# Cooperative Luminescence and Cooperative Sensitisation
## Upconversion of Lanthanide Complexes in Solution

Richard C. Knighton,*[a]† Lohona K. Soro,[a]† Laura Francés-Soriano,[b] Aurora Rodríguez-Rodríguez,[c]
Guillaume Pilet,[d] Marc Lenertz,[e] Carlos Platas-Iglesias,[c] Niko Hildebrandt,[b],[f] Loïc J.
Charbonnière*[a]

**Abstract:** Upconversion nanoparticles have led to various breakthrough applications in solar energy conversion, imaging, and biomedicine. One key impediment is the facilitation of such processes at the molecular scale in solution where quenching effects are much more pronounced. In this work, molecular solution-state cooperative luminescence (CL) upconversion arising from a Yb excited state is explored and the mechanistic origin behind cooperative sensitisation (CS) upconversion in Yb/Tb systems is investigated. Counterintuitively, the best UC performances were obtained for Yb/Tb ratios close to parity, resulting in the brightest molecular upconversion complexes with an quantum yield of 2.8 × 10⁻⁶ at a low laser power density of 2.86 W/cm².

## Introduction

Upconverting phosphors – media which can generate high energy photons through the combination of two or more low energy photons – typically comprise of solid-state materials[1] and nanoparticles (UCNPs),[2] with wide application to in vivo imaging)[3] and as energy conversion materials.[4] The key disadvantages in the case of bioconjugated imaging is that the relatively large UCNP size can induce deleterious effects on their biodistribution in biological media, an inability to cross the blood-brain barrier, and suffer from reproducibility issues.[5] Alternatively, molecularly upconverting biolabels would represent a panacea for these issues, allowing precise control of the size and composition of the UC material.

Molecular UC requires long-lifetime donor states to ensure that the first excited state persists for sufficient time to allow access to the higher lying excited state. At the molecular level, the main non-radiative quenching pathways involve vibronic deactivation through the overtones of the vibration absorption bands of the ligands or solvent molecules,[6] with vibrations spanning from 1000 to 4000 cm⁻¹ in the infrared region, while they rarely exceed 1600 cm⁻¹ for phonons in solid lattices.[7] Thus, the first and second overtones of some vibrations in metal complexes, typically encompassing OH, NH and CH bonds, occur in the NIR

domain and present excellent non-radiative deactivation pathways, reducing the excited state lifetime and the probability of UC.[8] However, a few discrete coordination complexes have revealed their potential as UC molecular systems in the solid-state,[9] and more rarely in solutions.[10] These rare examples are related to mechanisms of excited state absorption (ESA)[10a–d], energy transfer UC (ETU)[11], cooperative sensitisation (CS) (Figure 1),[10d,12] or other mechanisms relevant to triplet-triplet annihilation in organic dyes.[13]

Except for the case of ESA, a mandatory prerequisite to obtain a signal in sensitised UC is the spatial proximity of energy donors and acceptors at only few Å. Indeed, Auzel noticed that, for CL (Figure 1c) in solids, the UC emission arises from localised 'ion clusters' that exhibit short intermetallic distances (< 5 Å).[14] A strategy to achieve such properties at the molecular level can be found in the architectures of polynuclear complexes with high nuclearity, and multiple lanthanide ions at very close intermetallic distances.[12b,15] Here we present a CL phenomenon in a discrete Yb₉ complex in solution and interrogation and optimisation of the Tb centered CS in Tb/Yb mixed complexes.

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Upconversion mechanisms; A) Excited State Absorption B) Energy Transfer Upconversion C) Cooperative Luminescence from Yb₂; D) Cooperative Sensitisation of Tb in Yb₂Tb</figcaption></figure>

## Results and Discussion

The nonanuclear complexes of the general composition [Ln₉L₁₆(OX)₁₀](OX) (L = Acac or Acac-d⁷ = acetylacetonate, X = H

<footnote>[a] Dr. R. C. Knighton, L. K. Soro, Dr. L. J. Charbonnière, Equipe de synthèse pour l'analyse (SynPA), Institut Pluridisciplaire Hubert Curien (IPHC), UMR 7178, CNRS/Université de Strasbourg, ECPM, 25 rue Becquerel, 67087 Strasbourg cedex, France.
E-mail: knighton@unistra.fr, lcharbonn@unistra.fr</footnote>

<footnote>[b] Dr. L. Francés-Soriano, Prof. N. Hildebrandt, nanoFRET.com, Laboratoire COBRA (Chimie Organique, Bioorganique, Réactivite et Analyse), Université de Rouen Normandie, CNRS, INSA, 76821 Mont Saint-Aignan, France.</footnote>

<footnote>[c] Dr. A. Rodríguez-Rodríguez, Dr. C. Platas-Iglesias, Centro de Investigacións Científicas Avanzadas (CICA) and Departamento de Química, Universidade da Coruña, Campus da Zapateira-Rúa da Fraga 10, 15008 A Coruña, Spain.</footnote>

<footnote>[d] Dr. G. Pilet, Laboratoire des Multimatériaux et Interfaces (LMI) UMR 5615 CNRS, Université Claude Bernard Lyon 1, Avenue du 11 novembre 1918, 69622 Villeurbanne cedex, France.</footnote>

<footnote>[e] Dr. M. Lenertz, Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504, CNRS/Université de Strasbourg, 23 rue du Lœss, BP 43, 67034 Strasbourg Cedex 2, France</footnote>

<footnote>[f] Prof. N. Hildebrandt, Université Paris-Saclay, 91190 Gif-sur-Yvette, France</footnote>

<footnote>† The authors contributed equally</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 2.** A) synthetic protocol for the preparation of the nonanuclear complexes. B) Single-crystal X-ray structure of [Tb₄Yb₅L₁₆(OD)₁₀](OD) (D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub>) representing the arrangement of the nine Ln (green), ten oxygen (red) and ten hydroxyl H atoms (white) core. C) Single-crystal X-ray structure of [Tb₄Yb₅L₁₆(OD)₁₀](OD) (D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub>, ellipsoids plotted at the 50% probability level) representing the acac ligands in grey, with endo and exo acac positions pointed in red and blue respectively.</figcaption></figure>

or D, yttrium (Y) will be integrated in the abbreviation of Ln for brevity) were prepared according to adaptation of literature procedure (Fig. 2a).[16] The compounds were synthesised, varying the extent of deuteration by utilising hydrated or deuterated Ln salts, conducting the reaction in CH₃OH or CD₃OD, and/or by using protiated or deuterated acetylacetonet[17] (X<sub>Ln</sub>, X<sub>S</sub>, X<sub>L</sub> respectively; where X = H or D; full synthetic details are available in the Electronic Supplementary Information). All isolated complexes were fully characterised by elemental analysis, infrared (IR) spectroscopy (Fig. S1-S7), ICP-AES analysis and powder X-ray diffraction (Fig. S8) and by X-ray crystallography in the case of the [Tb₄Yb₅L₁₆(OD)₁₀](OD) complex (Measured at 150 K, CCDC n° 2079088, Fig. 2c). The solid-state structure contains nine metal centres defining two square pyramids with shared apices at the central octacoordinated square anti-prismatic (Θ ≈ 45°) Ln(III) ion (Fig. 2b, Table S9). The coordination of the central ion is via eight μ₃-OH ligands which bridge to the eight peripheral Ln(III) ions and cap the eight triangular faces of the pyramids. The remaining two upper and lower square faces are spanned by two μ₄-OH bridges. The coordinative saturation of the peripheral Ln ions (Coordination Number = 8) is provided by 16 bidentate acetylacetonate ligands present as endo and exo environments, shown in red and blue respectively (Fig. 2c). The eight endo bidentate ligands are bound to a single metal centre (μ₁) while the eight exo ligands display bridging between two metal centres (μ₂).[18] The composition of the heteronuclear complexes was determined using ICP-AES and displayed a deviation of less than 0.26 integer units away from the theoretical compositions in all cases.

The non-deuterated nonanuclear [Y₉L₁₆(OH)₁₀](OH) complex was prepared as a diamagnetic analogue of the Ln₉ complexes to probe the structure in solution. The ¹H and ¹³C NMR spectrum (600 MHz, 298 K CDCl₃) shows time-averaged C₄ symmetry with two independent methyl and CH resonances, corresponding to endo and exo acetylacetonate ligands (Fig. 2a, S11). The appearance of equivalent methyl resonances in each Acac ligand reveals local C₂ symmetry due to fast ligand rotation on the NMR timescale. Two OH resonances were observable, corresponding to μ₃-OH and μ₄-OH environments. The ¹H and ¹³C spectra of [Y₉L₁₆(OH)₁₀](OH) were recorded between 323 K and 223 K (600 MHz, CDCl₃). The broad methyl resonance of the endo Acac (H⁰) reaches the slow-exchange regime first, displaying decoalescence around 298 K, closely followed by the methyl resonance corresponding to exo acac (Hᵃ) at 273 K, producing four methyl resonances. This indicates limitation of Acac rotation below 298 K, corroborated by the absence of significant broadening of the CH Acac resonances (Hᵇ and Hᶜ) at this temperature. Line shape analysis of the spectrum provides an estimation of the activation energy for the rotation of the Acac ligands about the local C₂ axis, which was found to be similar for the endo and exo ligands (ΔG‡₂₉₸ = 62.0 ± 3.0 kJ mol⁻¹).

<figure><img src="image_3.png" /><figcaption>**Figure 3.** A) Truncated variable temperature ¹H NMR spectrum of [Y₉L₁₆(OH)₁₀](OH) (600 MHz, 323 – 223 K, CDCl₃). Red and blue respectively correspond to endo and exo acac ligands, a and α to methyl groups, b and β to the methylene protons. B) and C) Energy minimised structures of respectively the (Δ,Δ) and (Δ,Λ) isomers of [Y₉L₁₆(OH)₁₀] (TPSSh/LanL2Dz level).</figcaption></figure>

Upon further cooling the four methyl environments further split into eight independent resonances with major (M) and minor (m) components, which was mirrored by all other resonances (Hᵇ, Hᶜ, μ₃-OH and μ₃-OH) (Fig. 3a, S12) and the ¹³C NMR spectrum (151 MHz, 233 K, Fig. S13). Analysis using low-temperature correlation experiments permitted identification of four discrete Acac environments lacking local C₂ symmetry. These could be assigned (ROESY, HSQC, HMBC; 600 MHz, 223 K. Fig. S14-S18) and the eight CH₃ and eight C=O peaks correspond to two Acac ligands (endo and exo) in a major and minor set of isomers. The corresponding equilibrium constants (K=[M]/[m]) determined at different temperatures, were used to estimate the reaction enthalpy (ΔH⁰) and reaction entropy (ΔS⁰) for isomerisation reaction m → M (Table S19, Fig. S20-S21): ΔH⁰ = +5.0 ± 0.9 kJ mol⁻¹ and ΔS⁰ = +26.8 ± 0.4 J mol⁻¹ K⁻¹. The full line shape analysis as well as attribution of the peaks in ¹H- and ⁸⁹Y-NMR (Fig. S23-S24) can be found in the supplementary information (Section S3).
<figure><img src="image_4.png" /><figcaption>**Figure 4.** A) Schematic representation of the absorption (blue) downshifting (purple), cooperative luminescence (red) and cooperative sensitisation (orange) processes. B) main spectroscopic properties of the [Tb₄Yb₅(acac)₁₆(OD)₁₀](OD) (Dₙ:D₅:D₆) complex in CD₃OD representing the absorption spectrum of the acac ligands in the UV (light blue) and of Yb in the NIR (dark blue), the Tb centred emission (green) and the Yb centred emission (brown) upon ligand excitation at 350 nm (for ε values of the other complexes see Table 1). C) cooperative luminescence emission of the Yb₈ complex (Dₙ:D₅:H₉, [c] = 2.04 mM, CD₃OD, λexc = 980 nm, P = 10.8 W/cm²). D) UC intensity as a function of the incident pump power in a Log/Log scale. The black straight line represents the linear regression of the data. E) CL UC emission of the Tb₄Yb₅ complex (Dₙ:D₅:D₆, [c] = 1.4 mM, CD₃OD, λexc = 980 nm, P = 2.86 W/cm²). F) CL UC intensity as a function of the incident pump power in a Log/Log scale.</figcaption></figure>

With a non-deuterated ligand, the D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub> complexes display similar overall Yb quantum yields (Φ<sub>Yb</sub><sup>L</sup> = 0.5-0.6%) and lifetimes (τ<sub>Yb</sub> = 17-20 μs), indicating a relatively small effect of substitution of Yb for Tb. Contrastingly, the Tb quantum yield shows a small increase upon introduction of more Tb atoms, progressing from 2.7% in Tb<sub>1</sub>Yb<sub>8</sub> to 3.3% in Tb<sub>4</sub>Yb<sub>5</sub>, arising from the decrease in non-radiative Tb→Yb energy transfer due to a lower local concentration of Yb acceptors. For the fully deuterated complexes, D<sub>Ln</sub>:D<sub>S</sub>:D<sub>L</sub> the salient observation is the dramatic increase of the longevity of the Yb excited state, with Yb<sub>5</sub>Tb<sub>4</sub>D<sub>Ln</sub>:D<sub>S</sub>:D<sub>L</sub> displaying the longest lifetime of 41 μs (cf. 20 μs for Tb<sub>4</sub>Yb<sub>5</sub> D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub>). The Tb lifetime and quantum yields remain largely unperturbed within complexes containing proteo or deuterated Acac ligands with much smaller propensity of non-radiative multi-phonon quenching of Tb vs. Yb excited states (viz. substitution of CH for CD in D<sub>Ln</sub>:D<sub>S</sub>:D<sub>L</sub>).[6] Sensitisation efficiencies in the range of 10-28% were obtained upon ligand irradiation. The radiative lifetimes of Yb were calculated[21] varying from 665 to 900 μs.

The upconversion properties of the hetero-nonanuclear Yb/Tb clusters were subsequently studied in CD₃OD (Table 1). Irradiation of the ²F₅/₂ ← ²F₇/₂ Yb absorption band (λexc = 980 nm) gave rise to signals in the visible region, with the characteristic digitated visible emission of Tb (⁵D₄ → ⁷Fᵢ with i = 6 to 3) with a maximum at 545 nm (Fig. 4e, S36a-39a). The two photon behaviour of the UC emission was confirmed using Log/Log plots, whereby the intensity was found to have a quadratic dependence on the intensity of the incident light (Fig. 4f, S36b-39b). The slopes of the Log/Log plots were similar across the series and

found to be in the range of 1.63-1.87, pointing to the same mechanistic origins across all complexes – two excited ytterbium donors cooperatively sensitising one terbium acceptor, *via* the upconverted quasi-virtual Yb₂** excited state. Inspection of the UC quantum yield across the series of complexes reveals that two factors affect the efficiency of these systems. First, is the variation of the donor-acceptor ratio. The UCQY of the D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub> complexes increase with substitution of Yb for Tb, giving an UCQY of 1.0 × 10<sup>-7</sup> for Tb<sub>1</sub>Yb<sub>8</sub> D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub> which increases over tenfold for Tb<sub>4</sub>Yb<sub>5</sub> to 1.1 × 10<sup>-6</sup>. This is also borne out for the fully deuterated D<sub>Ln</sub>:D<sub>S</sub>:D<sub>L</sub> systems (cf. 6.0 × 10<sup>-7</sup> for Tb<sub>1</sub>Yb<sub>8</sub> vs. 2.8 × 10<sup>-6</sup> for Tb<sub>4</sub>Yb<sub>5</sub>), conclusively demonstrating that having a large number of donor atoms does not produce the most efficient systems. Instead, a Yb/Tb ≈ 1 gives the largest quantum yield in this class of complexes.[10d,11a] This is likely due to the presence of a higher local concentration of Tb acceptors in the Tb<sub>3</sub>Yb<sub>6</sub> and Tb<sub>4</sub>Yb<sub>5</sub> complexes, coupled with the fact that Tb→Yb energy transfer acts as a deactivation pathway for the Tb excited state, whereby more ytterbium atoms have a deleterious effect on the UCQY. The second factor affecting the efficiency of the UC emission is the level of deuteration of the complexes. It can be clearly seen that there is a significant increase in the UCQY for the fully deuterated (D<sub>Ln</sub>:D<sub>S</sub>:D<sub>L</sub>) systems in comparison to the partially deuterated (D<sub>Ln</sub>:D<sub>S</sub>:H<sub>L</sub>) congeners. For example, the efficiency is increased in the Tb<sub>1</sub>Yb<sub>8</sub> complexes from 1.0 × 10<sup>-7</sup> to 6.0 × 10<sup>-7</sup> when substituting Acac for Acac-d<sub>8</sub>.
<figure><img src="image_5.png" /><figcaption>**Figure 5.** A) Temporal emission profile ($\lambda_{em} = 542 \pm 10$ nm) of the $[Tb_{x}Yb_{y}Yz(acac)_{16}(OD)_{10}]^{+}$ ($y + z = 8$) complexes in $CD_{3}OD$ ($\lambda_{exc} = 980$ nm, excitation for 60 µs, the color code for the value of y is given in Fig. 4c). B) CL UC intensities of $[Tb_{x}Yb_{y}Yz(acac)_{16}(OD)_{10}]^{+}$ ($x + y + z = 9$) complexes in $CD_{3}OD$ as a function of increasing y Yb proportion (the color code for x is given in the inset of Fig. 5c). C) Amplitude-averaged decay times of $[Tb_{x}Yb_{y}Yz(acac)_{16}(OD)_{10}]^{+}$ ($x + y + z = 9$) in $CD_{3}OD$ as a function of the x Tb proportion (the color code for y is given in the inset.</figcaption></figure>

This behaviour is mirrored by the other complexes and can be rationalised in the decreased probability of multi-phonon quenching of Yb by ligand CD oscillators compared to CH oscillators. In general, the trend is an approximately threefold increase in the UCQY efficiency in the fully deuterated systems. Inspection of these UCQY values against $\tau_{Yb}$ of conventional downshifted photoluminescence spectra ($\lambda_{exc} = 290$ nm) shows a strong correlation; the lifetime of ytterbium is increased from ca. 17 µs in the ($D_{Ln}$:$D_{S}$:$H_{L}$) complexes to >40 µs for the best performing $Tb_{x}Yb_{y}$ ($D_{Ln}$:$D_{S}$:$D_{L}$) complex. This is as expected, as a longer $\tau_{Yb}$ increases the probability of reaching the doubly excited state capable of CS of the Tb excited state, with a concomitant increase in the UCQY.

To further investigate the CSUC of the nonanuclear complexes and the possibility and efficiency of cooperative sensitisation, we prepared a combinatorial library of complexes with different molar fractions of Tb, Yb, and Y ions. The Y ions play the role of a spectroscopically silent Ln surrogate, being considered as chemically equivalent to Ln atoms of the second part of the Ln series with a similar ionic radius,[28] and allowing to investigate the possible effect of self-quenching of Yb and Tb cations. With nine places to be occupied by one of the lanthanide ions, the complex can contain 55 different compositions of Ln ions ($[Tb_{x}Yb_{y}YzL_{16}(OD)_{10}](OD)$ with x, y and z integers and $x + y + z = 9$) and therefore, we prepared 55 different samples by mixing x/9 TbCl$_{3}$.6D$_{2}$O, y/9 YbCl$_{3}$.6D$_{2}$O, z/9 YCl$_{3}$.6D$_{2}$O, Et$_{3}$N, and Acac in the proportions used for the synthesis. We subsequently measured their UCL decay kinetics at the $^{5}D_{4} \rightarrow ^{7}F_{5}$ Tb transition (542 ± 10 nm) upon excitation of the $^{2}F_{5/2} \rightarrow ^{2}F_{7/2}$ Yb transition (980 nm) on a time-resolved fluorescence plate reader system (Fig. 5a, S40 and Table S41).

As expected, complexes without Tb or Yb did not show UCL at 542 nm. All other mixtures showed significant UCL, including the ones that contained only 1/9 of Yb. Although a single Yb is not expected to sensitise Tb, the statistical distribution of the three lanthanide ions results in a distribution of complexes with different amounts of Tb, Yb, and Y, resulting, for $y = 1$ in more than 26% of complexes with $y > 1$ (see Supporting Information, Section S2 for discussion of the lanthanide ion distributions inside the complexes). The UCL decay curves clearly show that both UCL intensities (Fig. 5b) and decay times (Fig. 5c) are dependent on both x and y.

Despite the distribution of complexes, it could have been expected that the UCL intensity increases with the amount of Yb ions because the probability of sensitising Tb acceptors increases with the number of surrounding Yb donors.[1a,11b] However, the UCL intensities (Fig. 5a, 5b) clearly increased with both the number of Yb and Tb ions and were highest for similar amounts of Yb and Tb (maxima for $3 < x < 6$ and $6 > y > 3$, $z = 0$), which

means that both equally influenced the UCL intensity. When normalising the distribution of complexes with x and y, one can also find that the highest intensities can be expected for x and y values between 3 and 6 (see Supporting Information, Section S5 and Fig. S42-S47 for discussion of the distributions). This assumes identical chemical reactivity between the three rare earth metals and within the nine coordination sites, and while not strictly true,[29] holds true as a first approximation in modelling the chemical compositions of the complexes.

Because luminescence decay times are concentration-independent, they can provide further insight into the influence of Tb and Yb on the UCL of the different nonanuclear complexes. All decay curves (after the 60 µs excitation pulse) were fitted with a bi-exponential decay function (two different emitting Tb species). The full set of data is presented in Table S41, while Fig. 5b, 5c display the amplitude-averaged UC decay times as a function of y and x. A general trend is that the higher the amount of Yb, the more important is the short decay component and the shorter the average decay time. Despite the statistical distribution of the actual number of ions for each molar proportion (x, y, and z), the UCL decay curves clearly show the transition from one single species (with a long lived mono-exponential UCL decay) for one Yb to another single species (emitting one short lived mono-exponential UCL decay) for 8 Yb. All other samples ($y = 2$ to 7) show a double-exponential decay composed of both the long and short components. The decreasing Tb decay time with increasing Yb amount can be the result of an increased probability of Tb-to-Yb energy transfer associated to an increase of non-radiative energy pathways. Another trend is that the average Tb decay time is approximately independent of Tb (x) (Fig. 5c) for x and $y \ge 2$. The x-independent decay time for each y means that the presence of Y atoms does not influence the UC efficiency and that quenching by non-radiative energy losses associated to intramolecular Tb-to-Tb energy transfer, as often invoked in doped solids,[30] does not occur. The detailed UCL decay analysis of the entire library of 55 Tb-Yb-Y nonanuclear complexes within a single microplate fluorescence experiment revealed that the UCL intensity is equally dependent on the number of Tb and Yb ions. Furthermore, the efficiency of sensitising a Tb acceptor by two Yb donors can be tuned via y (the amount of Yb). Notably, such experiments were only possible due to the outstanding molecular UCL brightness of the nonanuclear Yb-Tb-Y complexes and their very simple preparation and good stability over a few hours after preparation.
Conclusion

We have prepared a coherent series of homo- and hetero-
nonanuclear complexes and investigated their solution-state
properties. The diamagnetic yttrium complex was probed by NMR
spectroscopy and computational investigations, revealing that the
complex retains its solid-state structure and symmetry in solution.
Low temperature experiments expose the prevalence of
diasteromeric and enantiomeric pairs ((Δ,Λ), (Δ, Δ), (Δ,Λ), (Δ,Λ))
in the slow-exchange regime, corresponding to low-symmetry
complexes. Studies of the upconversion properties of the Yb₃
cluster in CD₃OD gave rise to an emission band at 503 nm upon
irradiation of 980 nm, corresponding to the cooperative
luminescence upconversion, the first time such a phenomenon
has been observed in solution. The hetero-nonanuclear
complexes TbₓYbᵧ exhibited cooperative sensitisation
upconversion arising from energy transfer from a doubly excited
state of Yb to sensitise Tb. The study of a library of complexes
with varying donor-acceptor ratios concludes that the most
efficient systems are those containing an average ratio close to
one. These results coupled with the use of highly deuterated
complexes demonstrated that the Yb₅Tb₄ (D₉n:Ds:D₉) compound
presents the most efficient molecular UC in solution to date.

Acknowledgements

Financial support is gratefully acknowledged (LJC and RCK) from
the French Ministère de l'Éducation Nationale et de la Recherche,
the French Canada Research Fund (KS), Frontier Research in
Chemistry Foundation (LabEx CSC, ANR-10-LABX-0026 CSC),
and the French National research agency (LAPIN project n° ANR-
20-CE09-0021-02; LUCAS project n°ANR-19-CE29-0014-01) for
financial support. NH and LFS thank University of Rouen
Normandy, INSA Rouen Normandy, the Centre National de la
Recherche Scientifique (CNRS), European Regional
Development Fund (ERDF), Labex SynOrg (ANR-11-LABX-
0029), Carnot Insitute I2C, XI-Chem graduate school (ANR-18-
EURE-0020 XL CHEM), Region Normandie, ANR (LAPIN project
n° ANR-20-CE09-0021-02), European Commission (Horizon
2020 - MSCA-IF) for financial support. ARR and CPI thank Centro
de Computación de Galicia (CESGA) for providing the computer
facilities and the Ministerio de Ciencia e Innovación (Grant
PID2019-104626GB-100).

[1] a) F. Auzel, *Chem. Rev.* **2004**, *104*, 139-173; b) D. R. Gamelin, H. U. Güdel, *Acc. Chem. Res.* **2000**, *33*, 235-242.

[2] a) G. Chen, H. Qiu, P. N. Prasad, X. Chen, *Chem. Rev.* **2014**, *114*, 5161-5214; b) C. Duan, L. Liang, L. Li, R. Zhang, Z. P. Xu, *J. Mater. Chem. B*, **2018**, *6*, 192-209; c) S. Wen, J. Zhou, K. Zheng, A. Bednarkiewicz, X. Liu, D. Jin, *Nat. Commun.* **2018**, *9*, 2415.

[3] a) B. Zhou, L. Yan, J. Huang, X. Liu, L. Tao, Q. Zhang, *Nat. Photonics* **2020**, *14*, 760-766; b) B. Tian, A. Fernandez-Bravo, H. Najafiaghdam, N. A. Torquato, M. V. P. Altoe, A. Teitelboim, C. A. Tajon, Y. Tian, N. J. Borys, E. S. Barnard, M. Anwar, E. M. Chan, P. J. Schuck, B. E. Cohen, *Nat. Commun.* **2018**, *9*, 3082.

[4] a) C. Lee, E. Z. Xu, Y. Liu, A. Teitelboim, K. Yao, A. Fernandez-Bravo, A. M. Kotulska, S. H. Nam, Y. D. Suh, A. Bednarkiewicz, B. E. Cohen, E. M. Chan, P. J. Schuck, *Nature* **2021**, *589*, 230-235; b) B. Zhou, B. Tang, C. Zhang, C. Qin, Z. Gu, Y. Ma, T. Zhai, J. Yao, *Nat. Commun.* **2020**, *11*, 1174; c) F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen, X. Liu, *Nat. Mater.* **2011**, *10*, 968-973; d) Y. Wu, J. Xu, X. Qin, J. Xu, X. Liu, *Nat. Commun.* **2021**, *12*, 2022.

[5] A. Domínguez, B. Suarez-Merino, F. Cerio, *J. Nanosci. Nanotechnol.* **2014**, *14*, 766-779; b) D. Bobo, K. J. Robinson, J. Islam, *Pharm. Res.* **2016**, *33*, 2373-2387.

[6] A. Beeby, I. M. Clarkson, R. S. Dickinson, S. Faulkner, D. Parker, L. Royle, A. S. de Sousa, J. A. G. Williams, M. Woods, *J. Chem. Soc., Perkin Trans.* **1999**, *3*, 493-504.

[7] M. Haase, H. Schäfer, *Angew. Chem. Int. Ed.* **2011**, *50*, 5808-5829;
*Angew. Chem.* **2011**, *123*, 5928-5950

[8] a) A. M. Nonat, L. J. Charbonnière, *Coord. Chem. Rev.* **2020**, *409*, 213192; b) W. D. Horrocks, D. R. Sudnick, *J. Am. Chem. Soc.* **1979**, *101*, 334-340.

[9] a) J. Kalmbach, C. Wang, Y. You, C. Förster, H. Schubert, K. Heinze, U. Resch-Genger, M. Seitz, *Angew. Chem. Int. Ed.* **2020**, *59*, 18804-18808; *Angew. Chem.* **2020**, *132*, 18966-18970; b) J. T. Mo, Z. Wang, P. Y. Fu, L. Y. Zhang, Y. N. Fan, M. Pan, C. Y. Su, *CCS Chem.* **2020**, *2*, 729-738.

[10] a) B. Golesorkhi, H. Nozary, A. Fürstenberg, C. Piguet, *Mater. Horiz.*, **2020**, *7*, 1279-1296; b) L. J. Charbonnière, *Dalton Trans.* **2018**, *47*, 8566-8570; c) B. Golesorkhi, A. Fürstenberg, H. Nozary, C. Piguet, *Chem. Sci.* **2019**, *10*, 6876-6885; d) Golesorkhi, B.; Naseri, S.; Guénée, L.; Taarit, I.; Alves, F.; Nozary, H.; Piguet, C. *J. Am. Chem. Soc.* **2021**, *143*, 15326-15334; e) A. Nonat, S. Bahamyirou, A. Lecointre, F. Przybilla, Y. Mély, C. Platas-Iglesias, F. Camerel, O. Jeannin, L. J. Charbonnière, *J. Am. Chem. Soc.* **2019**, *141*, 1568-1576.

[11] a) B. Golesorkhi, H. Nozary, L. Guénée, A. Fürstenberg, C. Piguet, *Angew. Chem. Int. Ed.* **2018**, *57*, 15172-15176; *Angew. Chem.* **2018**, *130*, 15392-15396; b) A. Nonat, C. F. Chan, T. Liu, C. Platas-Iglesias, K.-L. Wong, L. J. Charbonnière, *Nat. Commun.*, **2016**, *7*, 11978.

[12] a) N. Souri, P. Tian, C. Platas-Iglesias, K.-L. Wong, A. Nonat, L. J. Charbonnière, *J. Am. Chem. Soc.* **2017**, *139*, 1456-1459; b) R. C. Knighton, L. K. Soro, A. Lecointre, G. Pilet, A. Fateeva, L. Pontille, N. Hildebrandt, L. Francés-Soriano, L. J. Charbonnière, *Chem. Commun.* **2021**, *57*, 53-56.

[13] I. Roy, A. Garci, Y. Beldjoudi, R. M. Young, D. J. Pe, M. T. Nguyen, P. J. Das, M. R. Wasielewski, J. F. Stoddart, *J. Am. Chem. Soc.* **2020**, *39*, 16600-13609.

[14] F. Auzel, D. Meichenin, F. Pellé, P. Goldner, *Opt. Mater.* **1994**, *4*, 35-41.

[15] a) D. A. Gálico, J. S. Ovens, F. A. Sigoli, M. Murugesu, *ACS Nano* **2021**, *15*, 5580-5585; b) J. Wang, Y. Jiang, J.-Y. Liu, H.-B. Xu, Y.-X. Zhang, X. Peng, M. Kurmoo, S. W. Ng, M.-H. Zeng, *Angew. Chem. Int. Ed.* **2021**, *60*, 22368-22375; *Angew. Chem.* **2021**, *133*, 22542-22549

[16] D. Guettas, C. M. Balogh, C. Sonneville, Y. Malicet, F. Lepoivre, E. Onal, A. Fateeva, C. Reber, D. Luneau, O. Maury, G. Pilet, *Eur. J. Inorg. Chem.* **2016**, 3932-3938.

[17] D. H. Moseley, S. E. Stavretis, K. Thirunavukkuarasu, M. Ozerov, Y. Cheng, L. L. Daemen, J. Ludwig, Z. Lu, D. Smirnov, C. M. Brown, A. Pandey, A. J. Ramirez-Cuesta, A. C. Lamb, M. Atanasov, E. Bill, F. Neese, Z.-L. Xue, *Nat. Commun.* **2018**, *9*, 2572.

[18] S. Petit, F. Baril-Robert, G. Pilet, C. Reber, D. Luneau, *J. Chem. Soc. Dalt. Trans.* **2009**, *34*, 6809-6815.

[19] For the sake of simplicity only the (Δ,Λ) and (Δ, Δ) structures were calculated.

[20] G. D. R. Napier, J. D. Neilson, T. M. Shepherd, *Chem. Phys. Lett.* **1975**, *31*, 328-330.

[21] M. H. V. Werts, R. T. F. Jukes, J. W. Verhoeven, *Phys. Chem. Chem. Phys.* **2002**, *4*, 1542-1548.

[22] R. C. Benson, H. A. Kues, *Phys. Med. Biol.* **1978**, *23*, 159-163.

[23] J. Olmsted, *J. Phys. Chem.* **1979**, *83*, 2581-2584.

[24] N. Weibel, L. C. Charbonnière, M. Guardigli, A. Roda, R. Ziessel, *J. Am. Chem. Soc.* **2004**, *126*, 4888-4896.

[25] a) E. Nakazawa, S. Shionoya, *Phys. Rev. Lett.* **1970**, *25*, 1710-1712; b) W. P. Qin, Z.-Y. Liu, C.-N. Sin, C.-F. Wu, G.-S. Qin, Z. Chen, K.-Z. Zheng, *Light Sci. Appl.* **2014**, *3*, 1-6.

[26] L. Zhang, J. Yang, Z. Zhang, H. Yu, W. Pan, *Ceram. Int.* **2019**, *45*, 9278-9282.

[27] J.-C. G. Bünzli, *Chem. Rev.* **2010**, *110*, 2729-2755.

[28] R. D. Shannon, *Acta Cryst.* **1976**, *A32*, 752-767.

[29] N. Dall-Favera, J. Hamacek, M. Borkovec, D. Jeannerat, G. Ercolani, C. Piguet, *Inorg. Chem.* **2007**, *46*, 9312-9322.

[30] a) F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, *Opt. Mater.* **2003**, *24*, 103-109; b) F. Auzel, *J. Lumin.* **2002**, *100*, 125-130