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Article pubs.acs.org/accounts A Long Journey in Lanthanide Chemistry: From Fundamental Crystallogenesis Studies to Commercial Anticounterfeiting Taggants Olivier Guillou,* Carole Daiguebonne, Guillaume Calvez, and Kevin Bernot INSA Rennes, UMR 6226 “Institut des Sciences Chimiques de Rennes”, 20 Avenue des buttes de Coësmes, 35708 Rennes, France CONSPECTUS: Lanthanide ions have unique physical properties and are essential for numerous technological devices. Indeed, much research has been undertaken in order to understand and optimize their luminescent behavior. From a chemical and more specific point of view, lanthanides can be used to build coordination polymers (CPs). CPs are materials in which metal ions are associated with organic molecules (ligands) to form extended networks. They present great structural diversity and a wide range of unique properties such as great porosity, strong catalytic activities, and original magnetic and luminescent behaviors. In this Account, we highlight recent research advances obtained by our team in the field of lanthanide-based CPs. However, rather than present a simple chronological description of successive investigations, we have chosen present our own experience in order to show how standard academic studies can be successfully turned into applied research and finally into a viable startup that commercializes these products as anticounterfieting taggants. A taggant is a compound that can be dispersed in a host matrix at parts per million rates for it to be labeled. Its economic advantages over traditional anticounterfeiting techniques (labels, chips, etc.) are its very low cost and its ability to label a raw material at every stage of its processing, unlike traditional techniques that label only the final product. It thus permits traceability of a given material over a wide range of suppliers/subcontractors/sellers or customers at every step of its life. After 15 years of fundamental crystallogenesis research, we identified a very stable phase of lanthanide-based CPs in which strong lanthanide luminescence can be observed. We investigated this phase further and showed that a heteronuclear approach can give access to billions of different compositions and makes it possible to turn these powders into taggants. After the creation of a startup, we refocused on fundamental studies in order to ensure its future development. This permitted the design of future generations of taggants and provided brightness- or color-tunable compounds as well as temperature-sensitive and soluble taggants. We hope to demonstrate here that strong fundamental research is a very effective tool to create a technological breakthrough that allows the development of efficient applicative research and competitive products and finally contributes to economic growth. 1. INTRODUCTION The unique physicochemical properties of the lanthanide ions make them key to an extremely wide range of techniques and devices.1−4 This is why lanthanide-rich ore is nowadays a strategic and geopolitical concern.5−8 In such a competitive field, we would like to illustrate how academic studies are absolutely mandatory for the emergence of efficient applicative research and commercial devices. This approach can lead to better use of resources, improvement in the existing devices, and even the creation of brand-new technologies. Our group has been working for more than 15 years in the field of lanthanide-based coordination polymers (CPs). Here we retrace the evolution of this research and show how basic crystallogenesis studies can lead to a viable startup and how a close association between fundamental and applicative research is necessary to ensure its future development. Lanthanide-based CPs have attracted great interest in fundamental research because of their fascinating crystal structures and physical properties such as magnetism, luminescence, and porosity.9 Because of the shielded character © 2016 American Chemical Society of their valence orbitals, lanthanide ions establish bonds with ligands that have a very weak covalent character, and consequently, their coordination geometry is essentially driven by the structuring character of the ligands. From this point of view, benzenepolycarboxylates constitute prime ligands.10 Indeed, their carboxylate groups can be involved in hydrogenbonding networks, their phenyl rings can lead to π stacking, and their carboxylate clips fit well with the oxophilic lanthanide ions. Moreover, a given ligand in association with the same lanthanide ion can provide several different crystallographic phases depending on the crystallization process.11 2. FUNDAMENTAL STUDIES During the late 1990s, some of the authors were involved in the synthesis and crystallization of lanthanide-based CPs.12 Our academic studies aimed to provide a better understanding of Received: February 2, 2016 Published: April 15, 2016 844 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Scheme 1. Ligands Used in This Work Figure 1. Representation of [Ln2(bdc)3(H2O)4]∞: (a) projection view of an extended asymmetric unit; (b, c) perspective views along (b) the a axis and (c) the b axis; (d) schematic representation. Adapted from ref 19. Copyright 2008 American Chemical Society. the relationships between the crystallization process and the resulting crystallographic phase.13−15 In order to conduct this project, we developed the gel-medium crystallization of lanthanide-based CPs.16−18 Among the different ligands we used, terephthalate (hereafter bdc2−) (Scheme 1) has particularly caught our attention. Indeed, this nontoxic rodlike ligand offers a large variety of crystallographic phases upon coordination with lanthanide 845 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 2. (a) Pictures of [Tb2(bdc)3,(H2O)4]∞ and [Eu2(bdc)3,(H2O)4]∞ as powders or crystals in gel media under 312 nm illumination. (b) Colorimetric coordinates of [Ln2(bdc)3(H2O)4]∞ derivatives (Ln = Sm−Dy) under 312 nm illumination. Adapted with permission from ref 23. Copyright 2013 John Wiley and Sons. Figure 3. Emission spectra of [Ln2(bdc)3(H2O)4]∞ with Ln = Sm (a), Eu (b), Tb (c), and Dy (d). Adapted with permission from ref 23. Copyright 2013 John Wiley and Sons. ions.11 One of these phases is thermodynamically very stable and can be obtained almost quantitatively as a homogeneous microcrystalline powder (1 μm) simply by mixing, at ambient temperature and pressure, aqueous solutions of lanthanide chloride and sodium terephthalate.19 An isostructural series of formula [Ln2(bdc)3(H2O)4]∞ is obtained for all of the lanthanide ions between La3+ and Tm3+ (except Pm3+) plus Y3+.19,20 This crystal structure consists of a compact threedimensional (3D) framework. There is only one Ln3+ ion in the asymmetric unit that is eightfold-coordinated by oxygen atoms (Figure 1). The crystal structure can be described as a superimposition of planes that spread parallel to the bc plane at ∼10 Å from each other. Intermetallic distances in this plane are ∼5 Å along the c axis and ∼6 Å along the a axis. 846 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research All of the derivatives present similar thermal behaviors. Coordination water molecules are removed in one step, and the resulting anhydrous phase is stable up to a decomposition temperature that ranges between 250 and 500 °C depending on the involved lanthanide ion. The anhydrous phase reversibly binds water molecules when exposed to moisture, restoring the initial phase. Beyond our structural studies, we undertook a photophysical investigation of the series, as lanthanide ions can produce visible luminescence under UV excitation.21,22 Compounds based on Sm3+, Eu3+, Tb3+, and Dy3+ exhibit purple, red, green, and yellow luminescence, respectively (Figure 2).23 All of the excitation spectra show a broad excitation band below 320 nm that can be attributed to the bdc2− ligand and indicates an efficient “antenna effect”:24,25 the ligand has the ability to absorb the energy of the UV illumination and transmit it to the lanthanide ion, which is otherwise a very bad UV absorber (forbidden f−f transition).26 An excitation wavelength of 312 nm was chosen for comparison with commercially available UV lamps (Figure 3). The overall energetic scheme for the emission can be summarized by a Jablonski diagram (Scheme 2). excited singlet state (by referring to the wavelength of the UV− vis absorbance edge: 320 nm ≈ 31 250 cm−1) and triplet state (by referring to the shortest wavelength of the phosphorescence band of the ligand: 425 nm ≈ 23 530 cm−1).29−31 Reinhoudt’s empirical rules34 state that the intersystem crossing (ISC) becomes efficient when ΔE(1π1π*−1π3π*) is at least 5000 cm−1, which is actually the case here: ΔE ≈ 7720 cm−1. Moreover, Latva’s empirical rules35 stipulate that the lowest excited triplet state of the ligand (23 530 cm−1) favors efficient ligand-to-metal energy transfer (ηsens) without significant back-transfer for both the Eu- and Tb-based compounds (the energies of the respective emitting levels ( 5 D 0 and 5 D 4 ) are 17 300 and 20 500 cm −1 ). For [Eu2(bdc)3(H2O)4]∞, ηsens = 83%.36,37,28 These quite high overall quantum yields combined with the high molar absorption efficiency of the ligand (ε(λ = 312 nm) = 4500 L mol−1 cm−1 for aqueous solutions of Na2bdc)28 explain the strong luminance of the Eu- and Tb-containing compounds. Scheme 2. Simplified Jablonsky Diagram for [Tb2−2xEu2x(bdc)3(H2O)4]∞ A taggant is a material that can be dispersed in a host matrix at parts per million rates without alteration of the properties of the host matrix in order to permit its unambiguous identification and to fight against it being counterfeited. Even though the [Ln2 (bdc) 3(H 2 O) 4] ∞ series seemed to be particularly suitable for this purpose, it rapidly became clear that their effective use would be rather limited. In fact, with such “homonuclear” compounds (powders based on only one lanthanide ion), only five different colors are theoretically accessible in the visible domain26 and possibly only three (green, yellow, red) given the luminous efficiency for the human eye.38 This poor color variability has two evident drawbacks: (i) The properties of [Ln2(bdc)3(H2O)4]∞ are very similar to those of already efficient and cheap existing phosphors38 and do not have any commercial or technological advantage. (ii) Once the emissive color is observed and consequently the lanthanide is identified, counterfeiters can easily make and use their own fake taggants. The snake bites its own tail, and the anticounterfeiting taggant is in turn counterfeited. Consequently, multiplication of the accessible emission colors appears absolutely mandatory. This would permits a breakthrough in the use of the taggant: dating of the tagged material. 3. TOWARD THE REAL TAGGANT 3.1. Shortcomings of the Homonuclear Compounds as Taggants The photophysical parameters listed in Table 1 indicate that the Sm- and Dy-based compounds exhibit weak luminescence. This can be related to their small energy gaps, which favor nonradiative de-excitation.27,28 The Gd-based compound emits weak blue light that is due to the ligand fluorescence because its potentially emitting levels are higher than the feeding levels of the ligand. However, it allows the estimation of the lowest Table 1. Spectroscopic and Colorimetric Data for Homonuclear Compounds [Ln2(bdc)3(H2O)4]∞ [Sm2(bdc)3(H2O)4]∞ [Eu2(bdc)3(H2O)4]∞ [Gd2(bdc)3(H2O)4]∞ [Tb2(bdc)3(H2O)4]∞ [Dy2(bdc)3(H2O)4]∞ QLigand (%)a Ln τobs (ms) xb yb luminance (Cd m−2)c 0.08(1) 13.9(14) − 45.5(45) 0.28(3) 0.0021(2) 0.40(4) − 1.10(10) 0.0011(1) 0.43(1) 0.66(1) 0.26(1) 0.34(1) 0.35(1) 0.23(1) 0.34(1) 0.28(1) 0.57(1) 0.32(1) 0.2(1) 10.8(5) 1.6(1) 142(2) 0.3(1) a Ligand QLn is the overall quantum yield upon ligand excitation.28 b(x, y) are the CIE (1931) emission color coordinates under 312 nm UV light. With X nm 780 nm 780 nm = k × ∫ 780 380 nmIλxλ dλ, Y = k × ∫ 380 nmIλyλ dλ, and Z = k × ∫ 380 nmIλzλ dλ, where k is a constant for the measurement system, Iλ is the sample spectrum intensity, and xλ, yλ, and zλ are wavelength-dependent trichromatic values, the color coordinates are given by x = X/(X + Y + Z), y = Y/(X + Y + Z), and z = Z/(X + Y + Z).32,33 cLuminance is expressed in Cd m−2 and represents the luminous flux weighted by the spectral response of the human eye. 847 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research For instance, a raw material tagged in January would not be tagged in the same way as February’s “vintage”, etc. This fast variability of the taggant over the time and/or the batch and/or the customer would be almost impossible for the counterfeiter to copy and would protect the taggant’s efficiency against being itself counterfeited. 3.2. The Heteronuclear Approach 3.2.1. Structural Robustness upon Lanthanide Substitution. To generate a great variability of colors, a first and evident strategy would be to mix the homonuclear-based taggants in different proportions in order to produce new colors. However, it is clear that this is not an effective approach. First, it would be no different from mixing two already-existing phosphors. Second, under extreme dilution a mixture of Tbbased (green) and Eu-based (red) taggants would produce not a yellowish color but instead isolated red and green spots. Once again, the taggant could easily be counterfeited. We consequently chose another approach: the “heteronuclear” strategy. Rather than mixing the final CPs, we mixed the lanthanide precursors before their reaction with the ligand. This allowed us to obtain one single phase with several different lanthanides and opened the way to a great number of accessible colors. The preliminary and obvious question was whether we had a single and homogeneous phase. The similar chemical properties of the lanthanide ions make them easily interchangeable in the synthetic process. We demonstrated by a multiscale approach (Figure 4) that a homogeneous powder can be obtained even with lanthanides of very different radii, such as lanthanum and yttrium.39 Powders of formula [La2−2xY2x(bdc)3(H2O)4]∞ were synthesized for 0 ≤ x ≤ 1 with an x spacing of 0.1. The chemical homogeneity of the constitutive particles was checked using energy-dispersive spectrometry (EDS) measurements. The 11 powders were found to be isomorphous on the basis of their powder X-ray diffraction (PXRD) patterns, and a clear shift of the diffraction peaks was visible as the bigger lanthanide (La) replaced the smaller one (Y). The peaks were not split into two, as was the case for corresponding mixtures of the homonuclear species [La(bdc) 3 (H 2 O) 4 ] ∞ and [Y(bdc)3(H2O)4]∞.39 This confirmed the monophasic character of the crystallites that constitute the particles. Finally, the 89Y solid-state NMR spectra were similar regardless of the doping rate, indicating that the local environment of the Y3+ ions was identical in all of the compounds. This confirmed that these compounds were perfect solid solutions. Such structural robustness upon lanthanide substitution was ultimately tested in a phase combining all of the lanthanides from lanthanum to thulium (except promethium), and the resulting compound was once again isomorphous with the pure phases.39 Consequently, the number of compounds obtainable by mixing lanthanides in bi-, tri-, or even triskaidecaheteronuclear phases is extraordinary. In fact, with 13 lanthanides, if one considers that an accuracy of 1% in the doping rate can be experimentally obtained (which is reasonable), 4 × 1015 (i.e., C100 112) different compounds can be prepared. 3.2.2. Luminescence of the Heteronuclear Phases: Color Tuning. The luminescence of a heteronuclear phase does not vary linearly along a series. Whereas a 1:1 mixture of [Eu2(bdc)3(H2O)4]∞ and [Tb2(bdc)3(H2O)4 produces a yellowish color, [Eu1.00Tb1.00(bdc)3(H2O)4]∞ is red (Figure 5).23,39 The yellow emission is observed far from this doping Figure 4. Multiscale characterization of the [La2−2xY2x(bdc)3(H2O)4]∞ powders: (a) SEM image; (b) PXRD patterns; (c) 89Y NMR spectra. Adapted with permission from refs 23 and 39. Copyright 2013 John Wiley and Sons and 2009 American Chemical Society, respectively. rate for a very small amount of Eu3+ in [Eu0.06Tb1.94(bdc)3(H2O)4]∞. In fact, for a heteronuclear compound, the Jablonski diagram is somewhat modified (Scheme 2) because an intermetallic energy transfer (ηET) from the lanthanide with the highest emitting energy level toward the other ions can be observed28 (the yield of the intermetallic energy transfer is ηET = (1 − τobs)/τ0, where τobs and τ0 are the luminescence lifetimes with and without acceptor lanthanide ion, respectively).40 This feature is obviously not accessible in a mix of homonuclear powders.24,25 For [Eu1.00Tb1.00(bdc)3(H2O)4]∞, ηET is 95%. This extremely high value leads to an emission color close to pure red, as almost all of the energy from the emitting 5D4 level of Tb3+ is transferred to the 5D0 level of Eu3+. On the other hand, the color variation upon Tb3+ substitution is very sensitive for high Tb3+ content 848 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 5. (a) Luminescence spectra of the [Tb2xEu2−2x(bdc)3(H2O)4]∞ series with 0 ≤ x ≤ 1 under 312 nm irradiation. (b) Corresponding colorimetric coordinates. (c) Variation of the colorimetric coordinates (x in black and y in red). (d) Luminance (0.25 W m−2 irradiance) upon composition variation. (e) Schematic representation. Adapted with permission from refs 23 and 39. Copyright 2013 John Wiley and Sons and 2009 American Chemical Society, respectively. because of the high luminescence quantum yield of Tb3+ ions in these compounds. We adopted an empirical approach in order to scan the accessible colors in such series. Heterotrimetallic phases are particularly appealing, as one can try to reproduce classic tricolor association used in commercial devices.38 We chose to use Ce3+ ions, whose pure phase exhibits blue emission, and to combine it with the red (Eu3+) and green (Tb3+) emission. We were then able to elaborate some kind of ternary phase diagram where a great variety of colors (blue, yellow, purple, pink, etc.) are observed, including even white emission for [Eu0.20Tb0.10Ce1.70(bdc)3(H2O)4]∞ (Figure 6). 3.2.3. Luminescence of the Heteronuclear Phases: Brightness Tuning. The great advantage of the heterometallic CPs over traditional inorganic phosphors is that their brightness can easily be tuned and can in some cases give spectacular results. We have to recall here that the bdc2− ligand is relatively small, and a rough estimate of the mean volume occupied by a lanthanide leads to a mean Ln3+−Ln3+ distance of 8.3 Å.41 This intermetallic distance is small enough (<10 Å) to allow efficient nonradiative de-excitation by a Dexter intermetallic energy transfer mechanism, which induces a decrease in the overall brightness. With these considerations in mind, we chose to synthesize heterobimetallic compounds with one emitting lanthanide ion and one optically inactive (La3+, Y3+, Gd3+) lanthanide ion. As the doping rate of the latter is increased, the mean distance between the emissive ions increases, which diminishes the intermetallic energy transfer and thus increases the brightness, as illustrated by the [Gd2−2xLn2x(bdc)3(H2O)4]∞ series (Ln = Eu3+, Tb3+) (Figure 7). Whereas the color is not modified upon optical dilution, the brightness is enhanced for low doping rates of Gd3+, reaching a maximum efficiency at x = 0.20, where the luminance can be increased by up to 30% for [Gd0.4Eu1.6(bdc)3(H2O)4]∞ and reach 170 Cd m−2 under 0.25 W m−2 irradiance for [Gd0.4Tb1.6(bdc)3(H2O)4]∞, a significant value compared with existing phosphors.38 The brightness decreases at higher doping rates because the number of optically active ions per unit volume decreases. This brightness optimization has tremendous importance for two reasons: (i) Brightness-tuned taggants can be efficient to tag a matrix at extremely low doping rates. This is a key point for materials whose regulation rules do not tolerate high amounts of additives (e.g., drugs and food contact materials). (ii) Powders that are as bright as the pure ones can be obtained with only a few percent of optically active ions (e.g., [Gd1.6Eu0.4(bdc)3(H2O)4]∞ has similar luminance as [Eu2(bdc)3(H2O)4]∞). This offers an extremely high 849 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 6. Photographs of the luminescence of [(Ce2−2x−2yEu2xTb2y)(bdc)3(H2O)4]∞ with x + y = 1 under λexc = 312 nm. Adapted from ref 39. Copyright 2009 American Chemical Society. Figure 7. Luminance measurements (in Cd m−2 with 0.25 W m−2 irradiance) on the [Gd2−2xLn2x(bdc)3(H2O)4]∞ series with Ln = Tb (a) or Eu (b) and its schematic representation. Adapted with permission from ref 23. Copyright 2013 John Wiley and Sons. special handling procedures), (iii) uses few synthetic steps and offers high yields, and (iv) can be easily scaled-up. Moreover, the final compounds are (i) stable over long time periods (no photobleaching has been observed over 10 years) and require no special storage procedure; (ii) chemically and thermally stable, which allows their introduction in a wide number of industrial processes; (iii) nontoxic (FDA and EFSA approvals for food contact are under process); and (iv) easily disposable (heating to >1000 °C affords back the lanthanide oxide in very good yield). This green synthetic approach is a strength, but with all of these library of compounds in hand comes the big problem for fundamental researchers: how does one turn a powder obtained in a beaker into an efficient and competitive commercial material? Indeed, more than 300 security technologies going from simple labels to very complex ADN-based taggants are available. Each of these solutions is specific to a given demand. The big asset of heterolanthanide-containing CPs is that they open the way to the tagging of raw host materials. Hence, each competitive advantage for the taggant since optically inactive lanthanides are considerably cheaper than optically active ones.3,5,6,8 3.2.4. Luminescence of the Heteronuclear Phases: Combined Tuning. Finally, we combined brightness and color tuning by association of Tb3+ and Eu3+ ions (color) with La3+ (brightness). The resulting heterotrimetallic compounds of formula [Tb2xEu2yLa2−2x−2y(bdc)3(H2O)4]∞ offer colors that span the whole spectral range between red and green as for [Tb2−2xEu2x(bdc)3(H2O)4]∞ series, but as soon as some La3+ is used, the luminance of the given colors is significantly enhanced (Figure 8). Such optimization of the color and/or brightness properties has also been found to be efficient for CPs based on other ligands.41−45 4. THE TAGGANT...FINALLY! It is useful to recall here that the synthesis (i) uses low-cost and nontoxic ligands, (ii) is performed in water at ambient pressure and temperature and is nonhazardous (no solvent cost nor 850 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 8. Ternary phase diagram of the [Tb2xEu2yLa2−2x−2y(bdc)3(H2O)4]∞ heterotrimetallic series with x and y ranging from 0 to 1, with the corresponding pictures and some luminance values under 0.25 W m−2 irradiance at 312 nm. Reproduced with permission from ref 23. Copyright 2013 John Wiley and Sons. Figure 9. Colorimetric coordinates for [EuTb(hip)2(H2O)10·(hip)·4H2O]∞ at three different excitation wavelengths. Adapted with permission from ref 43. Copyright 2014 Royal Society of Chemistry. 4.1. Back to Academic Research: The Future Generation of Taggants taggant’s particles contain all of the labeling information; they can be introduced in various host matrixes such as plastics, paints, or plasters; their production cost is reasonable; and most importantly, they allow “vintage” traceability of the matrix. Moreover, customers and even startup funders are increasingly attentive to the environmental footprint of chemically elaborated products. Green-elaborated products are ecologically interesting but are also intrinsically cheap because no extra costs are generated by elaborated synthetic conditions, in particular to control of the safety of the process or for special packaging or adapted storage of the final products. These low technical and operation costs largely counterbalance the relatively high cost of the lanthanide oxides. All the above reasons convinced us that heterolanthanidecontaining CPs have their place among the available technologies. After patents were obtained,46,47 the company OLNICA was founded in 2010 in order to sell the taggants. At that point, the fast growth of the startup made it clear that the taggants answered customer demand. However, as in every market, a technological advantage is only temporary. As the startup developed and its knowledge of the market increased, an evolution of the taggant seemed to be necessary to feed customer demand. Beyond the need for more luminescent taggants (the brighter the taggant is, the lower is the necessary doping rate, and therefore, the tracing cost is lower and the identification of the taggant by counterfeiters is more difficult), two main areas of development were identified: (i) Customers need even more elaborated and customizable colorimetric signatures. (ii) Customers need taggants that are adaptable to different matrixes and not only solid raw materials. These apparently simple requisites forced us to go back to fundamental science, as it became evident that optimization or 851 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 10. Photophysical investigation of [EuTb(hip)2(H2O)10·(hip)·4H2O]∞. (a) 3D excitation/emission scan. (b) Schematic representation. (c) Schematic illustration of energetic pathways. From left to right: (i) λexc = 345 nm, resulting in excitation of the ligand. Green emission of the Tb3+ ion dominates because of the PET mechanism. (ii) λexc = 375 nm, resulting in direct excitation of both Eu3+ and Tb3+ ions. Both contribute to the greenyellow luminescence. (iii) λexc = 395 nm, resulting in direct excitation of the Eu3+ ions. Red luminescence dominates. Adapted with permission from ref 43. Copyright 2014 Royal Society of Chemistry. Figure 11. (a) Emission spectra and (b) colorimetric coordinates of [Eu(cpbOH)(H2O)2·(cpb)]∞ versus temperature (λexc = 303 nm). (c) Schematic representation. Adapted from ref 53. Copyright 2013 American Chemical Society. center. For this second generation, we chose to work on the ligand, but some of our exploratory results have already shown that optimization of the metal centers is also efficient.48−50 Benzenepolycarboxylate ligands were thus reconsidered, and we focused our work on a ligand that possesses an extra electron-donor group that is noncoordinating. The underlying idea was that UV illumination can force electron transfer toward the lanthanide and thus modify its emission properties. This photoinduced electron transfer (PET) is expected to be particularly efficient for europium and samarium, which have an accessible +II oxidation state. We first demonstrated that stable and efficient CPs can be obtained with aminoisophthalate (aip2−)44,51 and hydroxyi- engineering of the already existing products would not be enough to face these new challenges. 4.2. Second Generation of Taggant: Complexification of the Colorimetric Signature 4.2.1. Deliberate Introduction of Electron-Donor Groups for Multiple Color Emission. The necessary evolution of the taggant has to be somewhat tailored: not only the chemical and physical stabilities, brightness, and colorimetric versatility but also the ability to produce the taggant through a green synthetic approach have to be conserved. From the chemical point of view, we need to optimize one of the two building blocks of the CPs: the ligand or the metallic 852 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research Figure 12. Photographs under 312 nm illumination and schematic representations of (a) powders of formula [Tb2−2xEu2x(bdc)3(H2O)4]∞ with x = 1, 0.5, 0.1, and 0 and (b) the corresponding nanoaggregates in glycerol. Adapted with permission from ref 54. Copyright 2015 John Wiley and Sons. sophtalate (hip2−)43 ligands (Scheme 1). Their colorimetric and brightness tuning through heterometallic phases was also demonstrated. Second, as expected, PET was observed and induced a drastic modification of the emission color (the red emission was turned into a strong blue component). This result is strongly dependent on the illumination wavelength, as the ligands absorb only in a given wavelength range. Outside this range, direct sensitization of the lanthanide is possible without inducing PET. We therefore have two different illumination wavelengths that induce two different colors for a given compound (Figure 9). Multimodal emission is then possible. On heterometallic derivatives, this number is even bigger, as each lanthanide can be independently excited. A sort of bar code versus excitation wavelength is then obtained (Figure 10). 4.2.2. Deliberate Introduction of O−H Vibrators for Temperature-Dependent Color Emission. Another fruitful development was undertaken by using carboxyphenylboronate derivatives as ligands. These ligands are closely related to benzenepolycarboxylates and are considered particularly environmentally friendly. Most of all, they present two different acidic moieties: the carboxylic group acts as a Brönsted acid, and the boron atom of the B(OH)2 moiety has Lewis acidic character. This latter is turned into B(OH) 3 − upon coordination with the lanthanide.52 Such coordination of three O−H oscillators on the lanthanide drastically increases the possibility of nonradiative de-excitation of the lanthanide. Consequently, the obtained Eu 3+ derivatives are less luminescent than the corresponding terephthalate-based CPs but show a considerable advantage: their color shifts toward the blue as the temperature is lowered (Figure 11). This is the result of the combination of different mechanisms (ligand and/ or metal nonradiative de-excitation and/or ligand-to-metal energy transfer) that have different thermal dependences. This extremely strong temperature dependence can even be used to turn the compounds into molecular thermometers with sensibilities similar to the best reported to date.53 From the taggant point of view, these two modifications are breakthroughs because the colorimetric complexity is drastically enhanced and a new variable (temperature) comes into play. These types of taggants can, for instance, be useful for optical control of the temperature under operating conditions. 4.3. Third Generation of Taggant: Nanometrization of the Powders for Enhanced Solubility All of the investigated taggants are CPs and are very insoluble by nature. This feature contributes to their chemical and thermal robustness. However, some applications require some kind of solubility as, for example, a homogeneous dispersion in a liquid matrix such as a technological fluid or at every liquid step in the engineering process of a solid material. Two different pathways have been tested to achieve solubilization of the taggants: encapsulation into polyvinylpyrrolidone (PVP) nanoparticles19 and nanometrization by adapted solvents.54 The first route produces stable luminescent solutions, but their quantum yield and brightness are low, and the synthetic route is unlikely to be easily adapted to large-scale synthesis. The second route combines luminescence efficiency of the resulting solutions with a remarkably simple and cheap 853 DOI: 10.1021/acs.accounts.6b00058 Acc. Chem. Res. 2016, 49, 844−856 Article Accounts of Chemical Research synthetic process.54 The governing idea was to use a commercial dispersing solvent such as glycerol and to test its efficiency on the CPs (Figure 12). In fact, fragmentation of the compound into nanometric entities was easily achieved. Moreover, production of solutions with nanometric particles of controlled size and dimension has been found to be quite straightforward. This synthetic post-treatment of the taggants in the liquid phase avoids the hazardous handling of preelaborated nanometric powders and is adaptable to different families of taggants.54 This solvent is also environmentally friendly, and the FDA approved its use as a food additive (labeled as E422). This opens the way to new markets and permits further developments of taggant technology, especially when solubilization is required in a process, such as for thinfilm deposition. Biographies Olivier Guillou was born in Guérande, France. He received his Ph.D. in Chemistry in 1992 from Paris XI University. After holding various temporary positions, he became an Assistant Professor at INSARennes in 1994 then a Professor at INSA-Rennes in 2001. His research is focused on lanthanide coordination chemistry. Carole Daiguebonne was born in Thiers, France. She received her Ph.D. in Chemistry in 2000 from INSA-Rennes. After holding temporary positions between 2000 and 2002, she became an Assistant Professor at INSA-Rennes in 2002. Her research is focused on lanthanide coordination chemistry, including crystallogenesis and luminescence. Guillaume Calvez was born in Morlaix, France. He received his Ph.D. in Chemistry in 2009 from INSA-Rennes. After a postdoctoral stay in Rennes in 2010, he became an Assistant Professor at INSA-Rennes. His research is focused on lanthanide coordination chemistry, polynuclear lanthanide species, and luminescence. 5. CONCLUSION In this paper we have summarized 15 years of fundamental research on lanthanide-based CPs and demonstrated their outstanding luminescence properties. The heteronuclear approach offers considerable versatility in the accessible colorimetric area (Scheme 3) and makes these compounds suitable for anticounterfeiting applications. Kevin Bernot was born in Saint-Malo, France. He received his Ph.D. in Chemistry in 2007 from both INSA-Rennes and the University of Florence (Italy). After a postdoctoral stay in Florence in 2008, he became an Assistant Professor at INSA-Rennes. His research is focused on lanthanide coordination chemistry, molecular magnetism, and luminescence. ■ Scheme 3. Colors Accessible by Modification of the Composition of Heteronuclear Compounds (Solid Lines) or by Variation of the Temperature (Dashed Lines) for a Given Series of Lanthanide-Based CPs Most of all, we hope to have demonstrated that regular switching back and forth between fundamental research and applications is a very efficient way to create technological breakthroughs that lead to the development of both efficient applicative research and original fundamental research. ■ REFERENCES (1) Jones, N. The Pull of Stronger Magnets. Nature 2011, 472, 22− 23. (2) Pyykko, P. Magically Magnetic Gadolinium. Nat. Chem. 2015, 7, 680−680. (3) Bunzli, J.-C. Europium in the Limelight. Nat. Chem. 2010, 2, 696−696. (4) Samson, B.; Carter, A.; Tankala, K. Doped Fibres: Rare-Earth Fibres Power Up. Nat. Photonics 2011, 5, 466−467. (5) Ragnarsdottir, K. V. Rare Metals Getting Rarer. Nat. Geosci. 2008, 1, 720−721. (6) Beyond Mining. Nat. Geosci. 2011, 4, 653. 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