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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
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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
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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.
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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.
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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
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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
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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
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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
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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
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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
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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.
■
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Corresponding Author
*E-mail: [email protected].
Notes
The authors declare no competing financial interest.
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