Interactions and Supramolecular Organization of Sulfonated Indigo

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Interactions and Supramolecular Organization of Sulfonated Indigo and Thioindigo Dyes in Layered Hydroxide Hosts Ana Luisa Costa, Ana C. Gomes, Ricardo Costa Pereira, Martyn Pillinger, Isabel Sousa Gonçalves, Marta Pineiro, and J. Sérgio Seixas de Melo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03735 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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Interactions and Supramolecular Organization of Sulfonated Indigo and Thioindigo Dyes in Layered Hydroxide Hosts Ana L. Costa,†,‡ Ana C. Gomes,‡ Ricardo C. Pereira,† Martyn Pillinger,*,‡ Isabel S. Gonçalves,‡ Marta Pineiro,† and J. Sérgio Seixas de Melo*,† †

Coimbra Chemistry Centre, Department of Chemistry, University of Coimbra, Rua Larga, 3004-535 Coimbra, Portugal

‡

Department of Chemistry, CICECO - Aveiro Institute of Materials, University of Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

Keywords: Cointercalation; indigo; dyes/pigments; host-guest systems; supramolecular organization; organic-inorganic hybrid composites

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ABSTRACT: Supramolecularly organized host-guest systems have been synthesized by intercalating water-soluble forms of indigo (indigo carmine, IC) and thioindigo (thioindigo5,5’-disulfonate, TIS) in zinc-aluminum layered double hydroxides (LDHs) and zinc layered hydroxide salts (LHSs) by coprecipitation routes. The colors of the isolated powders were dark blue for hybrids containing only IC, purplish blue or dark lilac for cointercalated samples containing both dyes, and ruby/wine for hybrids containing only TIS. As-synthesized and thermally treated materials were characterized by FT-IR, FT-Raman and NMR spectroscopies, powder X-ray diffraction, SEM, elemental and thermogravimetric analyses. The basal spacings found for IC-LDH, TIS-LDH, IC-LHS and TIS-LHS materials were 21.9, 21.05, 18.95 and 21.00 Å, respectively, with intermediate spacings being observed for the cointercalated samples that either decreased (LDHs) or increased (LHSs) with increasing TIS content. UV-visible and fluorescence spectroscopy (steady-state and time-resolved) were used to probe the molecular distribution of the immobilized dyes. The presence of aggregates together with monomer units is suggested for IC-LDH, while for TIS-LDH, IC-LHS and TIS-LHS the dyes are closer to the isolated situation. Accordingly, while emission from the powder H2TIS is strongly quenched, an increment in emission of about one order of magnitude was observed for the TIS-LDH/LHS hybrids. Double exponential fluorescence decays were obtained and associated with two monomer species interacting differently with cointercalated water molecules. The incorporation of both TIS and IC in the LDH and LHS hosts leads to an almost complete quenching of the fluorescence, pointing to a very efficient energy transfer process from (fluorescent) TIS to (non-fluorescent) IC.


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INTRODUCTION Indigo is the mythic blue (molecule) and one of the oldest dyes used for dyeing textiles (Scheme 1).1-2 Until recently, the earliest known use of indigo as a blue dye was 4400 years ago in Egypt.3 Recent archaeological research on cotton fabrics from Peru has pushed back this date to 7800 years ago.4 These findings are all more remarkable given the fact that indigo is not easy to obtain, with several steps being required to extract the dye from plants. Synthetic indigo came to prominence at the beginning of the 20th century and has remained an important industrial product due mainly to its use for dyeing denim. Other potential applications of indigo and its derivatives are starting to emerge. For example, the recent finding that indigo displays good performance as an ambipolar semiconductor5 has inspired further studies of functional indigos as building blocks for organic electronics applications.6-7 While indigo is poorly luminescent, derivatives such as thioindigo (scheme 1) show strong fluorescence.8 Organic dyes based on thioindigo have recently been used as sensitizers in dye-sensitized solar cells.9

Scheme 1. Chemical structures of indigo, thioindigo, and of the sulfonated derivatives Indigo Carmine (IC) and Thioindigo-5,5’-disulfonate (TIS). The importance of indigo in ancient times was not limited to its use as a dyestuff. Sometime between 500 and 800 AD, the Maya civilization (Central America) developed the light blue or turquoise pigment known as Maya Blue (MB) by combining indigo with the clay mineral palygorskite.10-13 The extraordinary durability of MB14 is due in part to the high 3 ACS Paragon Plus Environment

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intrinsic photostability of indigo which has been attributed to rapid and efficient internal conversion involving ultrafast intramolecular proton transfer.7 In MB, the chemical and photochemical stability of indigo is enhanced even further by encapsulation of the dye molecules in the channels of the host, leading to a hybrid organic-inorganic nanocomposite.15 Investigations into the structure and properties of MB have inspired research on the synthesis of MB simulants and related organic-inorganic hybrid pigments. Apart from indigo-palygorskite materials,16-17 other indigo-based hybrids have been prepared, such as indigo-sepiolite,15, 18-22 indigo-silicalite20, 23 and indigo-kaolinite,20 as well as hybrids containing indigo derivatives such as thioindigo and indigo carmine (indigo-5,5’disulfonate, hereafter denoted as IC).8, 24-26 The anionic dye IC has been intercalated into layered double hydroxides (LDHs)24-26 and a layered hydroxide salt (LHS).27 In the former case the cointercalation of a surfactant suppressed the formation of dye aggregates, with high surfactant content favoring the isolation of dye molecules.24

Figure 1. Schematic representation of the structures of anion-exchangeable (an-) (a) layered hydroxide salts (LHSs) and (b) layered double hydroxides (LDHs).


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LDHs and LHSs are closely related anion-exchangeable materials with structures that can be derived from the brucite-type structure, M(OH)2, which consists of {M(OH)6} octahedra sharing edges to form infinite charge-neutral sheets (Figure 1). In LDHs, some of the divalent cations are replaced isomorphously by trivalent cations, leading to a positively charged layer that requires the presence of interlayer anions. The general formula of LDHs can be written as M2+1-xM3+x(OH)2(An-)x/n‧mH2O. In Zn2+ LHSs of the type Zn5(OH)8(An)2/n‧mH2O, one quarter of the octahedral sites in the brucite-type structure is vacant and these empty octahedra are capped on either side by {Zn(OH)3X} tetrahedra. The apex (X) of the tetrahedra can be occupied either by water molecules, as is the case with zinc hydroxide nitrate where unbound charge-balancing nitrate ions occupy the interlayer, or by anions such as Cl- as found for the mineral simonkolleite. The concept of cointercalation is fairly unexplored as far as hybrid pigments containing layered inorganic hosts are concerned. Most of the research performed to date has centered around dye-surfactant cointercalations similar to the one mentioned above for IC and LDHs.28-32 Only a few papers have reported on the cointercalation of two different dye molecules.33-36 Dianqing and co-workers described the synthesis of multicolour hybrid pigments by the cointercalation of either C.I. Acid Yellow and C.I. Acid blue or Acid Green 25 and Acid Yellow 25 in a Zn-Al LDH.34-35 Duan and co-workers successfully cointercalated 4,4diaminostilbene-2,2-disulfonate and 4,4-dinitro-stilbene-2,2-disulfonate anions into Zn-Al LDHs to produce composite materials with electron donor-acceptor character for photoelectrochemical water splitting.36 Motivated by recent research on MB-related materials and the potential of organic dye cointercalation for the synthesis of hybrids with tunable colors and photophysical properties, the work described herein is a comparative study of LDH and LHS materials containing either solely IC or thioindigo-5,5'-disulfonate anions (TIS), or a mixture of the two dyes in different molar ratios (Scheme 1). To the best of our knowledge this is the first report that describes hybrid materials containing the TIS anion. As-synthesized and thermally treated materials have been comprehensively characterized with a view to confirming genuine cointercalation and getting insights into the nature of the host-guest and guest-guest interactions.


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EXPERIMENTAL SECTION Methods. Details of instruments and procedures used for elemental analyses (C, H, N and S), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), FT-IR and Raman spectroscopy, solid-state 13C{1H} crosspolarization (CP) magic-angle spinning (MAS) NMR, and solid-state UV-Vis-NIR absorption and fluorescence spectroscopy have been described previously.24, 37 The uncertainty of the basal spacings, as calculated from the 00l diffraction lines, was estimated to be ± 0.1 Å. Before UV-Vis-NIR spectra of solid samples were acquired, a baseline, with NO3-LDH for LDH samples and NO3-LHS for LHS samples, was obtained. Fluorescence decays were measured by using a home-built time-correlated single photon counting (TCSPC) apparatus with an IBH PicoLED (with excitation at 451 nm) as excitation source. Triangular quartz cuvettes were used and the emission, collected at the surface front at right angle (90°) geometry and at magic angle polarization, was detected through a double subtractive Oriel Cornerstone 260 monochromator by a Hamamatsu microchannel plate photomultiplier (R3809U-50). The signal acquisition and data processing were performed with a Becker & Hickl SPC-630 TCSPC module. The fluorescence decays and the instrumental response function (IRF) were collected using 1024 channels in a 326 ps/channel scale, until 2000-5000 counts at maximum were reached. The full width at half-maximum (fwhm) of the IRF was 0.95-1.10 ns. Deconvolution of the fluorescence decay curves was performed using the modulating function method as implemented by Striker et al. in the SAND program38 which allows a value of ca. 10% of the fwhm (9 ps) as the time resolution of the equipment with this excitation source. Simulated PXRD patterns, structural models and representations were generated using CrystalMaker and CrystalDiffract software.39-40 Reagents and materials. The chemicals indigo carmine (abbreviated Na2IC; SigmaAldrich), thioindigo (TCI chemicals), chlorosulfonic acid (99%, Sigma-Aldrich), sodium bicarbonate (99.5%, JMGS), dichloromethane (99.9%, Sigma-Aldrich), Zn(NO3)2·6H2O (98%, Fluka), Al(NO3)3·9H2O (98.5%, Riedel de-Haën), 1 M NaOH (Fluka) and acetone (99.5%, Sigma-Aldrich) were obtained from commercial sources and used as received. A nitrateform Zn-Al LDH with the composition Zn4Al2(OH)12(NO3)2·2.5H2O (denoted NO3-LDH) was prepared as described previously.24 Details concerning the synthesis and characterization of 6 ACS Paragon Plus Environment

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the compound (E)-3,3'-dioxo-3H,3'H-[2,2'-bibenzo[b]thiophenylidene]-5,5'-disulfonic acid (H2TIS) and a nitrate-form LHS (NO3-LHS) are given in the Supporting Information. All LDH and LHS preparations were performed under nitrogen using deionized and decarbonated (DD) water. General synthesis of intercalated LDHs

(IC(n%)/TIS-LDH)). A solution of

Zn(NO3)2·6H2O (x mmol) and Al(NO3)3·9H2O (x/2 mmol) in DD water (30 mL) was added dropwise to a solution containing Na2IC (a mmol) and/or Na2TIS (b mmol) in DD water (z mL) with vigorous stirring (a + b = x/2; n% = 100 × a/(a + b); n = 100, 98, 90, 50, 0). The value of x was 3.33 for n = 100, and 2.50 for the remaining materials; the value of z was 80 for n = 100, and 30 for the remaining materials. Solutions containing the disodium salt Na2TIS were prepared using H2TIS and 2 equiv. of NaOH. During addition of the Zn2+/Al3+ solution the pH of the reaction mixture was maintained at 7.5-8.0 through dropwise addition of 0.25 M (n% = 0-98) or 0.5 M (n% = 100) NaOH. Once addition of the Zn2+/Al3+ solution was complete, the resultant gel-like slurry was stirred for 18 h at 65 °C. The solid product was isolated by filtration, washed several times with DD water and acetone, and finally dried at room temperature under reduced pressure in a vacuum desiccator. For simplicity, the materials containing solely IC or TIS are denoted as IC-LDH and TIS-LDH. General synthesis of intercalated LHSs (IC(n%)/TIS-LHS)). The procedure used is similar to that reported by Maruyama et al. for the preparation of an LHS material intercalated by IC.27 A solution of ZnCl2 (0.57 g, 4.18 mmol) in DD water (50 mL) was added dropwise to a solution containing Na2IC (a mmol) and/or Na2TIS (b mmol) in DD water (50 mL) with vigorous stirring (a + b = 1; n% = 100 × a/(a + b); n = 100, 98, 90, 80, 50, 0). Solutions containing the disodium salt Na2TIS were prepared using H2TIS and 2 equiv. of NaOH. During addition of the ZnCl2 solution the pH of the reaction mixture was maintained at 7.5-8.0 through dropwise addition of 0.17 M NaOH. Once addition of the ZnCl2 solution was complete, the resultant gel-like slurry was immediately filtered and the solid product washed several times (5 × 50 mL) with DD water, and finally dried at room temperature under reduced pressure. For simplicity, the materials containing solely IC or TIS are denoted as IC-LHS and TIS-LHS. 13

C{1H} CP MAS NMR chemical shift values (for IC-LDH, TIS-LDH, IC-LHS and TIS-LHS),

elemental analysis (CHN) data and a complete listing of the FT-IR and Raman spectroscopic bands for each material are given in the Supporting Information. 7 ACS Paragon Plus Environment

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Heat treatment of LHS samples. The samples IC-LHS and TIS-LHS (ca. 200 mg of each sample) were placed in a Petri dish and heated at 200 ºC for 30 min in an oven under static air atmosphere. The resultant heat-treated samples (IC-LHSht and TIS-LHSht) were stored under nitrogen while awaiting their respective characterization.

RESULTS AND DISCUSSION Synthesis and characterization. A constant pH (co)precipitation method was used to prepare Zn2Al-LDHs and Zn-LHSs containing solely IC (IC-LDH, IC-LHS) or TIS (TIS-LDH, TISLHS) anions, or a mixture of the two indigo derivatives (IC(n%)/TIS-LDH or IC(n%)/TIS-LHS, where n% is the initial molar percentage of IC anions in the reaction mixture). The procedures used are slightly modified versions of those reported recently for the synthesis of IC-LDH24 and IC-LHS27 materials. During (co)precipitation the pH of the reaction mixtures was maintained between 7.5 and 8.0. For the LDH materials, the coprecipitation step was followed by ageing of the gel-like slurry at 65 °C for 18 h. In the case of the LHS materials, the solid products were immediately filtered following complete addition of the NaOH solution since it was found that ageing of the gel, even at room temperature, led to the formation of an unidentified impurity phase. The synthesis of pure LHS phases was also promoted by fixing the net OH/Zn molar ratio at 0.5, in agreement with observations reported by Newman and Jones (higher ratios favor the formation of ZnO and/or Zn(OH)2 impurity phases).41 The colors of the isolated solids were very dark blue for the LDH intercalates with n = 50-100, dark blue for IC-LHS (n = 100), and ruby/wine for TIS-LDH and TIS-LHS (n = 0). The colors of the samples IC(80%)/TIS-LHS (purplish blue) and IC(50%)/TISLHS (dark lilac) were consistent with the expected IC and TIS contents. The PXRD patterns of the LDH and LHS materials are shown in Figure 2. The patterns for the LDH intercalates are typical of hydrotalcite-type materials with an expanded interlayer spacing, displaying up to seven equally spaced 00l basal reflections between 3.5 and 30° 2θ. The basal spacings for the end-members of the series are 21.90 Å for IC-LDH and 21.05 Å for TIS-LDH. The former distance is slightly lower than the 22.2-22.4 Å distances previously reported for IC-intercalated (Zn/Mg)-Al LDHs with a metal cation molar ratio of 3:1 prepared by the coprecipitation method.25-26 We previously reported a basal spacing of 17.6 Å for an IC-LDH prepared using the same procedure adopted in the present work.24 The 8 ACS Paragon Plus Environment

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difference between this distance and the current value of 21.9 Å may be justified by the fact that the former material had a final Zn/Al molar ratio slightly lower than 2. For samples containing both IC and TIS, the basal spacings lie between those for IC-LDH and TIS-LDH, and decrease with increasing TIS content: 21.80 Å for IC(98%)/TIS-LDH, 21.55 Å for IC(90%)/TISLDH, and 21.20 Å for IC(50%)/TIS-LDH. This trend is consistent with genuine cointercalation of the two indigo derivatives rather than the presence of a mixture of IC-LDH and TIS-LDH phases.

Figure 2. PXRD patterns of (a) IC-LDH, (b) IC(98%)/TIS-LDH, (c) IC(90%)/TIS-LDH, (d) IC(50%)/TIS-LDH, (e) TIS-LDH, (f) IC-LHS, (g) IC(98%)/TIS-LHS, (h) IC(90%)/TIS-LHS, (i) IC(80%)/TIS-LHS, (j) IC(50%)/TIS-LHS, and (k) TIS-LHS. Basal (00l) reflections are indicated for IC-LDH and IC-LHS. The asterisk in pattern (j) indicates a shoulder that is attributed to the 002 reflection for a TIS-LHS phase.

The crystal structure of a polymorphic form of Na2IC was recently described.42 In the structure, layers composed of sodium ions and IC molecules are alternately stacked. Figure 3 depicts one IC molecule in the structure viewed along the 010 direction. The vertical distance L shown in Figure 3 is a good estimate of the longest dimension of the molecule (that includes van der Waals radii for the S-bound oxygen(s)). Based on the unit cell


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parameters [a = 17.7145(6) Å, β = 97.287(2)°], the value of L is 17.6 Å, which is in good agreement with previously reported DFT calculations that yielded a value of 17.5 Å for the longest dimension of the molecule.27 The distance of 17.6 Å can also be considered as a good estimate for the longest dimension of the TIS anion. Assuming that the thickness of a brucite-type layer as 4.8 Å, the gallery heights for the LDH materials fall in the range of 16.25-17.10 Å, which suggests that the indigo derivatives adopt a near-vertical orientation with an angle of 65-80° with respect to the host layers.

Figure 3. Crystal structure of Na2IC42: (a) View of one IC molecule along the b axis. (b) Twodimensional packing of the IC molecules in the bc-plane. The circle illustrates the calculated estimate of 5.34 Å (= circle radius) for the average distance between neighboring IC anions (see discussion for details). In proposing interlayer arrangements in LDHs and related materials, the area available to each anion must also be considered. For LDHs with layers of the type [M2+1-xM3+x(OH)2]x+, the area per unit charge is (1/x)a02sin60, where a0 is the unit cell parameter. Zn2Al-LDHs have unit layer charge areas of about 24.5 Å2 (a0 ≈ 3.07 Å, x = 1/3). In the structure of Na2IC, the IC anions are quite efficiently close-packed in the bc plane (Figure 3b), and the average cross-sectional area occupied by each IC anion is 24.7 Å, which corresponds to an area per unit negative charge of 12.35 Å2, well below the unit layer charge area for a Zn2Al LDH. If the cross-sectional area of 24.7 Å is, for simplicity, associated with a hexagonal array, the 10 ACS Paragon Plus Environment

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average distance between neighboring IC anions in Na2IC can be estimated as 5.34 Å (depicted in Figure 3 as a circle with radius 5.34 Å). For an LDH with the ideal composition [Zn4Al2(OH)12][TIS], each interlayer anion will, on average, occupy an area of 49 Å2, from which we can derive a mean anion…anion separation of 7.5 Å, significantly larger than that calculated for Na2IC. Figure 4 illustrates a model for intercalated TIS anions that reproduces the observed basal spacing of 21.05 Å and the estimate for the average distance between neighboring TIS anions. Similar arrangements can be envisaged for the other LDH intercalates.

Figure 4. Model of a possible guest packing mode in TIS-LDH featuring an offset face-to-face alignment of TIS molecules. A side-on perspective view of (a) is shown in (b). The average distance between neighboring TIS molecules (ca. 7.5 Å) was calculated from the theoretical packing density of 1 TIS molecule per 49 Å2 for an LDH with the ideal composition Zn4Al2(OH)12(TIS), a0 ≈ 3.07 Å, and a hexagonal array of TIS molecules. Interlayer water molecules have been omitted for clarity. Similar arrangements can be envisaged for the other hybrids denoted IC(n%)/TIS-LDH.

The crystal packing of indigo derivatives tends to be mediated by strong intermolecular hydrogen bonding and/or π-π stacking interactions. For example, indigo,43-44 monothioindigo45 and thioindigo46 all display a molecular packing in which the aromatic rings of adjacent molecules are in a displaced face-to-face alignment, with a ring separation of about 3.4 Å being evidence of a strong π-π interaction. For indigo and monothioindigo, 11 ACS Paragon Plus Environment

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molecules are also linked by strong intermolecular hydrogen bonds involving the NH and carbonyl groups. In the structure of thioindigo, the molecules exhibit only van der Waals interactions and molecular stacking. While N-H···O interactions are present in the crystal structure of Na2IC, the π-stacked columnar structure is absent (Figure 3b), which may be largely due to unfavorable electrostatic repulsion between the sterically demanding sulfonate groups.42 Based on the structure of Na2IC and the rather large estimate of 7.5 Å for the mean anion…anion separation in the TIS/IC-LDH structural model (Figure 4), π-π interactions between neighboring guest molecules are unlikely to be significant. This factor, in combination with the host-guest interaction, probably disfavors guest aggregation, especially in the case of TIS anions that do not possess NH groups capable of forming intermolecular hydrogen bonds. The situation may be different with IC anions that may have a strong tendency to form N-H···O interactions involving amine and carbonyl groups of adjacent molecules. For the LHS materials, the diffractograms are not dissimilar to those for the LDH intercalates, displaying four basal reflections up to about 20° 2θ and some non-basal peaks towards higher angles. The pattern and basal spacing of 18.95 Å for IC-LHS are in agreement with those reported by Wypych and co-workers.27 Curiously, the basal spacing tends to follow the opposite trend to that observed for the LDH materials, i.e. it increases with increasing TIS content: 19.10 Å for IC(98%)/TIS-LHS and IC(90%)/TIS-LHS, 19.25 Å for IC(80%)/TIS-LHS, 19.80 Å for IC(50%)/TIS-LHS, and 21.00 Å for TIS-LHS. For the 002 reflection in the pattern of IC(50%)/TIS-LHS, a distinct shoulder on the low-angle side may be due to a pure TIS-LHS phase, suggesting that genuine cointercalation of the two indigo derivatives to give a single intercalated phase is only possible for an IC content higher than 50%. Wypych and co-workers performed DFT simulations to model the anion arrangement in their LHS material intercalated by IC molecules.27 The calculations were performed by considering that the structure of the positively charged zinc hydroxide layer was similar to that for the known compound Zn5(OH)8(NO3)2·2H2O (Figure 1a with An‒ = NO3‒),47 and that IC anions would interact with the LHS layers through electrostatic attractions and hydrogen bonds to zinc-bonded water molecules. The predicted basal spacing of 19.30 Å was similar to the experimental value of 19.07 Å. The structural model presented by Wypych and coworkers was used in the present work as a starting point to model the anion arrangement in TIS-LHS. Figure 5 shows an arrangement of TIS molecules that gives rise to a basal spacing 12 ACS Paragon Plus Environment

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equal to the observed value of 21.0 Å. There is reasonable agreement between the experimental PXRD pattern for TIS-LHS and a simulated pattern for the model, especially concerning the basal reflections (Figure S4 in the Supporting Information). The TIS molecules in the model are inclined at an angle of about 50° with respect to the host layers, which is slightly higher than the value of 45° for the reported IC-LHS model.27

Figure 5. Possible arrangement of intercalated TIS anions in TIS-LHS and TIS-LHSht. The model

for

TIS-LHS

corresponds

to

the

ideal

stoichiometric

composition

of

Zn5(OH)8(TIS)(H2O)2 (see discussion). The structural model for TIS-LHS (and IC-LHS reported by Wypych and co-workers27) corresponds to the ideal stoichiometric composition of Zn5(OH)8(TIS)(H2O)2. Visualization of this model in space-filling mode (van der Waals radii) shows that the guest molecules are quite tightly close-packed. LHSs have higher layer charge densities than LDHs. In the structural model used for TIS-LHS the unit layer charge area is about 17.8 Å2, which is slightly higher than the minimum area per unit negative charge for IC anions, estimated above as 12.35 Å2 from the cross-sectional area. From the mean area occupied by each TIS anion in the structural model for TIS-LHS (35.6 Å2), the average distance between neighboring TIS molecules can be estimated as 6.4 Å (Figure 5). As will be discussed below, the actual IC and TIS contents in the LHS materials were significantly lower than those for the idealized stoichiometric composition, pointing to a lower packing density and a higher lateral separation of neighboring molecules. 13 ACS Paragon Plus Environment

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The intercalation of the indigo derivatives in IC-LDH, IC-LHS, TIS-LDH and TIS-LHS was further confirmed by FT-IR, Raman and

13

C{1H} CP MAS NMR spectroscopies (see the

Supporting Information for a detailed discussion of these data). Regarding the cointercalated samples, clear FT-IR evidence for the presence of both IC and TIS anions was only obtained for the samples with n = 50%. For the samples obtained using higher values of n (lower TIS contents), the infrared spectra in the 300-1800 cm-1 range were dominated by the bands due to IC, and no bands due to TIS could be discerned. Chemical compositions for the intercalated samples were estimated by elemental (CHNS) and thermogravimetric analyses (Table S1, Supporting Information). For the LDH samples, the experimental data consistently indicated that (i) the combined IC and TIS contents were slightly lower than the ideal stoichiometric value for an LDH with Zn/Al = 2, and (ii) the final bulk IC:TIS molar ratios for the cointercalated samples were, to within experimental error, consistent with the expected values based on the initial molar ratios used in the syntheses. The combined IC and TIS contents indicated that the anionic dyes balanced 91-95% of the positive charge of the hydroxide layers. The remaining 5-9% may be balanced by (co)intercalated carbonate, nitrate and/or hydroxide anions. As found previously for IC-LDH,24 FT-IR and

13

C{1H} CP MAS NMR data indicated that the samples

contained minimal or insignificant amounts of carbonate or nitrate. Hence, the major fraction of the residual positive charge may be counterbalanced by hydroxide anions, as suggested previously for composites of perylene bisimide dyes and LDHs.48 The general composition for the LDH composites is therefore written as Zn4Al2(OH)12(ICn/100TIS(100n)/100)z(OH)2(1-z)(H2O)m,

where z = 0.91-0.95 and m = 4.5-5.5.

While elemental analyses for the cointercalated LHS samples indicated that the final bulk IC:TIS molar ratios were close to the starting values, the combined IC and TIS contents for these samples and also IC-LHS and TIS-LHS were significantly lower than those expected for the stoichiometric composition of Zn5(OH)8(ICn/100TIS(100-n)/100)(H2O)m. The presence of significant amounts of interfering carbonate and/or chloride ions in the composites was not supported by FT-IR spectroscopy and energy dispersive X-ray spectrometry analyses for chlorine, respectively. In analogy with what was suggested above for the LDH samples, one possibility is that the LHS samples contain cointercalated hydroxide ions, which could be coordinated (instead of water molecules) to the tetrahedrally coordinated zinc atoms. Another possibility is that 14 ACS Paragon Plus Environment

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the layer charge densities of the LHS composites are lower than the ideal value due to the fraction of tetrahedrally coordinated Zn2+ ions being lower than the expected value. In other words, upon completion of the coprecipitation and self-assembly process, not all empty octahedral sites in the resultant brucite-type layers are capped above and below by tetrahedrally coordinated zinc atoms. For such materials, the layer composition may be more generally expressed as [Znocta3Zntetra(2-y)(OH)8(H2O)(2-y)](2-2y)+ where octa/tetra indicate octahedrally/tetrahedrally coordinated zinc atoms, and y gives the number of missing Zntetra (1 < y < 2). Lang and co-workers uncovered a similar phenomenon for layered zinc hydroxide salts intercalated with dodecyl sulfate anions and suggested that LHS materials may have a general propensity to exhibit this kind of variability of the zinc ion distribution within the hydroxide layers.49 When the lower than expected IC+TIS contents for the LHS composites are associated exclusively with missing Zntetra, the general composition is Zn5y(OH)8(ICn/100TIS(100-n)/100)1-y(H2O)m,

where y = 0.32-0.50 and m = 2.5-3.5 (Table S1). This

would mean a Zntetra removal of 16-25% (consistent with a reduction of the layer charge by 32-50%). Taking into account the compositional data and structural modifications proposed above, the estimates for the net areas occupied by each IC/TIS anion (49.0 Å2 for the Zn2AlLDHs and 35.6 Å2 for the Zn-LHSs) and the mean separation of neighboring anions (7.5 Å and 6.4 Å, respectively) are corrected to the following values: 51.6 Å2 and 7.7 Å for IC-LDH, 53.3 Å2 and 7.9 Å for TIS-LDH, 60.4 Å2 and 8.3 Å for IC-LHS, and 52.4 Å2 and 7.8 Å for TIS-LHS. The striking thing to note is that the lower IC/TIS contents for the LHS materials (associated with a decrease in the layer charge density) suggest areas and mean separations that are similar to those estimated for the LDH materials.


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Figure 6. Representative SEM images (15000× magnification) of (a) IC-LDH, (b) IC(90%)/TISLDH, (c) IC(50%)/TIS-LDH, (d) TIS-LDH, (e) IC-LHS, (f) IC(80%)/TIS-LHS, (g) IC(50%)/TIS-LHS, (h) TIS-LHS, (i) IC-LHSht, and (j) TIS-LHSht. The scale bar shown in (a) applies to all images. Images recorded at 500× magnification are provided in the Supporting Information (Figure S6). SEM images of the IC(n%)/TIS-LDH samples recorded at low magnification revealed randomly oriented particles with irregular shapes (Figure S6 in the Supporting Information). High magnification images of IC-LDH revealed a sheet-like morphology with regular stacking, in agreement with the lamellar character of the structure (Figure 6). Individual platelets have irregular forms and sizes, with no well-defined corners. This type of morphology is generally present for IC(90%)/TIS-LDH, IC(50%)/TIS-LDH and TIS-LDH (Figure 6 – b, c and d), although the latter clearly presents a slightly different morphology consisting of finer, smaller-sized crystallites. The sample IC(50%)/TIS-LDH seems to present a morphology that is intermediate between that for IC-LDH and TIS-LDH (Figure 6 – a and d), which is consistent 16 ACS Paragon Plus Environment

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with the formation of a single cointercalated phase in which IC and TIS guest ions are distributed evenly. High magnification SEM images of IC-LHS showed a flake-like structure with irregular plates of various sizes (Figure 6 - e). A similar morphology was found for IC(80%)/TIS-LHS (Figure 6 - f). Somewhat in parallel with that observed for the LDH samples, the sample TISLHS comprises finer, smaller-sized crystallites (Figure 6 - h), while IC(50%)/TIS-LHS seems to exhibit an intermediate morphology (Figure 6 - g). The thermal stability of TIS-LHS was investigated by TGA to determine whether thermally-induced guest-to-host covalent grafting would be possible as reported previously for a zinc hydroxide salt intercalated by IC.27 Heat treatment of TIS-LHS results in a gradual mass loss of 9.4% up to 150 °C, attributed to removal of physisorbed/intercalated water (Figure 7). This is followed by a more abrupt loss of 5.4% up to 190 °C (DTGmax = 177 °C), which may be due to partial decomposition of the LHS structure. Decomposition of intercalated TIS anions manifests itself as a single step between 430 and 570 °C (DTGmax = 540 °C), leaving a residual mass of 47.2%, which is good agreement with the expected value of 46.8% calculated on the basis of the proposed chemical composition (Table S1) and the assumption that the residue is ZnO. It is noteworthy that the thermal stability of intercalated TIS anions is enhanced in relation to that for H2TIS, which displays the onset of decomposition at about 325 °C (DTGmax = 370 °C). The TGA curve for IC-LHS was also measured and found to be similar to that for TIS-LHS (up to 350 °C) and to the curve reported by Wypych and co-workers for their IC-LHS material (Figure S5, Supporting Information).27 In contrast to that observed for the thioindigo derivative, intercalated IC anions display lower thermal stability (onset = 360 °C, DTGmax = 430 °C) than those in the sodium salt Na2IC (onset = 400 °C, DTGmax = 470 °C). The samples TIS-LHS and IC-LHS were thermally treated at 200 °C under air in a manner similar to that described previously for IC-LHS.27 The PXRD patterns of the resultant materials, denoted as TIS-LHSht and IC-LHSht, indicate the retention of a layered structure, albeit with reduced crystallinity, and the basal spacings are estimated as 21.5 Å for TIS-LHSht and 19.3 Å for IC-LHSht, which represent slight expansions (0.35-0.50 Å) with respect to the initial spacings (see Figure 5). Wypych and co-workers obtained similar results upon thermal treatment of IC-LHS and, with the help of DFT calculations and FT-IR spectroscopy, concluded that the intercalated IC anions became grafted to the host structure via covalent 17 ACS Paragon Plus Environment

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bonds between Zntetra atoms and sulfonate oxygen atoms. The structural model presented by Wypych and co-workers was used as a starting point to model the anion arrangement in TIS-LHSht. Figure 5 shows a possible arrangement of grafted TIS molecules that reproduces the observed basal spacing of 21.5 Å. In Figure S4 (Supporting Information) the experimental PXRD patterns for TIS-LHSht and IC-LHSht are compared with the simulated patterns calculated for the corresponding structural models. Differences observed between the FT-IR spectra of IC-LHS and IC-LHSht, most notably in the 1100-1250 cm-1 region, match those reported previously,27 and support thermally-induced guest-to-host grafting via the sulfonate oxygens (Figure S7, Supporting Information). On the other hand, no significant differences were observed between the FT-IR (or Raman, Figure S8) spectra for TIS-LHS and TIS-LHSht, and therefore some caution is required in drawing conclusions about any changes in the nature of the host-guest interaction. The covalent grafting of TIS molecules in TISLHSht is nevertheless supported by the similarity of the TGA curves (up to 350 °C) for TISLHSht and IC-LHSht (Figure 7, Figure S5 in the Supporting Information). Decomposition of the organic molecules in the heat-treated samples occurred over the same temperature ranges (identical DTGmax) as those for TIS-LHS and IC-LHS.

Figure 7. TGA curves of H2TIS (- · · - · ·), TIS-LHS (———), and TIS-LHSht (- - - -).


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SEM images for the thermally treated samples IC-LHSht and TIS-LHSht do not reveal substantially different morphologies from those observed for IC-LHS and TIS-LHS, respectively (Figure 6, Figure S6). Photophysical characterization in the solid state Absorption in UV-Vis region. The absorption spectra of Na2IC and H2TIS together with those of IC and TIS solely intercalated in the LDH and LHS hosts, including the thermally treated samples IC-LHSht and TIS-LHSht, are shown in Figure 8. The spectra of the reference compounds NO3-LDH and NO3-LHS are distinguished by a complete absence of bands in the visible region with, however, a clear band in the UV region, at 300 nm for NO3-LDH and 296 nm for NO3-LHS, attributed directly to the interlayer NO3- ions.50 Accordingly, this band is absent in the spectra of the intercalated samples, while a new band at 265 nm is present and is attributed to the SO3- group of the indigo derivatives.51 Towards higher wavelengths a very well-resolved band is observed at 340-350 nm for TIS-LDH, IC-LHS and TIS-LHS, and is associated with the IC/TIS aromatic units of the chromophore.52 This band only appears as a shoulder in the spectrum of IC-LDH.

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The absorption spectrum of Na2IC in solution (DMF) exhibits a fairly narrow band in the visible region with λmax = 615 nm.24 In comparison, IC-LDH presents spectral broadening such that its absorption spectrum is more similar to that of Na2IC in the solid-state (both have λmax = 538 nm), although the former is broader still. The spectral broadening indicates an increase in intermolecular interactions as a consequence of the IC molecules being arranged in ordered structures. As discussed above, there is no π-stacking for crystalline Na2IC due to the distance between IC units as well as their relative orientation; intermolecular interactions are restricted to N-H‧‧‧O hydrogen bonds. The packing arrangement of intercalated IC molecules in IC-LDH may be different due to guest-guest interactions being affected by H-bonding interactions involving water molecules, a lower packing density (as discussed above, ca. 1 IC unit per 51.6 Å2), and the host-guest electrostatic interaction. In fact, in the LDH galleries the orientation of IC molecules may favour the presence of aggregates together with monomer units. For IC-LHS, the absorption spectrum seems to be better resolved than Na2IC and even more than IC-LDH, with a wavelength maximum at 565 nm and a shoulder around 730 nm. As discussed above, we propose that the host structure in IC-LHS is modified such that the mean IC packing density (1 molecule per 60.4 Å2) and separation (ca. 8.3 Å) are higher than the values calculated for the ideal structural model (35.6 Å2, 6.4 Å). Comparison of the diffuse reflectance absorption spectrum of H2TIS (powder) with that of TIS-LDH (Figure 8-B) shows that the former displays a wavelength maximum at 507 nm whereas in the hybrid the value is red-shifted (538 nm). However, the main difference lies in the fact that the bandwidth in TIS-LDH decreases relative to that observed in the spectrum of the powder. The TIS-LHS absorption spectrum is quite similar to that of TIS-LDH. This indicates that in the powder (H-type) aggregates are present whereas in TIS-LDH (and TIS-LHS) the dye is closer to the isolated (in comparison with, for example, the solution behaviour) situation. For the cointercalated IC(n%)/TIS-LDH materials, some observations can be made about the absorption spectra (Figure 9-A): (i) The solid with the lower amount of TIS (IC(98%)/TIS-LDH) displays a broad band in line with that observed for IC-LDH; (ii) for IC(90%)/TIS-LDH, the spectrum, although close in shape to IC(98%)/TIS-LDH, shows a slight increase in the visible band, particularly evident between 540 and 750 nm. This may indicate


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that further increment of TIS in the mixture leads to the formation of a new dimer (involving the interaction between TIS and IC). A good quality spectrum could not be obtained for IC(50%)/TIS-LDH. From Figure 9-B it can be seen that all the IC(n%)/TIS-LHS materials show similar spectra with an increase in vibronic resolution compared with cointercalated LDH systems and a red-shift of the wavelength maxima (560 nm) when compared to Na2IC. The spectrum of IC(50%)/TIS-LHS (the sample with higher amount of TIS) shows a broad band at about 450 nm, more intense than that observed for the other samples, which is a direct consequence of the TIS dye content in the hybrid. Fluorescence emission. Although we were unable to obtain absolute fluorescence quantum yield values, the fluorescence spectra provide additional relevant information on the systems studied, particularly in the case of TIS-LDH and TIS-LHS. Figure 9 (C, D) shows the emission spectra of the studied samples with excitation at 530 nm (TIS emission wavelength maxima). Emission from the powder H2TIS is strongly quenched, a characteristic of aggregates (and in particular H-type) that are known to be poorly emissive. The most interesting feature, which is in line with the above proposal of more individualized TIS monomer molecules in TIS-LDH, is the fact that the emission is augmented by a factor of ~10 for the hybrid solid with intercalated TIS (although no absolute quantum yields could be obtained, an increment in emission of about one order of magnitude is observed on going from the pure solid (H2TIS – purple line) to the TIS-LDH hybrid (pink line in Fig. 9-C)). For all of the IC(n%)/TIS-LDH samples the presence of IC strongly quenches the emission of TIS, revealing an efficient energy transfer process similar to that reported by Zheng et al. with the cointercalation of 4,4-diaminostilbene-2,2-disulfonate and 4,4-dinitro-stilbene-2,2disulfonate in LDH hybrids.36


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Figure 9. (A, B) Diffuse reflectance spectra (Kubelka-Munk scale) and (C, D) fluorescence emission spectra of LDH and LHS samples in the solid state with λexc = 530 nm. Figure 9 (D) shows the emission spectra of all LHS samples with excitation at the same wavelength referred to above (530 nm). An increase of the fluorescence intensity of TIS when incorporated into the LHS vs. in the solid/powder (H2TIS) is observed. This constitutes, once more and as seen with TIS-LDH, an indication that the dye becomes more isolated when intercalated into the LHS, as also seems to be the case with LHS-intercalated IC. The simultaneous presence of IC and TIS in the cointercalated LHS samples leads to an efficient energy transfer (from TIS to IC) which culminates in a strong decrease (or even absence) of fluorescence, as was also observed for the IC(n%)/TIS-LDH samples. It should be noted that self-absorption (particularly in the case of the simultaneous presence of IC and TIS in the cointercalated LHS and LDH samples) is likely to be present.53 This typically depends on the degree of overlap between the absorption and emission spectra. In the case of TIS-LDH the overlap between the emission and absorption (as can be seen in Fig. S13) is low and the selfabsorption effect is therefore negligible.


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The charge density calculations described above are consistent with the spectral observations in supporting an increasing tendency (relative to the pure solids Na2IC and H2TIS) for the guest indigo derivatives to be isolated when intercalated in the LDH and LHS host materials since they suggest that the IC/TIS molecules can be spaced quite far apart (mean separations of 7.7-8.3 Å) and still balance the host layer charge. As mentioned above, isolation of TIS molecules is likely promoted by the impossibility of intermolecular C=O···H-N interactions as well as strong π-π interactions. Cointercalated water molecules probably also play a very important role in all the hybrid solids by forming a strong water-water hydrogenbonding network that may “solvate” individual dye molecules, with dye-water hydrogenbonding interactions (H‒N···HOH and/or C=O···HOH) being involved. To further assess the influence of water, the photophysical properties of the heat-treated samples IC-LHSht and TIS-LHSht were studied and are described in the next section. Dehydrated LHS samples. Figure 8 compares the absorption spectra of hydrated and dehydrated (heated to 200 ºC) samples. From this figure, two observations can be made: (i) in the case of TIS-LHSht (violet line in Figure 8-B), the absorption spectrum is unchanged relative to that for TIS-LHS; (ii) the absorption spectrum for IC-LHSht (cyan line in Figure 8-A) is different from that for the as-synthesised hydrated sample IC-LHS, with the appearance of a new band at longer wavelengths suggesting an increase in the level of aggregation. Hence, loss of interlayer water seems to increase the level of aggregation. In other words, for ICLHS (hydrated state), the molecules are more isolated, with C=O···H-N hydrogen bonding interactions being mainly intramolecular, supplemented as discussed above by waterbridged intermolecular such as C=O···H2O···H-N. Upon heat treatment to give IC-LHSht, the loss of water promotes further aggregation via intermolecular C=O···H-N interactions of neighbouring IC units. This cannot occur with TIS due to the lack of an NH group, and so the guest molecules remain “isolated” even after thermal treatment, with the molecular arrangement being mainly determined by the host-guest interaction. The bathochromic shift observed in the absorption spectra for IC-LHSht relative to IC-LHS was also seen by others with IC-LHS,27 similar to the shift presented in less polar solvents and for methyl orange (MO) in a LDH where the loss of water promoted a higher interpenetration of MO units in the interlayer cavity.54 The emission spectra of optically matched TIS-LHS samples were obtained with excitation at 610 nm (spectra not shown). A quenching of the fluorescence was observed for 23 ACS Paragon Plus Environment

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the dehydrated hybrid, clearly indicating that TIS is sensitive to the water content of the media/sample. In the case of the IC-LHS samples, no apparent change was observed in the fluorescence spectra (and intensity). Time-Resolved measurements. Figure 10 shows the fluorescence decay of TIS-LDH (hydrated) best fitted with a double exponential decay law. This bi-exponential decay is attributed to the simultaneous presence of structures that are likely close to what can be considered two types of monomer: one that interacts (or “sees”) water molecules and another that does not interact with water molecules. Indeed, the shorter decay time (81 ps) is close to the value obtained in water for TIS (130 ps). This represents the major component/species in the ground-state (excitation at 451 nm, therefore direct excitation of TIS) and with a pre-exponential factor of 0.74. The second (and dominant emissive species with ca. 60% of the emission at 700 nm) is associated with the emission of a monomer that does not probe water (the reason for the longer lifetime and contribution to the overall fluorescence emission) within the galleries of the TIS-LDH hybrid.


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Figure 10. Fluorescence decays collected at 700 nm for TIS-LDH (top panel) and TIS-LHS (bottom panel) in the solid state at room temperature (λexc = 451 nm). For a better judgment of the quality of the fits, autocorrelation (A.C.) functions, weighted residuals, and χ2 values are provided as insets. The fluorescence decay of TIS-LHS shows a similar decay law, also fitted with a double exponential. The shorter component has a value (95 ps) again comparable to that found in the solution. The lifetime value of the second component is basically equal to that found in TIS-LDH. The relative amount of these two monomeric species (as seen by the preexponential factors) is similar to that found for the TIS-LDH system. These results seem to be fully consistent with estimates of the TIS densities for TIS-LDH and TIS-LHS (1 TIS molecule per 52.4-53.3 Å2) which, as detailed above, point to mean separations of neighboring TIS molecules in the range of 7.8-7.9 Å.


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CONCLUSIONS The encapsulation of dyes within inorganic hosts is of interest for the synthesis of hybrid pigments with tunable colors as well as photofunctional materials that display improvements in the dye physicochemical properties such as luminescence quantum yields, lifetime, thermal stability and photostability. In the present work we obtained new thioindigo-based hybrid materials by the intercalation of thioindigo-5,5’-disulfonate (TIS) into zinc-aluminum LDH and zinc LHS hosts by the coprecipitation method. The intercalation of TIS into the LHS host enhanced the thermal stability of the dye relative to the pure compound H2TIS. Hybrid pigments with tunable colors (from ruby/wine to dark blue) were successfully prepared by cointercalating TIS and indigo carmine in different molar ratios. The basal spacings for all materials range between 19 and 21 Å and are consistent with the intercalated indigo derivatives being inclined at angles between 50 and 80° relative to the host layers. On the basis of the chemical compositions and charge density calculations, it is estimated that the average distance between guest anions is in the range of 7.7-8.3 Å. This is well in line with the photophysical spectral data, especially for the TIS-containing hybrids, which display enhanced emission (relative to the pure compound H2TIS) and absorption spectra that are more indicative of isolated molecules than aggregates. Biexponential fluorescence decays are attributed to the presence of two types of monomer, one that interacts with water molecules and another that does not. Hence, in synergy with hostguest/guest-guest interactions, interlayer water molecules may actively contribute to the separation and stabilization of monomer species. This high degree of supramolecular organization seems also to be present for the cointercalated samples where a uniform distribution of IC and TIS molecules promotes fluorescence quenching through an efficient energy transfer process.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.


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Synthesis description and characterization data for H2TIS, synthesis description, PXRD pattern, TGA data and SEM images for NO3-LHS, listing of FTIR, Raman and 13C{1H} CP MAS NMR peaks for IC(n%)/TIS-LHS/LDH hybrids, simulated and experimental PXRD patterns, supplementary TGA curves, description of the spectroscopic characterization of intercalated samples, supplementary SEM images (500× magnification), chemical compositions of IC(n%)/TIS-LHS/LDH hybrids, supplementary FTIR, Raman and 13C{1H} CP MAS NMR spectra, diffuse reflectance spectra of IC-LDH hybrids.

AUTHOR INFORMATION Corresponding author. *E-mail: [email protected]; [email protected]. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS We acknowledge funding by FEDER (Fundo Europeu de Desenvolvimento Regional) through COMPETE (Programa Operacional Factores de Competitividade). National funding through the FCT (Fundação para a Ciência e a Tecnologia) within the project Coimbra Chemistry Centre (PEst-OE/QUI/UI0313/2014) is thanked. This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement. The FCT is acknowledged for a PhD grant to A.L.C. (ref. SFRH/BD/88806/2012) and a post-doctoral grant to A.C.G. (ref. SFRH/BPD/108541/2015).

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Seixas de Melo, J. S., The molecules of colour. In Photochemistry: Volume 45, Fasani,

E.; Protti , S.; Albini, A., Eds. The Royal Society of Chemistry: 2018; Vol. 45, pp 68-100. 2.

Melo, M. J.; Ferreira, J. L.; Parola, A. J.; Seixas de Melo, J. S., Photochemistry for

Cultural Heritage. In Applied Photochemistry: When Light Meets Molecules, Bergamini, G.; Silvi, S., Eds. Springer International Publishing: Cham, 2016; pp 499-530. 27 ACS Paragon Plus Environment

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3.

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