imine (C=N) group15. With excitation at 785 nm the indigo contribution to the Raman spectrum is much larger, due to the pre-resonance enhanced, than the enhancement expected for DHI (530 nm) at such wavelength. At this point, resonance Raman spectroscopy seems to be the technique able to address this issue: excitation at 632.8 nm provides the vibronic spectrum associated with the intense indigo ππ* transition, whereas excitation at 457.9 nm is mainly in resonance with the electronic absorption band at 500 nm. The spectra of unheated and heated (2 h at 130 °C) ind-paly excited at 632.8 nm are shown in Figure 4. The Raman spectrum of the unheated sample matches the spectrum of indigo (not shown) but differences are observed when the unheated sample is compared with the heated one. A remarkable feature is an overall improvement in the signal-to-noise ratio of the heated samples (the spectra shown in Figure 4(II) were baseline corrected) due to a lowering in the background intensity, which comes from the dye; the interaction with the clay, therefore, is somehow creating other energy relaxation path or enhancing the efficiency of an already existing route.


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(I)

(II)

Figure 4. Raman spectra (λ0 = 632.8 nm) (I) without baseline correction and (II) baseline corrected of (a) unheated and (b) heated (130 °C for 2 h) ind-paly mixture.

Although the assignment of the indigo Raman bands supported by theoretical calculations was already reported in the literature43, DFT calculations (B3LYP, 6-31G*) were performed and the vibrational frequencies are in agreement with the literature, except for the bands at 754, 1571 and 1582 cm-1 bands, as will be discussed below; it is also important to mention that the results are in a much better agreement with the experimental data for the heated ind-paly system than for the pure indigo. This is somehow unsurprising considering that in the unheated sample the presence of indigo microcrystals in the mixture can be anticipated, whereas, in the heated one, isolated molecules are expected. Accordingly, in the low frequency region (Figure 4(II)) the most significant changes are a shift in the 544 cm-1 band, assigned to δ (O=C-C=C-C=O), that shows up at 552 cm-1 in the heated sample, and the disappearance of the 633 cm-1 band which has a large contribution from δ (N-H) and δ (C-C) ring vibration. Below 300 cm-1 the narrow bands at 250, 264 and 276 cm-1 were replaced by a broad feature at 255 cm-1 with a shoulder at 239 cm-1, indicating that the indigo



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lattice (microcrystals in the unheated sample) was no longer present in the thermally treated ind-paly. In the 700 – 1300 cm-1 spectral window several bands are slightly shifted (4–12 cm-1) and a new band appear at 754 cm-1, which was assigned in the literature to δ (N-H)14,44 but, using the DFT calculation here reported, it was found that an in-plane deformation involving the central C=C bond (δ (C-C=C-C)) is, visually, a better description (potential energy distribution was not calculated). Above 1300 cm-1, the most important changes caused by heating in the ind-paly Raman spectrum are the coalescence of the 1621 and 1633 cm-1 bands to a single feature at 1634 cm-1, the coalescence of the 1571 and 1582 cm-1 bands to a single band at 1574 cm-1 with a weak shoulder at 1595 cm-1 and the downshift of the 1700 cm-1 that shows up at 1680 cm-1 in the heated ind-paly sample. Such changes are in agreement with the literature15,24 although it must be taken into account that some differences can be credited, in most cases, to different excitation wavelengths used. The thermal treatment seems to facilitate the rupture of intramolecular interactions in the indigo microcrystals, as demonstrated by the low frequency region in the Raman spectra shown in Figure 4(II) and, in fact, the Raman spectrum of the dye obtained as DMSO solution (at 532 nm due to the intense dye fluorescence background at longer wavelengths) shows the coalescence of the same bands described above (Figure 5); such conclusion is in agreement with a previously reported investigation on genuine and MB simulant samples13.


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Figure 5. Raman spectrum of (a) indigo in DMSO solution (λ0 = 532 nm) after subtraction of the solvent spectrum. The Raman spectrum of ind+paly heated at 130 °C for 2 h (b) (λ0 = 632.8 nm) was inserted for comparison purposes.

As already cited in this work, in the first Raman investigation on MB18, the authors claimed that a decrease in the indigo molecular symmetry caused the show up of new bands and the band at 1595 cm-1, which was assigned to a Ag+Bu mode; furthermore, the band at 1380 cm-1 was ascribed to a Bu δ(N-H) vibration and the 1017 and 1128 cm-1 features (Raman) were correlated with the 1038 and 1128 cm-1 modes in the infrared absorption spectrum. Notwithstanding a lowering in the indigo symmetry upon interaction with the clay surfaces is a very probable event, the band assignment presented by the authors is not complete (only the 1380 and 1595 cm-1 were assigned) and a comparison with the IR absorption would be necessary to give support to such interpretation.



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Furthermore, theoretical calculations showed that the rotation along the central C=C bond cannot account for the spectral changes because they are observed only for rotation angles higher than 30°, what is improbable within the micropores24. The spectral changes observed in the Raman spectra of thermally treated ind-paly mixtures are assigned by some authors17,21,23 to the formation of hydrogen bonds involving the indigo C=O, central C=C and N-H groups with coordinated water molecules. The coordinated water molecules are present inside the clay channels, directly linked to Mg+2 or Al+3 ions26 and their release from the clay framework occur between 150 and 250 °C (as determined by DTG); at lower temperatures (50-130 °C), the release of zeolitic and weakly adsorbed water can also be detected by DTG45,46. Originally, such proposed interaction through hydrogen bond with coordinated water was made based on theoretical calculations12 and, FTIR and Raman (λ0 = 325 nm) measurements21, however, in the latter the interaction was mainly inferred from the effect of indigo molecules on the O-H stretching vibration of the coordinated water molecules, because the indigo concentration was too low to be detected by FTIR, therefore, as far as the authors are aware, a direct evidence of such interaction was not presented yet. Accordingly, if indigo interacts with the coordinated water molecules, it is expected a dependence of the Raman spectrum of the ind-paly samples with heating temperature, because the release of zeolitic water would make the coordinated water molecules available for interaction with indigo. In the Raman spectra excited at 632.8 nm (Figure 6(I)), it can be noticed that the band intensities are relatively insensitive to the heating temperature (Figure 6(I) b-g) and, therefore, cannot help to clarify this point.


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(I)

(II)

Figure 6. Raman spectra obtained at (I) 632.8 nm and (II) 457.9 nm of (a) ind-paly mixture unheated and heated at (b) 90 °C, (c) 130 °C, (d) 170 °C, (e) 200 °C, (f) 220 °C and (g) 250 °C.

Considering that the intensity of the 500 nm absorption band increases with heating temperatures up to 180-200 °C, the Raman spectra obtained under resonance condition with such absorption band is expected to be more sensitive to the dye interaction with the clay surface and, furthermore, to detect DHI if present. When the 457.9 nm laser line is used (Figure 6(II)) two different responses to temperature are observed: one from 90 °C to 170 °C and other from 200 °C to 250 °C. At lower temperatures, the most significant enhancement is observed for the band at 1595 cm-1; above 200 °C other bands which are worth mentioning are at 1420 cm-1, 1461 cm-1, 1490 cm-1 and 1700 cm-1. At this point, before moving into an in-depth analysis of the spectra obtained at 457.9 nm it is important to discuss the nature of the 1595 cm-1 band. In the resonance Raman spectrum of indigo (632.8 nm), two bands are found close to this position (1571 cm-1 and 1582 cm-1) which coalesce into a single band at 1578 cm-1 in DMSO and at 1574 cm-1 in the MB simulant; according to the DFT calculations here reported, the 1571 cm-1 band has a 17 ACS Paragon Plus Environment


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larger contribution from aromatic ring stretching vibration and N-H in-plane deformation, whereas the 1582 cm-1 is better described as arising from the central C=C stretching vibration, together with C=O vibration and N-H deformation. With excitation at 457.9 nm, at 250 °C the band at 1574 cm-1 is still observed as a weak feature when compared to the much stronger band at 1595 cm-1, which probably corresponds to a very weak (or forbidden) band (ν C=C) that gained intensity with the lowering in the indigo molecular symmetry upon interaction with the clay. Confirming the sensitivity of the resonance Raman spectroscopy in probing the nature of the 500 nm absorption band, in two studies published in the literature using 325 nm as excitation wavelength21,22 the most intense band appears at 1580 cm-1 and not 1595 cm-1 indicating that the latter band is only enhanced when the excitation is in resonance with the 500 nm band, whose nature is disclosed by the resonance Raman spectral pattern. As a matter of fact, in the FT-Raman spectrum (not shown) of ind-paly heated at 130 °C, a weak band can be observed at 1595 cm-1, hence giving support for such explanation. The relative intensities of the 1420 cm-1 and 1595 cm-1 bands were obtained (as integrated area) using the 1250 cm-1 band (δ(C-H)) as standard, and the A1595/A1250 and A1420/A1250 ratios were plotted as a function of the heating temperature and are shown in Figure 7. The two plots show a change in behavior at ca. 200 °C in agreement with the first derivative curve of the thermogravimetric analysis (DTG), where the release of coordinated water molecules starts45. This is also the temperature where the color of the ind-paly samples is stabilized as shown by the a*/b* vs. temperature graph (Figure 3) and where the relative intensity of the 500 nm band (I500/I660) reaches a maximum (Figure 2). All of these experimental facts give a firm evidence that hydrogen bond involving the indigo molecules and coordinated water molecules which are inside the micropores is the interaction


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responsible for the changes in the UV-VIS and Raman spectra of MB simulants when compared to indigo.

(I)

(II) -1

-1

-1

Figure 7. Plot of the (I) 1595 cm /1250 cm and (B) 1420 cm /1250 cm-1 area ratio as a function of heating temperature.

As the temperature is raised above 200 °C other changes are observed in the Raman and in the UV-VIS spectra. In the Raman spectra the changes are: (i) a new band is observed at 1700 cm-1 whose intensity increases with temperature with the concomitant decrease in the intensity of the band at 1680 cm-1; (ii) the relative intensity of the bands at 1420, 1465, 1492 and 1595 cm-1 increases with temperature; and (iii) all the bands seem to broaden as the temperature is raised. In the UV-VIS spectrum, above 200 °C a broad feature shows up at ca. 850 nm, which progressively gain intensity as the temperature is raised (Figure 1). The intensities of the 1680 cm-1 (ν (C=O) of indigo hydrogen bonded to the clay) and 1700 cm-1 (ν (C=O)) bands were obtained, in this case as integrated areas after peak fitting, and the A1700/A1680 ratio was also plotted against temperature, which is shown in Figure 8 that also depicts the DTG data for palygorskite, in the same temperature range.



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The involvement of the indigo carbonyl groups in the formation of MB has been recognized since the first studies using vibrational spectroscopy in the 2000’s13, however, Figure 8 shows that when the coordinated water molecules are released (from 200 °C), the

ν(C=O) vibration is upshifted by 20 cm-1, reaching the position observed for the unheated mixture. This fact clearly indicates that the indigo molecules were not interacting directly with the metal ions of the clay mineral framework, but rather reinforces the hypothesis of interaction through hydrogen bond with the water molecules coordinated to such ions when the indigo clay mixture was treated at temperatures up to 200 °C. Higher temperatures probably benefits the direct interaction between the dye and the metal ions, causing changes in the Raman spectra which did not follow the trend observed for lower temperatures. In fact, the same explanation was given by Giustetto et al. discussing the interaction of methyl red and palygorskite: up to 300 °C the dye and clay interaction is mediated by hydrogen bond between coordinated (structural) water molecules and from 300 °C the methyl red molecules are directly bond to octahedral Mg atoms47.

Figure 8. Plot of the 1700 cm-1/1680 cm-1 area ratio as a function of heating temperature. The dotted line is the DTG curve from pristine palygorskite.


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The other changes observed in the Raman spectra obtained when heating temperatures above 200 °C were used could be compatible with complexation with the metal ions from the clay mineral framework. The formation of complexes involving indigo and metals (Pt2+ and Pd2+) was reported in the literature48 and although their Raman spectra was not obtained, the infrared absorption spectra is in agreement with the results reported in this work for higher temperatures: in the Pd2+/indigo complex the force constant of the C=O bond increases (1650 cm-1 in the complex and 1625 cm-1 in the pure dye) and in the UV-VIS spectrum an absorption band is observed at 748 nm48. The data here reported, i.e., the shift of the 1680 cm1

Raman band to 1700 cm-1 and a new band at ca. 850 nm in the electronic absorption

spectrum, goes in the same direction, suggesting that the release of the coordinated water molecules facilitates the interaction of the dye molecules directly with the metals (Mg2+ and Al3+) inside the channels, thus causing the observed spectral alterations. In spite of these evidences, the possibility that the thermal degradation of the indigo is initiated above 200 °C cannot be discarded, however, because the color of the ind-paly system also changes to a less attractive greyish tint, the thermal event (or events) at higher temperatures is less significant from the MB pigment investigation point of view. So far in this investigation, the 457.9 nm excitation produced direct evidence that indigo interacts through hydrogen bound with coordinated, but the possibility of DHI formation is yet to be addressed. The resonance Raman spectrum of DHI (λ0 = 457.9 nm) is reported in the Figure 9 and the two most intense bands show up at 1378 cm-1 (aromatic ring CC stretching + dihedral N=C-C=N deformation) and 1530 cm-1 (N=C-C=N stretching vibration). However, these and the other DHI features are not observed in any of the ind-paly samples, irrespective of the heating temperature.



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Figure 9. Resonance Raman spectra (λ0 = 457.9 nm) of (a) DHI, ind-paly (b) unheated and (c) heated (130 °C for 2h).

3.2. Indigo and other clays: Laponite, montmorillonite and sepiolite It could be argued that the enhancement, although present, could be too small to permit the observation of the DHI bands and to verify this possibility, the same procedure used to prepare ind-paly was adopted for MB simulants using other clays, namely, Laponite RD, montmorillonite and sepiolite; all of them resulted in a greenish blue solid after heating. In Figure 10, the UV-VIS absorption spectra and the resonance Raman spectrum (457.9 nm) of ind-lap and heated ind-lap are shown. The UV-VIS absorption spectrum of indlap, although not similar to the ind-paly one, shows the same general trend; in spite of this, ind-lap does not present the same chemical and photochemical stability exhibited by ind-paly, in a clear indication that color (green hue) and stability are not directly related, as already pointed out in the literature32; the same is observed for ind-mont24. Comparing the Raman spectra of DHI, ind-lap and heated ind-lap it is clear that DHI is formed in both heated and unheated samples, although the DHI features are more pronounced in the heated one. It is, 22 ACS Paragon Plus Environment

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therefore, indisputable that if DHI is present in the ind-paly system, its characteristic bands should be observed in the Raman spectrum obtained in resonance with its most intense absorption band in the 480 - 520 nm region (λ0 = 457.9 nm or 488 nm). It can be concluded, therefore, that DHI is not formed (or is formed in amounts which does not permit its detection by resonance Raman spectroscopy) in the ind-paly samples, despite the green hue presented by them, which does not necessarily imply in enhanced chemical stability.

(I)

(II)

Figure 10. (I) UV-VIS absorption spectra of (a) DHI, (b) indigo and ind-lap (c) unheated and (d) heated (130 °C for 2h) and (e) ind-mont heated at 130 °C for 2h. (II) resonance Raman spectra of the same samples (except ind-mont) excited at 457.9 nm.

In order to confirm that color and chemical stability are not necessarily related in MB simulants, two other clays were tested: sepiolite (sepi) and montmorillonite (mont). Under the experimental conditions here employed in the preparation of the simulants (130 °C and 200 °C for 2 h), the Raman spectrum (632.8 nm) of the ind-sepi sample was similar to ind-paly, however, the UV-VIS absorption spectrum was not, presenting a behavior similar to ind-lap: the 657 nm band shows an hypsochromic shift (642 nm) when compared to pure indigo 23 ACS Paragon Plus Environment


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(solid) instead of the red shift observed for ind-paly (667 nm, Figure 11). In all the cases (indigo with paly, lap, sepi and mont), after heating the solids presented a greenish-blue color but, curiously, in the case of ind-mont and ind-lap the Raman spectra excited at 632.8 nm do not match the ind-paly one, but the ind-sepi does. Unlike ind-lap and ind-mont, the MB simulant prepared with sepiolite presented an enhanced chemical stability (although not as extensive as with palygorskite) as reported already in the literature49,50 what means that, beyond any doubt, the green hue is not associated with stability. This fact raises the question of how the clay structure affects the dye UV-VIS absorption spectrum and chemical stability; considering the key role played by the water molecules coordinated to the metals inside the clay channels in the ind-paly interaction, their presence was first taken as an explanation for the different behavior presented by the clays.

(I)

(II)

Figure 11. (I) UV-VIS absorption spectra of (a) solid indigo, ind-sepi (b) unheated and (c) heated at 130 °C for 2 h and (d) ind+paly heated at 130 °C for 2 h. (II) resonance Raman spectra (457.9 nm) of (a) indigo, (b) ind-sepi heated at 130 °C for 2h and (c) ind-paly heated at 130 °C for 2 h.


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When the structures of paly, sepi, mont and lap are compared, the relevance of coordinated water molecules becomes evident: they are present only in palygorskite and sepiolite clays, although in small relative amount in the latter which has larger micropore width (10.6 Å and 6.4 Å for sepi and paly, respectively51). The experimental results here reported indicate that it is possible to discriminate interaction sites for indigo on the clay surface using resonance Raman spectroscopy and UV-VIS absorption spectroscopy. The technique should, therefore, be able to provide insights on the reason why the UV-VIS absorption spectra of ind-paly and ind-sepi are not similar. It seems that there are two main different interaction sites in these clays: the hydrophilic environment inside the channels, where the indigo molecules are hydrogen bonded to coordinated water molecules, and the external clay surfaces which are mostly represented by siloxane (Si-O-Si). In sepiolite, the larger channel size has two implications: a smaller amount of coordinated water molecules and a higher surface area (300 m2/g).51 It seems, therefore, that the intense ππ* electronic transition at ca. 660 nm (indigo) is responding to the siloxane groups in a similar way that it responds to solvents dielectric constant,52 whereas the weaker band at ca. 500 nm appears as a consequence of specific interactions, which are responsible for a decrease in the indigo molecular symmetry. This conclusion explains why the ind-sepi spectrum excited at 632.8 nm is not significantly sensitive to the heating temperature, while a completely different behavior is observed with 457.9 nm excitation. Using the same rational in the case of montmorillonite, which is a cationic layered clay, the interlayer space cannot be occupied by indigo molecules (because of its electrical neutrality) and considering that it contains no channels, both Raman and UV-VIS spectra of ind-mont are dominated by the externally adsorbed indigo molecules and, accordingly, do not match the spectra from the ind-paly samples, but the UV-VIS spectrum matches the ind-sepi one.



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Regarding Laponite, its surface area is the largest among the clays here investigated and it is composed of disk-shaped particles of nanometric dimensions (ca. 25 nm in diameter and 1 nm in height), therefore, the relative concentration of silanol groups (-Si-OH) present is the disks edges is high and in this case specific hydrogen bonds are expected to be formed with the indigo molecules, what is supported by theoretical calculations.53 On the other hand, contrarily to ind-paly, in the heated ind-mont and ind-lap samples, Raman spectroscopy using 457.9 nm excitation detected the presence of DHI, whose formation cannot be credited to the oxidizing metal ions (Fe3+ and Mn4+, for example) present in the naturally occurring montmorillonite because Laponite, although showing the same behavior, is a synthetic clay. It can thus be concluded that the interaction with the siloxane groups causes hypsochromic shift in the indigo 657 nm band, although such surfaces have adsorbed water molecules which can interact with indigo through hydrogen bonding, which supposedly are weaker than the interaction with the coordinated water molecules inside the micropores. Finally, perhaps the most important issue yet to be addressed here is the reason for the chemical and photochemical stability of indigo in MB simulants. Different works assigned such stability to the formation of hydrogen bonding between the C=O moieties in the indigo molecule with the coordinated water inside the channels12,54, while others suggested the direct interaction with the cation (Al3+ or Mg2+) at the edges of the clay tunnels as the main cause for the stability of the pigment32,55,56. Fois et al. credited the high stability of Maya blue to the tight fit of indigo inside the channel.57 In a previous publication, time-resolved spectroscopies were used in an attempt to understand the MB simulants (ind-paly) photostability58. Transient absorption and time resolved infrared spectroscopies evidenced a dramatic decrease in S1 excited state lifetime when the dye interacted with the clay mineral (3 ps), in comparison with a DMSO solution (170 ps)58. Such decrease indicates that the indigo molecules have a more efficient energy relaxation path when interacting with the inorganic


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matrix, therefore, the organic molecule does not merely diffuse inside the palygorskite microchannels. Intermolecular hydrogen bond is recognized as an important route for radiationless decay process59,60, what has been ascribed to the significant anharmonicity of the H-stretching vibrations61. However, only hydrogen bonding does not explain the stabilization because it is also observed in other systems such as ind-lap, for example,62 where the dye and clay interaction involves hydrogen bond but, despite of it, the indigo molecule does not gain chemical or photochemical stability when interacting with Laponite, as it was here shown. Therefore, the answer for such stability must be somewhere else. It is known that the first electronic excited state of indigoid molecules exhibits a geometry distortion and the aromatic rings become nearly perpendicular63. Considering (i) the size of the clay microchannels, (ii) the observed reduction in S1 excited state lifetime and (iii) the formation of hydrogen bond between the water molecules coordinated to the metal ions of the clay framework and the indigo molecules, it is here proposed that inside the palygorskite microchannels the indigo molecule experiences steric constrain which restricts the molecular geometry rearrangement expected for the excited state, and a fast energy relaxation, probably assisted by hydrogen bond, becomes the major deactivation route.

CONCLUSIONS In the above paragraphs, a consistent discussion of the resonance Raman spectra of MB simulants were given and shed light into some controversial issues about this important historical pigment. As already reported in the literature, the spectral changes observed in the MB simulants when compared to the pure dye are compatible with interaction through hydrogen bond but the use of different excitation laser lines permitted the observation of two different spectral patterns which are dependent on the temperature used to process the indigo



Page 28 of 38

and clay mixture. It is clear from the Raman and UV-VIS absorption spectroscopies that the spectral profile changes in the 200 – 220 °C range and such change is clearly associated with the coordinated water molecules release, as shown by DTG, providing a direct evidence of the participation of the coordinated species inside the clay microchannels in the MB formation. Above 200 °C the Raman spectra are compatible with a complex formation between indigo and metal, however, considering that stability was already achieved at lower temperatures, such complexation does not contribute to the indigo stability in the MB simulants. Commonly, the greenish tint produced by heating has been linked in the literature with the enhanced chemical stability presented by MB and MB simulants32, however, it was here shown that despite ind-lap and ind-mont are greenish-blue solids, they do not resist to solvent extraction nor to nitric acid attack. This proves that color and chemical stability are not necessarily related. From the UV-VIS spectroscopy data recorded as a function of temperature it seems that the green shade results from an increase in the intensity of a band at 500 nm, together with the bathochromic shift and narrowing of the 657 nm band (indigo). Resonance Raman spectra obtained with the 457.9 nm laser line indicate that a decrease in the indigo molecular symmetry is responsible for the showing up of the 500 nm band in ind-paly and not to the formation of DHI. On the other hand, in the case of ind-lap and ind-mont the DHI features are clearly observed but, as mentioned above, this finding is not paralleled by the increased chemical stability observed in ind-paly. It can be concluded, therefore, that DHI is not related to MB color or to its chemical stability. It has to be remembered, however, that clay minerals are commonly a mixture of different mineral phases and the results here presented indicate that some of them


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(montmorillonite, commonly found in admixture with palygorskite, is an example) can produce DHI although without impact in the MB pigment chemical stability.

AUTHOR INFORMATION Corresponding authors Dalva L. A. de Faria ([email protected]) Molecular Spectroscopy Laboratory Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo Av. Prof. Lineu Prestes, 748 – Butantã São Paulo, SP, CEP 05508-000, Brazil

ACKNOWLEDGEMENT The authors wish to express their gratitude to Dr. Erick Bastos for providing the Cary WinUV Color Software and Isabella L. Freire for some spectral measurements. Research grants from Fapesp (2012/05643-4 and 2012/13119-3 (DLAF and NDB) and 11/50318-1 (VRLC)) and CNPq (309288/2009-6 (DLAF) and 312384/2013-0 (VRLC)) are greatly acknowledged by the authors.

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