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Upconversion of light with molecular and
supramolecular lanthanide complexes
Aline Nonat, Loic Charbonniere
To cite this version:
Upconversion of light with molecular and supramolecular lanthanide
complexes
Aline M. Nonat and Loïc J. Charbonnière*
Equipe de synthèse pour l’analyse (SynPA), IPHC, UMR 7178, CNRS/Université de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex, France. E-mail :
l.charbonn@unistra.fr
Abstract. Upconversion (UC) is the process by which the energy of multiple photons is
absorbed by a compound and restored in the form of a photon of higher energy than the incident light, resulting in an anti-Stokes process. Although studied theoretically since the middle of the last century and experimentally observed in the 1960’s, the process was up to recently mainly restricted to solid state devices and ultimately to nanoparticles at the end of the century. At the same period, different researches were directed towards the possibility to observe UC at the molecular level and it is only recently that the phenomenon could be observed in discrete molecular entities in solution with still very few examples. This review aims at explaining the difficulties encountered at the molecular level compared to the solid state and summarizes the results reported to date on UC at the molecular scale.
Introduction
was deeply studied by George Stokes [1], who investigated numerous luminescent solutions, among which quinine sulfate, and the loss of energy resulting in an increased emission wavelength compared to the absorption one was dedicatedly called Stokes shift.
In rarer cases, the compound in its excited state can absorb the energy of a second photon, reaching a higher excited state (L**, Figure 1). If this energy is released in the form of a photon, the luminescence occurs at a lower wavelength than the incident one, resulting in an anti-Stokes process called upconversion (UC) [2].
200 400 600 800 1000 Wavelengths ( / nm) Excitation Photoluminescence (Stokes process) Upconversion (Anti-Stokes process)
b)
L L* L’ L**a)
h
UC h
lum h
exc h
excFigure 1. a) schematic representation of the phenomenon of photoluminescence and
upconversion. b) Wavelength dependence of the luminescence phenomena.
The UC mechanisms can be divided into two main categories. The first one entails phenomena related to non-linear processes such as two-photon absorption excitation, second harmonic generation and cooperative luminescence [2]. These phenomena were theoretically studied by Maria Göppert-Meyer, who developed the concept of two photon absorption as soon as 1931 [3], concept experimentally demonstrated thirty years later with the observations of second harmonic generation [4] and two photon fluorescence [5], thanks to the income of high energy excitation sources, the MASERs (Microwave Amplification by Stimulated Emission of Radiations) [6]. However, these mechanisms are based on virtual excited states and are generally weakly efficient.
were first postulated by Nicolaas Bloembergen in 1959 [7], who proposed the concept of quantum counters, in which a first excited state is reached by absorption of an infrared photon, followed by the optical pumping to a higher energy excited state. Figure 2 summarizes these mechanisms: excited state absorption (ESA); energy transfer UC (ETU) and cooperative sensitization (CS).
CS
ESA
ETU
Figure 2. UC processes based on successive linear optical processes: a) ESA; b) ETU;
and c) CS (color code: blue = absorption, red = energy transfer and green = emission).
ESA consists in the absorption of a first photon leading to an intermediate excited state, which consequently absorbs a second photon to reach the emitting UC luminescent level. It was first reported in 1961, in crystals of La:PrCl3, upon excitation at two distinct wavelengths [8].
In ETU, the absorption of the second photon occurs on another ion in the material and an energy transfer occurs between the two ions to feed the upper excited state. This mechanism was first reported by François Auzel in two distinct papers [9,10]. This supplementary step is the hallmark of ETU and is at the basis of the distinction between ESA and ETU mechanisms. When recording time-resolved UC intensity upon pulsed excitation, ETU is characterized by a slow rising edge attributed to the supplementary energy transfer step [11]. Finally, a last mechanism is the CS in which two donor atoms transfer their energy to a third one which emits light [12], experimentally observed in 1969 in Tb and Yb doped crystals of CaF2 and SrF2 [13].
These different processes have been described in details in the relevant reviews of the literature [2,14-16]. It should be noted that another UC mechanism exists called photon avalanche [17], but it will not be detailed here as it hasn’t been observed for molecular systems up to now.
mandatory as the two donors should have a single excited state level situated at half the energy of the emitting element. This very particular case can be observed for example for the CS of Tb (the 5D
4 excited state of which is situated at ca 20600 cm-1) by Yb (which has a single 2F5/2
excited state at ca 10200 cm-1 [18]).
The second necessary requirement is that the intermediate excited states must have long lifetimes, so that the excited state can be subject to the process leading to the upper state (second absorption for ESA or energy transfer for ETU) before it decays back to the ground state. These two criteria largely explain why lanthanide (Ln) cations are so frequently implicated in UC materials. The radiative lifetimes of Ln ions, r, i.e. the lifetime of an excited state in absence of non-radiative decay processes, vary from few tens of µs to tens of ms [19]. Providing they are well shielded from non-radiative processes, Ln ions are particularly long-lived and consequently adapted to UC processes. Additionally, the [Xe]4fn (n = 0 to 14) electronic
configuration of Ln ions leads to the presence of (14!/n!(14-n)!) electronic levels and their energy diagrams generally displays a plethora of intermediate steps in the visible and near-infrared (NIR) regions, that is rich ladder like energy levels [20]. The major drawback of Ln ions is that these f-f electronic transitions are for most of them forbidden by selection rules and the corresponding absorption coefficients are very low, resulting in the need for powerful excitation sources to get enough population of the excited states.
Considering that vibrations of molecules in solutions bring far more de-excitation pathways of the excited states than phonons in the solid materials (vide infra), it is foreseeable that the first observations of UC materials were made with solids [8-10,12] and more recently in the two last decades for nanoparticles [21,22], with numerous bioanalytical applications [23]. However, considering the difficult batch to batch reproducibility and potential toxicity of doped nanoparticles [24], there is an evident interest in a possible downsizing of UC materials to the molecular scale with stable and perfectly controlled molecular assemblies.
Solid vs solution: the question of the intermediate excited state lifetime.
of the excited state with the surrounding and the ability of the excited state to transfer part of its energy to the lattice (for solids) or to the ligand and solvent molecules (for complexes in solution). In the solid state, phonons designate quasi particles possessing a quantum of energy corresponding to a vibration of the solid matrix. Interaction of the excited states with phonons leads to energy loss at a rate that was estimated by Englman and Jortner to depend on the energy of the phonons (more generally of the vibrations in their approach) with the relation [27]:
knr = A.exp(-BEem/Ep) (1)
in which knr is the non-radiative rate constant, A and B are constants that depend on the molecule, Eem is the energy difference between the ground and excited state of the molecule and Ep is the energy of the implicated phonon. From equation (1), one can clearly notice a strong exponential dependence of the probability of non-radiative decay with the phonon energy. The smaller the phonon energy, the smaller the non-radiative loss of the excited state. Figure 3 schematizes this principle in the case of the 2F
5/2 → 2F7/2 transition of Yb at ca 10200
cm-1, which would be placed in a matrix of water, deuterated water or a phosphate glass [28].
Yb* (2F 5/2) Ep ≈ 3500 cm-1 Eem= hem ≈ 10200 cm-1 knr H2O D2O Phosphate glass kr Ep ≈ 2500 cm-1 ≈ 1200 cmEp -1 Yb (2F 7/2)
Figure 3. Schematic representation of the non-radiative transition probabilities of the Yb
excited state in water, deuterated water and phosphate glasses.
To illustrate this point, when Yb is doped at one percent in a phosphate glass, the luminescence lifetime was determined to be superior to 1 ms [29], whereas for complexes in water solution, this value rarely exceed few microseconds for the best of them [30], and can be notably improved in D2O [31]. Considering that the Yb centered luminescence quantum yield
Yb is related to the luminescence lifetime, lum, and to the radiative lifetime r by the relation:
and that the estimated radiative lifetime of Yb is 1.449 ms for the phosphate glass and range from 0.5 to 1.3 ms for complexes in solutions [19], one can see that the luminescence efficiency dropped from 76% in the glass to only few percent for the best complexes in solution.
Accordingly, in the case of NIR emitting Ln complexes, the strategy to improve the luminescence quantum yield will be governed by a multistep process: 1) designing polydentate ligands that fulfill the first coordination sphere of the Ln cations [32]; 2) replacing when possible CH, NH and OH bonds by CD, ND and OD ones [33] or even CF bonds [34,35]; introducing large aromatic parts to repel solvent and water molecules as far away as possible from the Ln cation [36]; using low phonon liquids or deuterated solvent [37]; and ultimately, playing on the coordination sphere of Yb to decrease the radiative lifetime [38,39].
The premises of molecular upconversion with lanthanide complexes.
As mentioned above, the solvent plays a critical rule in the case of molecular UC and it was not surprising that the first evidences of a possible observation were looked for in low phonon liquids. These liquids, such as selenium oxychloride (SeOCl2) or phosphorous
oxychloride (POCl3) [40], do not possess high energy vibrations but have dielectric constant
high enough to allow for the solubilisation of Ln salts. By dissolving ErCl3 salts at a
concentration of 0.2 M in a 10:1 mixture of POCl3 and SnCl4 it was possible to observe for the
first time the green emission of the 4S
3/2 → 4I15/2 of Er in solution upon excitation at 800 nm
[41]. Figure 4a represents the UV-Vis absorption spectrum of the solution, on which one can observed the characteristic f-f transitions of Er. Among those, the 4I
15/2 → 4I9/2 absorption band
a)
b)
c)
Figure 4. UV-Vis absorption spectrum (a), green UC spectrum (exc = 800 nm, P = 500 mW, b), and energy level diagram (c) for a 0.2 M solution of ErCl3 dissolved in a POCl3:SnCl4
solution [41]. (Reproduced with permission from the optical society (OSA)).
Upon excitation into this absorption band, the emission spectrum in the visible domain displayed a narrow emission band at 550 nm, which was ascribed to the 4S
3/2 → 4I15/2 emission
band of Er (Figure 4b). Considering the energy level diagram of Er (Figure 4c), it was postulated that the excitation at 800 nm leads to the population of the 4I
9/2 state, which non-radiatively
decays to the 4I
11/2 and/or 4I13/2 states. The absorption of the second photon then allows the
climbing to the 4F
3/2 or 2H11/2 states, which themselves non-radiatively decay to the 4S3/2 state.
This state can then radiatively decay to the ground state with emission of green photons at 550 nm. The evidence of an upconversion process was found in the plot representing the logarithmic values of the emitted intensity I as a function of the logarithmic values of the incident pump power P (Log/Log plot). In UC process, I is directly proportional to the nth power of P , where
In 2002, a deep investigation of the spectroscopic properties of Er, Tm and Yb complexes of dipicolinic acid (DPA) was run to investigate the possible UC in such complexes [43]. Unfortunately, it appeared to be impossible to observe any UC in these molecules, presumably because the non-radiative processes were too efficient. Despite this failure, the same ligand, dipicolinic acid, together with EDTA (ethylenediamine tetraacetic acid) and DTPA (diethylenetriamine pentaacetic acid), were used in a subsequent study with Nd, Er and Tm salts in water and D2O [44]. The complexes were formed with Ln/ligand stoichiometric
coefficients of 1/3; 1/2 and 1/1 respectively for DPA, EDTA and DTPA. Instead of exciting at a single wavelength, the authors used a combination of two synchronized lasers at two different tunable wavelengths. Such a combination allowed to tune the energy of the photons for the successive steps of an ESA process as the climbing may involve different energy differences for the two successive steps. So doing, UC could be observed for [Nd(EDTA)2], [Er(DPA)3]
and [Tm(DPA)3] in water and D2O, even if the exact solvent was never explicitly mentioned in
the description of the measurements. Noteworthy, the case of Nd salts was particularly enthusiastic, displaying three bands in the UV region, while Tm and Er were shown to emit at 480 and 550 nm respectively. It should however be mentioned that the LASER used in the experiment were particularly powerful with peak LASER powers of near 100 kW focused on a
ca 100 µm spot with concentrated solutions (20 mM in Ln). Despite the unclear composition
of the solvent (water or D2O), these results were the first observation of UC on Ln compounds
in aqueous solutions at room temperature.
In similar studies performed in d6-DMSO on different Ln salts, Sørensen, Faulkner and coworkers demonstrated that a highly focused laser could engender two photon excitation of Sm, Eu and Tb at 10 mM concentrations [45]. When studying the case of Tm salts of triflate, they could notice the observation of an emission band at 476 nm (21000 cm-1, 1G
4 → 3H6
transition of Tm) upon excitation at 790 nm (12650 cm-1). This observation was clearly not
related to two photon excitation and was ascribed to an ESA process in Tm [46].
into the dye absorption band at 808 nm, the visible part of the emission spectrum displayed two narrow emission peaks and a shoulder centered around 540 nm (Figure 5c), which were ascribed to the 2H
11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of Er. The LogI/LogP plot of this emission
band clearly showed a slope of 1.87, close to the expected value of 2 for a biphotonic process. This example is the first and lonely one in which UC photosensitization is obtain through the excitation of ligand absorption bands. However, the proposed mechanism for UC is not that clear, postulating successive energy transfer steps to the Er levels from the triplet state of the ligand. Assuming that the triplet state of the IR-806 dye is situated around 900 nm, as assumed by the authors, a triplet-triplet annihilation process (also called TTA in the literature [49], but not to be confused with the TTA ligand) of the IR-806 sensitizer should lead to the singlet excitation of the TTA ligand, which was shown to be excitable above 450 nm in Eu complexes [50].
a) b)
c) d)
Figure 5. a) Chemical structure of the [Er(TTA)4](IR-806) complex. b) absorption (black) and
emission spectrum of IR-806 (red) and fluorescence of the [Er(TTA)4](IR-806) complex (blue,
CDCl3, exc = 808 nm). c) UC emission spectra of the [Er(TTA)4](IR-806) complex (black
the UC process [48]. (Reprinted with permission from I. Hyppänen, S. Lahtinen, T. Ääritalo, J. Mäkelä, J. Kankare, T. Soukka, ACS Photonics 1 (2014) 394–397. Copyright (2019) American Chemical Society).
Considering the potential of Er for UC processes, Piguet and co-workers have recently reported the preparation of complexes of Er chelated by three tridentate ligands (Figure 6a). The complexation allows to partly protect the Er atom from non-radiative processes originating from solvent molecules which are repelled from the Er centers by the large aromatic parts of the ligands. The protection of the Er cation allowed the authors to evidence green ESA from Er upon excitation at 801 nm at room temperature in the solid state [51], and more recently in acetonitrile solution at a concentration of 10 mM [52], close to those used for Ln salts [46] but with a more reasonable pump intensity of 21 W.cm-2. The UC quantum efficiencies for these
systems were reported to vary from 3.9×10-9 (Ligand L
2) to 1.6×10-8 (Ligand L1) in the solid
state. b) N N N N N N N N R R Er(CF3SO3)3 [Er(L1)3](CF3SO3)3 L1 R = H, Me and Et L2, L3and L4 a)
Figure 6. Aromatic tridentate ligands for the formation of [ErL3] complexes (a) [51,52] and
X-ray crystal structure of the [YbEr(HIP)6(DME)2] dimers (b) [55].
to decrease the concentration of active luminescent centers in order to avoid concentration quenching effects. By preparing films of different compositions with mixtures of Yb and Tb or Yb and Eu, excitation into the Yb absorption bands at 980 nm led to the observation of the green emission of Tb (5D
4 → 7F5 transition at 545 nm) or the red (5D0 → 7F2 transition) emission
of Eu. These two cases are very particular mechanisms of UC as for both Tb and Eu there doesn’t exist any intermediate excited states corresponding to the energy of the Yb levels that should be populated by energy transfer. The mechanism is typically a cooperative sensitization in which the energy of two excited Yb centers is simultaneously transferred to one Tb or one Eu. Such a mechanism will be described in more details in the next section.
Regarding the particular case of Er, Balashova and coworkers reported a series of Ln dimers of general formula [Ln2L6], where the ligands L are
3-(2-benzothiazol-2-yl)-2-naphtholate, pentafluorophenolate and hexafluoro-iso-propoxide (HIP) [55]. For HIP, the dimers obtained showed a very close intermetallic distance of 3.74 Å, the two Ln cations being bridged by two HIP anions (Figure 6b). Although the authors were targeting the absence of CH bonds in the vicinity of the cations, they could not avoid a molecule of dimethoxyethane to coordinate to each Ln. In the heterodimer obtained from 67% of Yb and 33% of Er, excitation at 980 nm on the solid at r.t. resulted in the observation of green upconversion attributed to the Er emission band (2H
11/2 → 4I15/2 and 4S3/2 → 4I15/2). Interestingly, this UC emission is absent
in the pure Yb and Er homodimers, which clearly points to an assistance of the Yb cation in the UC process. This point is important as Er possess an absorption band at ca 980 nm (4I
11/2 → 4I
15/2) and one might have postulated direct ESA to occur in the Er dimer (see next paragraph).
The absence of UC in the Er dimer thus points to the Yb acting as a tank to feed the Er excited states, the larger absorption cross section of the Yb 2F
5/2 → 2F7/2 absorption band (up to few M -1.cm-1 units [56]) potentially compensating the weak absorption of Er at 980 nm.
When supramolecular chemistry comes into the game…
related with the presence of what Auzel called “the clustering effect” [57]. He defined two kinds of clusters: interactions clusters, inherent of the doping of the material and with long range (up to µm) effects; and “chemical clusters” combining luminescent centers (donors or acceptors) into small (few Å) chemical entities, the later arising from the synthetic protocol used for the preparation of the material. The short intermetallic distances in these last kind of clusters can tremendously increase ion-ion interactions with large effects on energy transfer migration processes of all kinds (energy transfer, self-quenching…). At the molecular level, the issue of gathering multiple donors around an acceptor can be addressed by a careful design of the ligand or by the introduction of secondary interactions affording the possibility to control the complex architecture. The comparison of UC probabilities in molecular systems composed of a mononuclear Ln acceptor displaying ESA, a heterodinuclear donor-acceptor system based on ETU and a heterotrinuclear system with two donors around an acceptor was theoretically described by Piguet and coworkers, confirming the initial assumptions of Auzel [58].
Concerning the second point, the improvement of energy transfer efficiency between the energy donor (or sensitizer) and the energy acceptor, WSA, the theory developed by Förster for non-radiative dipole-dipole energy transfer [59] evidenced that the efficiency can be described by the following equation:
(2) 𝑊𝑆𝐴= 𝑅 6 0 𝑅6 0+ 𝑟60
In which R0 is the Förster distance (or Förster radius or critical radius) at which the chance of radiative decay and of energy transfer are equiprobable, and r is the distance between the donor and the acceptor [60,61].It clearly appears that the choice of the donor/acceptor pair is critical, and that the distance between them should be the shortest possible, R0 values rarely exceeding few Å for pairs of Ln cations [62].
Supramolecular assemblies for controlled heteropolynuclear UC complexes.
supramolecular [Cr2LnL3]7+ heterotrinuclear triple helicate was formed by a self-assembly
process. At that stage, oxygen bubbling into the solution led to the oxidation of the labile Cr(II) into inert Cr(III) thereby freezing the helical architecture [65]. The choice of Cr(III) was not only dictated by the induced inertness of the helicate upon oxidation of Cr(II), but also because the as induced ligand field around Cr(III) resulted in the presence of d-d transitions in the red region, with emission of a narrow band at 747 nm upon ligand excitation, resulting from the 2E
→ 4A
2 transition of Cr (Figure 7c). Selective excitation into this transition with a tunable
Ti-sapphire laser resulted in the observation of green emission from the 4S
3/2 → 4I15/2 transition of
Er at 452 nm. The LogI/LogP plot confirmed the quadratic dependence of the emission intensity with the laser power, ascertaining the UC process. The excitation into the d-d transition of Cr resulted into energy transfer to the Er which can climbe the successive levels up to the emitting
4S
3/2 level. This is the very first example of molecular UC with an ETU mechanism. The only
drawback of the system was the rather poor energy transfer efficiency, which may be related to a large intermetallic Cr-Er distance (8.8 Å on average), necessitating the observation of the UC in frozen solutions at very low temperatures (30.6 K in CH3CN). The thermodynamic, kinetic
N N N N N N N N N N N
1) Ln(CF
3SO
3)
3Cr(CF
3SO
3)
22) O
2a)
b)
c)
Figure 7. a) Heterotritopic ligand developed by Piguet and coworkers for the formation of
heteropolynuclear [ErCr2L3]9+ triple helicates. b) UC emission spectrum of a 10 mM solution
of the helicate in CH3CN at 30.6 K upon excitation at 748 nm and c) Jablonski energy diagram
for the UC excitation of Er by Cr. (Reproduced with permission from reference [67], copyright Elsevier).
complexes were forming dimers in the solid state in which two Ln cations were bridged by a fluoride anion [69]. The supramolecular organization was guided by the concomitant presence of the fluoride bridge, aromatic stacking interactions between the two complexes and hydrogen bonds between the fluoride bridge and some hydrogen atoms of the ligands. However, the presence of the dimer could only be observed in the solid state, dissolution in water solutions leading to the disassembling of the dimers. Taking advantage of these findings, further engineering of the ligand structure was engaged to reinforce the different secondary interactions in solution, leading to the design of an indazolyl based ligand that strongly encapsulated the Ln cations and led to the formation of Ln dimers in aqueous solutions in the presence of F- anions
(Figure 8a, [70]) which could be used for fluoride sensing in solution. Noteworthy, the intermetallic Ln distance was found to be very short, amounting to less than 4.5 Å as evidenced by X-ray crystallography of the Yb dimer, allowing for Tb to Eu energy transfer in the case of the corresponding heterodimer [71]. Such a short distance is particularly appealing for possible Ln based energy transfer in the case of an ETU process.
Figure 8. a) Structure of the ligand and complexes for the formation of fluoride dimers in
aqueous solutions. b) UC visible emission spectra (D2O, exc = 980 nm, P = 5 W) of the [ErL(D2O)]+ complex without fluoride (red) and in the presence of 0.5 equivalent of fluoride
anions (Inset: evolution of the UC intensity as a function of the quantity of fluoride anions). c) LogI/LogP plot of the Er dimer in D2O [72].
Noteworthy, the case of the Er complex was particularly intriguing. In D2O, the
spectroscopic properties were particularly interesting as the Er complex was shown to be a rare case of Er luminescent complex [72]. Excitation into the ligand absorption bands resulted in the observation of ligand centered emission, together with admixture of Er based f-f transitions observable in the visible (at 550 nm, 4S
3/2 → 4I15/2 transition of Er) and NIR (at 980 nm for the 4I
11/2 → 4I15/2 and around 1550 nm for the 4I13/2 → 4I15/2 transitions of Er). Upon excitation of
the Er monomer in D2O at 980 nm, the visible emission spectrum displayed some visible
emission bands typical of the Er emission at 525 nm (2H
11/2 → 4I15/2 transition of Er), 550 nm
(4S
3/2 → 4I15/2 transition of Er) and 650 nm (4F9/2 → 4I15/2 transition of Er, Figure 8b, red
spectrum). The observation of this UC is to be related to an ESA mechanism in the Er monomer. More interesting was the evolution of the UC emission when fluoride anions were added to the solution. The UC dramatically increased up to 0.5 equivalent with a 7.7 fold improvement and slowly decreased for larger excesses, as a result of the formation of a [ErLF] monomer. The UC mechanism was unambiguously demonstrated by a slope of 2.1 of the LogI/LogP plot. The mechanism for UC in the dimer is still not clear as the overall luminescence quantum yield of the complex (obtained upon excitation into the ligand centered band with emission in the NIR region) only displayed a two fold increase. Assuming the most favorable option in which this increase is entirely due to an increase of the Er centered luminescence (no change of the sensitization efficiency of the ligand), one would expect a four (22) fold increase of the UC
quantum yield based on an ESA mechanism in the dimer, which is obviously smaller than the almost height fold experimentally observed. On this basis, it is not unrealistic to propose that the exact mechanism may be a mixture of ESA and ETU, although time resolved experiments were unsuccessful to support this assumption.
These two first examples revealed that coordination chemistry and supramolecular chemistry are very promising tools for the construction of polynuclear and heteropolynuclear Ln assemblies containing multiple donor atoms and an energy accepting one. However, for heteropolynuclear complexes, the design of such edifices is confronted to a severe constraint: the similar chemical properties of the Ln3+ cations along the series. Considering that the
coordination chemistry of Ln cations is mainly directed by a subtle equilibrium between electrostatic interactions, bulkiness of the ligands and hardness/softness of the coordinating atoms [73], Ln do not display stereoelectronic preferences and the only differences arose from the change in ionic radius along the series, i.e. the lanthanide contraction [74]. It results from these very similar coordination behaviors along the series that the thermodynamic parameters governing coordination such as the stability constant for the formation of LnL complexes with a ligand L are generally also very similar.
A consequence of these similarities is that the production of pure heteropolynuclear complexes is generally very difficult when not impossible. Let’s imagine a ditopic ligand possessing two identical coordination sites for the complexation of two Ln cations, each site being unaffected by the coordination or not at the other one. The coordination with two distinct Ln cation (Ln1 and Ln2) will lead to a statistical formation of 25% of each homodimers ([Ln1
2L]
and [Ln2
2L]) and only 50% of the heterodimer [Ln1Ln2L]. One way to displace this equilibrium
is to tune the two coordination sites for each of the Ln, but as mentioned above, the differences in stability are generally very modest and this strategy proved to be rather unsuccessful up to now, except for the construction of some heterodinuclear helicates [75]. Even in these cases, the best performances were for the formation of 90% of a pure heterodinuclear complex of La and Lu, the two extremes in the Ln series, while the deviation from statistics were very modest for neighboring cations. It should however be noted that where elegant and sophisticated synthetic strategies hardly meets the requirement for the production of pure heteropolynuclear complexes, serendipity sometimes opens new perspectives, as it was the case for simpler ditopic ligands which were shown to produce almost pure heterodinuclear complexes in the solid state [76,77]. In these cases, coordination of the first Ln has an impact on the entry of the second one, allowing for a subtle size discrimination.
favorable, being complicated by the localization of the cations for low symmetry complexes (a [Ln1Ln2Ln1L] complex probably responding differently from a [Ln1Ln1Ln2L] one).
To circumvent the failure of the thermodynamic approach, some authors developed approaches based on a kinetic control of the assemblies. Figure 9 schematized the different possibilities which will be further exemplified in the text. The first strategy, illustrated by an example developed by Faulkner and coworkers, rely on the step by step preparation of the coordinating site (Figure 10a, [78]). An orthogonal protection of the coordination sites allows for the freeing of a first one, complexation of the first lanthanide, deprotection of the second site and complexation of the second lanthanide cation. This achievement is only possible thanks to a thermodynamic and kinetic inertness of the first complexes formed and the synthetic strategy has to take seriously this point into account. In the example of Figure 10, one can easily imagine that the final complex would probably suffer transmetallation in acidic conditions, the scrambling of the cations finally resulting in the undesired statistical mixture.
1)
2)
3)
4)
deprotection
Figure 9. Different strategies developed for the formation of heteropolynuclear complexes by
1) step by step freeing of the coordination sites [78]; 2) Thermodynamically induced site selective coordination [79]; 3) chemical bonding of inert Ln complexes [81]; and 4) coordination to a preformed complex [86,87].
gathered a very stable DOTA like coordination sites (DOTA = 1,4,7,10-tetraaza-cyclododecane tetraacetic acid) with a DTPA one (diethylene triamine pentaacetic acid) linked by a probe, a tryptophan residue in their case (Figure 10b). Complexation of an excess of Ln(III) cation first led to the formation of the homodinuclear Ln complex, but the acidic conditions used for the chromatography purification of the complex revealed the kinetic lability of the DTPA site, with the release of the Ln from this site. The kinetically stable mononuclear complex thereby obtained could be complexed with a different Ln cation to afford the pure heterodinuclear species. The same strategy was employed to produced heterotetranuclear complexes of [Eu(GdL)3] general formula [80].
a)
b)
+
c)
N N N N O O O O O O HN O NH NH O NH2 O N H O N N N O O O O O O O O LnB LnA 1) TFA ; Ln1 2) NaOH ; Ln2 Ou 1) NaOH , Ln1 2) TFA; Ln2 or N N N N O EtO O EtO O OEt NH O N N N N O tBu-O O tBu-O O O-tBu NH O N N N N O O O O O O NH O Ln1 N N N N O O O O O O NH O Ln2 N N N N O O O O O O Ln2 Ln2 O O O O O O N N N N HO N N N N O O O O O O Ln 2 Ln2 O O O O O O N N N N HO N N N N O O O O O O Ln1 N N N N N O O O O O O Ln1 N N N N N O O O O O O Ln1 H2N N N N N O O O O O O Ln1Figure 10. Representation of the synthetic approaches towards heteropolynuclear lanthanide
complexes. a) step by step freeing of the coordination sites [78]. b) Thermodynamically induced site selective coordination [79]. c) chemical bonding of inert Ln complexes [81].
complex was oxidized with sodium nitrite to generate a diazonium cation which reacted with the phenol based dinuclear complex to produce the tetranuclear species. Having two different Ln cations in the phenol and aniline based dinuclear complexes allows for the preparation of heterotetranuclear species of [Ln1
2Ln22L] formula, with Ln1 and Ln2 being Yb and Nd or the
reverse. This strategy was also used to produce heterodinuclear complexes of Y and Gd (here Y is abusively included in the lanthanide series) [82], or of Eu and Tb [83], heterotrinuclear complexes of Tb and Yb [84], and heterotrimetalic tetranuclear Ln complexes [85].
A last approach is to use a firstly formed Ln complex as a template for the preparation of larger polynuclear species. This strategy is illustrated in Figure 11 and was utilized by different groups to form different species such as heterotetranuclear Yb2Er2 and EuTb3 or Eu3Tb
compounds [86], heterodinuclear Eu6Lu heptameric wheels [87], or heteropolynuclear tri-,
tetra- or pentanuclear complexes [88]. This strategy is particularly appealing as it often relies on the bridging of different Ln complexes by phenoxo, carboxylate or phosphonate functions, with concomitantly very short intermetallic distances propitious to efficient energy transfer, but it is also endowed with a large part of serendipity.
6 [EuL2](Tf) 1 [Lu(Tf)3] N N -O O N L = a) b)
Figure 11. Examples of strategies for the formation of (a) heterotetranuclear Tb2Er2 complexes
[86] and (b) heptameric LuEu6 wheels [87] by coordination to a preformed complex. (Reprinted
Phosphonated ligands, such as diphosphonic acids, have been involved in the formation of supramolecular coordination networks for a long time, revealing an amazing structural diversity of 1D, 2D and 3D frameworks [89]. This attribute is mainly due to their versatile binding ability, in a monodentate, bidentate or bridging coordination mode, and to their capacity to promote strong electrostatic interactions with cations. Moreover, they are hard Lewis bases, and as a consequence, are excellent chelators for lanthanide cations [90]. As a consequence, we turned towards the synthesis of a family of bis- tris- and tetra- phosphonated ligands with imino(methanephosphonate) units, such as ligands L1 to L6 [56, 91-94](Figure 12).
N N N PO3H2 H2O3P L1 N N N PO3H2 PO3H2 H2O3P L2 N N N PO3H2 PO3H2 H2O3P H2O3P L3 N N N N PO3H2 H2O3P PO3H2 H2O3P L4 N N N N PO3H2 H2O3P PO3H2 H2O3P L4D D D D D D D D D DD N N N N N N PO3H2 H2O3P PO3H2 L5 N N N N PO3H2 H2O3P PO3H2 L6
Figure 12. Structure of phosphonated ligands developed in our group. [56, 91-94]
For all ligands, mononuclear complexes were isolated when using equimolar concentrations of lanthanide salts and ligand. The increase of the number of phosphonate functions was shown to significantly increase the stability of these [LnL] complexes and their corresponding thermodynamic stability constants follow a monotonous trend from log βLaL1= 10.14(4) for the
pentadentate ligand L1, to log β
LaL2= 16.3(1) for hexadentate L2 and finally log βLaL3= 25.5(4)
for heptadentate L3 [91]. This last complex is extremely stable, as it can be judged by its pM =
20.6 (pM = -Log[Mfree], pH = 7, [Ln3+] = 1 μM, [L3] = 10 μM), which is similar to macrocyclic
of coordination bonds and electrostatic interaction with lanthanide cations and negatively charged complexes. The formation of discrete supramolecular complexes such as [Eu3L32] was
evidenced by ESI mass spectrometry while following the addition of Eu3+ in a ligand solution.
Similar information could also be deducted when following the evolution of the absorption and emission spectra of the ligands as a function of added cations [91]. In the light of these very interesting properties, it was thus though that heteronuclear supramolecular assemblies, and more particularly containing Yb3+ and Tb3+, could be obtained through stepwise addition of
Yb3+ complex and TbCl
3 salt. Nevertheless, in view of the application as luminescent complexes
for upconversion, it was also foreseen that such scaffold would not be optimal since the coordination sphere of the mononuclear complexes encompasses at least one water molecule (in the case of L3) and up to two (in the case of L1) [91].
A significant increase of the luminescent properties and in particular the lifetime of the Yb3+
complex was expected from the replacement of the central pyridine by a bipyridine backbone such as in ligand L4 (Figure 12). This was experimentally confirmed by the synthesis of the
[YbL4] complex. When exiting the bipyridyl antenna (at 310 nm), characteristic Yb3+ emission
is measured (H2O = 0.16%, D2O = 1.3%) and the luminescence lifetimes confirm the absence of metal-coordinated water molecules [56]. It is to be noticed that, despite the lack of inner-sphere water molecule, a significant increase of the luminescent lifetime is observed when replacing water (H2O = 2.3 μs) by heavy water (D2O = 35 μs). This was ascribed to the strong contribution of the second hydration sphere in the non-radiative deactivation processes. This phenomenon is not surprising as the second sphere mechanism was found to be almost as important as the inner-sphere contribution while finding 1H NMRD profile measured for
[GdL1(H
2O)] (q= 1 , q2nd = 4) [91]. Furthermore, considering the low excited state of Yb3+, the
influence of C-H oscillators in close proximity cannot be neglected [38]. Therefore, a deuterated analogue L4 , L4D (Figure 12) was synthesized in which all methylenic positions were
deuterated. Deuteration had a strong influence on both the excited state lifetime (D2O = 65 μs) and quantum yield (D2O = 3.8 %) of the complex. Noteworthy, this lifetime range [YbL4D]
Indeed, the formation of species with [(LnL4)
2Lnx] (x = 1 - 3 ) stoichiometry was observed in
the presence of more than one equivalent of Ln3+ salt. This observation is clearly corroborated
by the follow-up of the addition of LnCl3 into a ligand stock solution by luminescence
spectroscopy or ESI mass spectroscopy [56]. DFT modelling suggested that the mononuclear [YbL] complex is octacoordinated by the ligand and that the distribution of the negative charge potentials is highly localized on the phosphonate functions, therefore promoting strong electrostatic interactions between the complex and additional Ln3+ cations, hence leading to the
formation of polynuclear species. Trinuclear, tetranuclear and pentanuclear assemblies were also modelled and their calculated structures are displayed in Figure 13. In all cases, the two first Yb3+ ions are octacoordinated by the ligand and their coordination spheres do not include
any water molecule. Additional Yb3+ ionsare sandwiched between the two [YbL4] complexes
Figure 13. DFT models of the trinuclear ([(YbL4)
2Yb; a,b], tetranuclear ([(YbL4)2Yb2]) and
pentanuclear ([(YbL4)
2Yb3]; e,f) complexes viewed along (a, c, e) and perpendicular to (b, d, f)
the Yb1-Yb2 axis. H atoms are omitted for the sake of clarity except for water molecules [56]. Moreover, the kinetic inertness of the assembly was also checked, demonstrating the absence of exchange between metal ions in the ligand cavity (Yb3+) and in the sandwich (Tb3+) [56]. It
was foreseen that heterometallic structure of similar stoichiometry and with similar structures might be obtained by stepwise addition of TbCl3 on a [YbL4] complex [93].
Because of the close proximity of two Yb3+ centers around one or more Tb3+ ion and of
the excellent spectroscopic properties of the [YbL4D] complexes, such assemblies are excellent
candidates to probe their upconversion emission properties, when exciting into the 2F
5/2 → 2F7/2
transition of Yb3+ at 980 nm. The evolution of the visible emission spectra of a solution of
[YbL4D] upon addition of TbCl
3.6H2O (D2O, pD =7) when exciting at 980 nm (P = 1.0 W) is
displayed in Figure 14. Typical Tb3+ emission spectra were observed, hence proving that
upconversion emission processes occur with these heteropolynuclear assemblies. In addition, the [(YbL4D)
2Tb2] species was found to be the most efficient.
Figure 14. Evolution of the visible emission spectra of a solution of [YbL5] upon addition of
TbCl3.6H2O (D2O, pD = 7.0, TRIS/DCl), when exciting at 980 nm (P = 1.0 W) [56]. Inset:
evolution of the intensity at 545 nm as a function of the [Tb]/[YbL4D] ratio.
Yb* (GSA), (ii) absorption of the second Yb3+ of the same molecule (Yb*Tb
xYb*, x = 1 to 3)
depending on the species considered) and finally (iii) intramolecular energy transfer (ETU) between two Yb* ions to Tb3+ ion to reach Tb* excited state (YbTb*Tb
yYb, y = 0 to 2).
G
S
A
G
S
A
Yb*TbYb*
Yb*TbYb
E
T
U
YbTb*Yb
U
C
2F
7/2 7F
6 7F
0 2F
5/2 5D
4E
ne
rg
y
/c
m
-1 0 5000 10000 15000 20000 25000Figure 15. Principle of cooperative UC photosensitization of Tb by Yb (GSA = Ground State
Absorption; ETU = Energy Transfert Upconversion; UC = Upconversion).
Similar studies have been performed with ligand L6 (Figure 12), another phosphonated ligand
based on the 1,4,7-triazacyclononane (tacn) ring [98]. In line with previous research on picolinate derivatives [99,100] the use of pyridyl phosphonate gives rise to the formation of highly stable nonacoordinated mononuclear [LnL6] complexes with C
3 symmetry (Figure 16).
a
b
Figure 16. X-ray crystal structure of the [YbL6] complex viewed along (a) and perpendicular
Theses complexes benefit from an excellent shielding of the Ln cation from the solvent molecules (q = 0 for Ln = Yb, Tb, Eu, Lu) and therefore, rather long-lived excited states (H2O = 2.8 μs, D2O = 10.2 μs for Ln = Yb). As a comparison, a radiative lifetime of the [YbL6] complex
of 1.36 ms was determined from the measurements of the electronic absorption spectrum in D2O. Finally, the negative charges of the phosphonate functions and their second sphere
interactions can be used to drive the construction of higher order polynuclear and heteropolynucler species of [(LnL6)
2Lnx] (x = 1 and 2) [98,101]. These supramolecular
assemblies have been observed in solution by concomitant analysis of UV-absorption spectrometry, spectrofluorimetry, 1H and 31P NMR, DOSY and ESI-mass spectrometry. The
absence of intramolecular exchange or decomposition was also confirmed by NMR experiments [98]. In terms of structural parameters, it is anticipated that intermetallic distances are sufficiently short to promote energy transfer processes. Interestingly, single crystals of the [La(L6)La(H
2O)9] were obtained upon cooling an aqueous solution containing ligand L6 and
1.5 equivalent of LaCl3. The crystal structure of this ion-pair complex (Figure 17) indicates a
short LaL6-La(H
2O)9 distance of 5.7 Å due to the presence of a strong hydrogen bonding
network (9 H-bonds) between water molecules and phosphonate groups. In addition, weak interactions are also observed between [La(H2O)9] and the proton atoms of the tacn moiety of
another [LaL6] complex, creating a repetition of the [(LaL)La(H
2O)9] pattern with La(H2O)9
-(LaL6) distances of 8.5 Å).
The ultimate proof of the formation of these assemblies with short intermetallic distances, relies in the observation of Tb3+ upconversion emission, when exciting a solution of [YbL6] at 980
nm (P = 1.08 W), in D2O (pD = 7.1), upon addition of aliquots of TbCl3 (Figure 18). In these
conditions, the [(LnL6)
2Ln2] species was again found to be the most efficient.
450 500 550 600 650 700 Iem / a rb . u ni t λ / nm 5D 4→ 7F6 5D 4→ 7F5 5D 4→ 7F4 5D 4→ 7F3 0 0,5 1 1,5 2 Iem / a rb . u ni t [Tb]/[YbL]
Figure 18. Emission spectrum measured upon addition of 1.0 equivalent of TbCl3.6H2O to a
1.25 mM solution of (YbL) in D2O at pD 7.1 (exc = 980 nm, P980 = 1.08 W). Inset: Evolution of the emission intensity as a function of the [Tb]/[YbL6] ratio [101].
Figure 19. UC emission spectrum of an 11 mM solution of [YbL6] in the presence of one
equivalent of TbCl3.6H2O upon excitation at 980 nm (P = 1.08 W) [101].
In order to further elucidate the upconversion energy transfer mechanism, time-resolved luminescence measurement were performed on solutions of [YbL6] containing 0.5 and 1
equivalents of TbCl3, respectively. In both cases (see Figure 20 for 0.5 eq), the rise of the Tb
emission signal is slow (few milliseconds), which is typical of a slow kinetic step related to the energy transfer occurring in a cooperative energy transfer mechanism. Based on the assumption that the [(YbL6)
2Tb2] is the most efficient, as deduced from upconversion luminescence
titrations, a cooperative photosensitization mechanism can be postulated and is represented on Figure 20. From this model, the evolution of the populations of the different states could be calculated, from which the rate of the energy transfer step (kUC) can be deduced. The efficiency
of the upconversion energy transfer can then be calculated from Equation 3, which in good agreement with the value determined from the variation of the lifetime of the Yb donor in presence or in absence of a Tb acceptor (ηUC = 2.5 ± 0.2 %, Equation 4). As a comparison with
other upconverting systems, ηUC = 3.2% was previously measured for triplet-triplet-annihilation
where kUC represents the UC energy transfer rate, kr the radiative rate constant of the Yb complexe, knr the non-radiative rate constant knr, τYbTb the Yb lifetime in presence of Tb and τYb, the Yb lifetime in absence of transfer.
0,0001 0,001 0,01 0,1 1 0 0,005 0,01 0,015 0,02 0,025 0,03 0,035 Laser off Laser on 0 YbTb1Tb2Yb kexc kexc 1/Yb 1/Yb 1/Tb1 kUC1 1 2 3 Yb*Tb1Tb2Yb
Yb*Tb1Tb2Yb* YbTb1*Tb2Yb
1/Tb2
4 YbTb
1Tb2*Yb kUC2
Figure 20. Evolution with time of the Tb centered UC emission intensity at 550 nm (in blue)
upon time gated excitation at 975 nm (schematized in pink) for a 1.53 mM solution of [YbL6]
in D2O (pD = 9.2) containing 0.5 equivalent of TbCl3.6H2O. The green curve represents the fit
of the data according to a two Tb sites model presented on the right [101].
Conclusion
The recent decade has seen the emergence of the possibility to create UC phenomenon at the molecular level with discrete molecular entities in which Ln cations are often playing a major role. Because of their outstanding spectroscopic properties, such as long lived excited state and ladder like energy levels, these elements are particularly appealing for the piling up of photons. However, the design of efficient Ln based UC molecular entities is still confronted to major drawbacks such as the difficulty to control the formation of polynuclear entities and the deleterious effects of second sphere solvent molecules on the de-excitation of the excited states.
understanding of the molecular UC phenomena [58], will allow for further advances in the efficiency of the assemblies, potentially leading to molecular probes with practical applications, which would alleviate the actual inconvenients of currently used nanoparticles (batch to batch reproducibility, toxicity,…). The field is still in its infancy and it is to be bet that the inventiveness, creativity or luck of molecule makers will be challenged for the design of ever better UC molecular systems.
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Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Upconversion of light with molecular and supramolecular lanthanide
complexes
Aline M. Nonat and Loïc J. Charbonnière*
Equipe de synthèse pour l’analyse (SynPA), IPHC, UMR 7178, CNRS/Université de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex, France. E-mail :
l.charbonn@unistra.fr
Abstract. Upconversion (UC) is the process by which the energy of multiple photons is
absorbed by a compound and restored in the form of a photon of higher energy than the incident light, resulting in an anti-Stokes process. Although studied theoretically since the middle of the last century and experimentally observed in the 1960’s, the process was up to recently mainly restricted to solid state devices and ultimately to nanoparticles at the end of the century. At the same period, different researches were directed towards the possibility to observe UC at the molecular level and it is only recently that the phenomenon could be observed in discrete molecular entities in solution with still very few examples. This review aims at explaining the difficulties encountered at the molecular level compared to the solid state and summarizes the results reported to date on UC at the molecular scale.
Introduction
was deeply studied by George Stokes [1], who investigated numerous luminescent solutions, among which quinine sulfate, and the loss of energy resulting in an increased emission wavelength compared to the absorption one was dedicatedly called Stokes shift.
In rarer cases, the compound in its excited state can absorb the energy of a second photon, reaching a higher excited state (L**, Figure 1). If this energy is released in the form of a photon, the luminescence occurs at a lower wavelength than the incident one, resulting in an anti-Stokes process called upconversion (UC) [2].
200 400 600 800 1000 Wavelengths ( / nm) Excitation Photoluminescence (Stokes process) Upconversion (Anti-Stokes process)
b)
L L* L’ L**a)
h
UC h
lum h
exc h
excFigure 1. a) schematic representation of the phenomenon of photoluminescence and
upconversion. b) Wavelength dependence of the luminescence phenomena.
The UC mechanisms can be divided into two main categories. The first one entails phenomena related to non-linear processes such as two-photon absorption excitation, second harmonic generation and cooperative luminescence [2]. These phenomena were theoretically studied by Maria Göppert-Meyer, who developed the concept of two photon absorption as soon as 1931 [3], concept experimentally demonstrated thirty years later with the observations of second harmonic generation [4] and two photon fluorescence [5], thanks to the income of high energy excitation sources, the MASERs (Microwave Amplification by Stimulated Emission of Radiations) [6]. However, these mechanisms are based on virtual excited states and are generally weakly efficient.
were first postulated by Nicolaas Bloembergen in 1959 [7], who proposed the concept of quantum counters, in which a first excited state is reached by absorption of an infrared photon, followed by the optical pumping to a higher energy excited state. Figure 2 summarizes these mechanisms: excited state absorption (ESA); energy transfer UC (ETU) and cooperative sensitization (CS).
CS
ESA
ETU
Figure 2. UC processes based on successive linear optical processes: a) ESA; b) ETU;
and c) CS (color code: blue = absorption, red = energy transfer and green = emission).
ESA consists in the absorption of a first photon leading to an intermediate excited state, which consequently absorbs a second photon to reach the emitting UC luminescent level. It was first reported in 1961, in crystals of La:PrCl3, upon excitation at two distinct wavelengths [8].
In ETU, the absorption of the second photon occurs on another ion in the material and an energy transfer occurs between the two ions to feed the upper excited state. This mechanism was first reported by François Auzel in two distinct papers [9,10]. This supplementary step is the hallmark of ETU and is at the basis of the distinction between ESA and ETU mechanisms. When recording time-resolved UC intensity upon pulsed excitation, ETU is characterized by a slow rising edge attributed to the supplementary energy transfer step [11]. Finally, a last mechanism is the CS in which two donor atoms transfer their energy to a third one which emits light [12], experimentally observed in 1969 in Tb and Yb doped crystals of CaF2 and SrF2 [13].
These different processes have been described in details in the relevant reviews of the literature [2,14-16]. It should be noted that another UC mechanism exists called photon avalanche [17], but it will not be detailed here as it hasn’t been observed for molecular systems up to now.
mandatory as the two donors should have a single excited state level situated at half the energy of the emitting element. This very particular case can be observed for example for the CS of Tb (the 5D
4 excited state of which is situated at ca 20600 cm-1) by Yb (which has a single 2F5/2
excited state at ca 10200 cm-1 [18]).
The second necessary requirement is that the intermediate excited states must have long lifetimes, so that the excited state can be subject to the process leading to the upper state (second absorption for ESA or energy transfer for ETU) before it decays back to the ground state. These two criteria largely explain why lanthanide (Ln) cations are so frequently implicated in UC materials. The radiative lifetimes of Ln ions, r, i.e. the lifetime of an excited state in absence of non-radiative decay processes, vary from few tens of µs to tens of ms [19]. Providing they are well shielded from non-radiative processes, Ln ions are particularly long-lived and consequently adapted to UC processes. Additionally, the [Xe]4fn (n = 0 to 14) electronic
configuration of Ln ions leads to the presence of (14!/n!(14-n)!) electronic levels and their energy diagrams generally displays a plethora of intermediate steps in the visible and near-infrared (NIR) regions, that is rich ladder like energy levels [20]. The major drawback of Ln ions is that these f-f electronic transitions are for most of them forbidden by selection rules and the corresponding absorption coefficients are very low, resulting in the need for powerful excitation sources to get enough population of the excited states.
Considering that vibrations of molecules in solutions bring far more deexcitation pathways of the excited states than phonons in the solid materials (vide infra), it is foreseeable that the first observations of UC materials were made with solids [8-10,12] and more recently in the two last decades for nanoparticles [21,22], with numerous bioanalytical applications [23]. However, considering the difficult batch to batch reproducibility and potential toxicity of doped nanopartilces [24], there is an evident interest in a possible downsizing of UC materials to the molecular scale with stable and perfectly controlled molecular assemblies.
Solid vs solution: the question of the intermediate excited state lifetime.
