Lanthanide Ion and Tetrathiafulvalene-Based Ligand as a âMagic

Article pubs.acs.org/accounts

Lanthanide Ion and Tetrathiafulvalene-Based Ligand as a “Magic” Couple toward Luminescence, Single Molecule Magnets, and Magnetostructural Correlations Fabrice Pointillart,*,† Boris le Guennic,† Olivier Cador,† Olivier Maury,‡ and Lahcène Ouahab† †

Institut des Sciences Chimiques de Rennes UMR 6226 CNRS-UR1, Université de Rennes 1, 35042 Cedex Rennes, France Laboratoire de Chimie, UMR 5182 CNRS-ENS Lyon, Université Lyon 1, 46 Allée d’Italie, 69364 Cedex 07 Lyon, France

‡

CONSPECTUS: The synthesis of molecules featuring different properties is a perpetual challenge for the chemists’ community. The coexistence and even more the synergy of those properties open new perspectives in the field of molecular devices and molecular electronics. In that sense, coordination chemistry contributed to the development of new functional molecules through, for instance, single-molecule magnets (SMMs) and light emitting molecules with potential applications in high capacity data storage and OLEDs, respectively. The appealing combination of both electronic properties into one single object may offer the possibility to have magnetized luminescent entities at nanometric scale. To that end, lanthanides seem to be one of the key ingredients since their peculiar electronic structures endow them with specific magnetic and luminescence properties. Indeed, lanthanides cover a wide range of emission wavelengths, from infrared to UV, which add up to a large variety of magnetic behaviors, from the fully isotropic spin (e.g., GdIII) to highly anisotropic magnetic moments (e.g., DyIII). In lanthanide complexes, ligands play a fundamental role because on one hand they govern the orientation of the magnetic moment of anisotropic lanthanides and on the other hand they can sensitize efficiently the luminescence. The design of appropriate organic ligands to elaborate such chemical objects with the desired property appears to be essential but remains a perpetual challenge. In this Account, we describe the design of lanthanide-based complexes that emit light, behave as SMMs, or combine both properties. We have paid peculiar attention to the design of ligands based on the tetrathiafulvalene (TTF) moiety. TTF and its derivatives are well-known chemical entities, stable at different oxidation states, and employed mainly in the synthesis of molecular conductors and superconductors. In addition to their redox properties, TTF-based derivatives act as organic chromophores for the sensitization of visible and near-infrared (NIR) luminescence of lanthanides. The mechanism of sensitization involves either antenna effect (energy transfer from the excited state) or photoinduced electron transfer. TTF-based ligands act also as structural agents in the conception of SMM in crystals. Such objects are obtained with the highly anisotropic DyIII ion in crystalline phase as well as in frozen solution with magnetic memory at helium-4 temperature (4 K). We highlight the influence of the magnetic dilution (both in amorphous solution and in diamagnetic crystalline matrix) and, particular case of dysprosium based SMMs, the effect of metal-centered isotope enrichment on the SMM properties. Our aim is not only to realize functional molecules but to rationalize both luminescence and magnetic properties on the basis of the structure of the molecules. These two properties are intimately intricate and governed by the electronic structure, which can be calculated and interpreted using modern quantum chemistry tools. plethora of potential applications in nanoscale data storage6,7 and spintronic devices.8 Lanthanides are also intensively studied for their specific luminescence that covers a large spectroscopic range from the visible to the near-infrared (NIR) with easily recognizable lineshape emission bands. These optical properties are at the origin of applications in the conception of OLEDs,9 bioimaging, sensors,10,11 time-resolved luminescent immunoassays,12 or imaging microscopy.13 The f−f transitions are Laporteforbidden,14 resulting in both advantages and disadvantages: the emitting excited states are characterized by long lifetimes but with very weak absorption coefficients leading to ineffective

1. INTRODUCTION Before becoming one of the cornerstones of molecular magnetism, rare earth elements have been employed successfully to produce very hard magnets1 and light emitters.2 Lanthanide ions possess strong magnetic moments that make them good candidates to elaborate single molecule magnets (SMMs).3 Magnetic anisotropy is one of the crucial parameters for a molecular magnet to retain its magnetization in a specific direction (Ising system). Of course the axial anisotropy of the metal center is not enough to guaranty the existence of a SMM. It has been recently demonstrated that the crystal field and its symmetry as well as ligand donation and electronic distribution play an important role.4,5 All efforts of chemists and physicists to deeply understand the SMM behavior are motivated by the © 2015 American Chemical Society

Received: June 16, 2015 Published: October 22, 2015 2834

DOI: 10.1021/acs.accounts.5b00296 Acc. Chem. Res. 2015, 48, 2834−2842

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Accounts of Chemical Research Scheme 1. Molecular Structures of TTF-Based Ligandsa

a

L1, bis (2,6-di(pyrazol-1-yl)-4-methylthiolpyridine)-4′,5′-ethylenedithiotetrathiafulvene; L2, tetrathiafulvalene-amido-2-pyridine-N-oxide; L3, 4tetrathiafulvalene-2,6-pyridinedicarboxylic acid dimethyl ester; L4, 4-(2-tetrathiafulvalenyl-ethenyl)pyridine; L5, 2-{4,5-[4,5-bis(propylthio)tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine; L6, 2-{1-methylpyridyl-4,5-[4,5-bis(propylthio)tetrathiafulvalenyl]-1H-benzimidazol-2-yl}pyridine; L7, 4,5-bis(2-pyridyl-N-oxidemethylthio)-4′,5′-ethylenedithiotetrathiafulvene; L8, 4,5-bis(thiomethyl)-4′-ortho-pyridyl-N-oxide-carbamoyltetrathiafulvalene; HL9, 4,5-bis(thiomethyl)-4′-carboxylictetrathiafulvalene.

direct excitation processes in dilute solution. To tackle this drawback, indirect sensitization processes have been developed mediated by organic chromophores acting as antennae, such as (i) triplet excited-state sensitization processes, (ii) induced triplet metal−ligand charge transfer (MLCT), or (iii) singlet excited-state pathways with intraligand charge transfer (ILCT) mechanism. We particularly focused our attention on the ILCT process since it needs push−pull type ligands such as functionalized tetrathiafulvalene (TTF) derivatives.15 TTF analogues have been employed first to design conducting materials such as organic metals, semiconductors, and superconductors.16,17 Concomitantly, the functionalization of the TTF core (donor) by an acceptor moiety participated in the development of photofunctional materials for applications as fluorescence switches, metal ion sensors, photovoltaic cells, and nonlinear optics.18−20 The acceptor moiety was adapted first to the coordination of d metals,21,22 while the first TTF-f system was obtained using a “through space” approach, that is, without a covalent bond between the TTF-based ligand and the lanthanide ion.23 The first TTF−LnIII coordination complexes were published during the last decade,24,25 and their production increased dramatically during the last five years with the emergence of TTF-f SMMs.24,26−28

All coordination complexes exposed in this Account are redox active and based on the neutral functionalized TTF (Scheme 1). The general strategy consists in functionalizing the TTF core with an oxygenated or nitrogenated acceptor moiety able to coordinate lanthanide ions. Mono-, bis-, and tridentate neutral ligands are then synthesized and associated with neutral [Ln III (hfac) 3 ] (hfac − = hexafluoroacethylacetonate) or [LnIII(tta)3] (tta− = 2-thenoyltrifluoroacetonate) precursors (Ln = Nd, Eu, Gd, Dy, Er, or Yb). Mononuclear as well as polynuclear TTF-based lanthanide complexes are obtained. This Account is organized as follows. In the first part, we concentrate on lanthanide luminescence sensitization processes, while in the second part, the production of TTF-based SMMs is discussed. In the last part, we illustrate how luminescence and magnetism are used to probe electronic energy levels and how these levels are related to the molecular structure.

2. VISIBLE TO NEAR-INFRARED LANTHANIDE LUMINESCENCE SENSITIZATION In lanthanide coordination complexes involving functionalized TTF-based ligands, ILCT absorption bands are observed in the visible region. The ILCT energy must lie at about 2−3000 cm−1 above those of emissive lanthanide-centered states to optimize 2835


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Figure 1. Structures of complexes 1−4 with their corresponding emission spectrum. Color codes: gray = C, blue = N, red = O, yellow = S, and light green = F; H atoms and solvent molecules of crystallization are removed for clarity. The same code and simplifications are applied in the whole Account.

acceptor moieties is increased with respect to L1 thanks to the presence of an amido bridge. Consequently compound 2 is red. Irradiation in the intense visible HOMO → LUMO + 1 and LUMO + 2 ILCT centered at 19600 cm−1 leads to a broad fluorescence band centered at 13160 cm−1, which avoids the sensitization of visible lanthanide emitters. This irradiation induces 2F5/2 → 2F7/2 YbIII line-shape emission in the NIR (Figure 1). The extended Rehm−Weller equation shows that the electron-transfer process is not thermodynamically favored (ΔGET = 0.112 eV), so the sensitization occurs through the antenna.33 The efficiency of the sensitization mechanism is due to a donating state localized in the energy range that perfectly matches with the YbIII luminescence (2−3000 cm−1 above the 2 F5/2 multiplet state).34,35 Finally, the ILCT band of this ligand is further red-shifted upon directly connecting the strong acceptor 2,6-pyridinedicarboxylic acid dimethyl ester (DPA) to the TTF core. The coordination of the Er(hfac)3 precursor to L3 leads to purple single crystals of [Er(hfac)3(L3)]2 (3) (Figure 1).36 The ILCT

the energy transfer and avoid thermally activated back-energy transfer.2,29,30 Let us first concentrate on complex [Eu2(tta)6(L1)]·(1) (Figure 1), obtained as yellow single crystals. Two EuIII ions are coordinated to the tris-chelating nitrogenized 2,6-di(pyrazol-1yl)-4-pyridine (dpp) moieties.31 The weak conjugation between the TTF and dpp moieties results in relatively high HOMO → LUMO (21100 cm−1) and HOMO → LUMO + 1 (27100 cm−1) ILCT and triplet state (20000 cm−1) energies of the ligand. They are compatible with a sensitization of the EuIII luminescence (17250−14250 cm−1) using L1 as an organic chromophore. The irradiation of 1 at 27780 cm−1 and at 77 K in solid state produces the characteristic EuIII emission profile with 5D0 → 7FJ (J = 0−4) transitions (Figure 1). Complex 1 illustrates the possibility to sensitize for the first time visible lanthanide luminescence with a TTF-based antenna. In the mononuclear complex [Yb(hfac)3(L2)2] (2), two L2 ligands complete the coordination sphere of YbIII (Figure 1).32 In L2, the electronic communication between donor and 2836


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Accounts of Chemical Research bands are localized at around 18750 cm−1, and their excitation induces two emissions: a ligand-centered emission at 13300 cm−1 and a metal-centered emission at 6540 cm−1 (4I13/2 → 4 I15/2) characteristic of ErIII (Figure 1). The zero-phonon transition is estimated at 16600 cm−1, and the presence of the low-energy ILCT transition strongly suggests that the sensitization occurs via an energy transfer from the charge transfer state. Such direct sensitization process is scarce for ErIII especially with such a low-energy antenna donating state.37,38 The three previous examples, and others unexposed,39,40 demonstrate the efficiency of neutral TTF-based ligands to sensitize lanthanide luminescence. The sensitization of the NIR luminescence of lanthanide ions with TTF derivatives in their oxidized forms is also accessible as demonstrated by the salt {[Nd(hfac)4(H2O)][((L4)•+)]}2 (4), which was obtained as dark blue single crystals (Figure 1).41 The ILCT1 was attributed to SOMO − n (centered on 4-pyridine) → SOMO (centered on TTF•+) (16000 cm−1) and the ILCT2 SOMO → LUMO (26500 cm−1) transitions.42 ILCT2 is the analogue of the classical HOMO → LUMO transition described in 1, 2, and 3. The NdIII luminescence is observed and composed of the three emission bands (4F3/2 → 4In n = 9/2, 11/2, 13/2) whatever the irradiation in the energy range 26500−14300 cm−1. However, the additional broad organic fluorescence (light gray curve) is mainly observed at low-energy excitations (ILCT1) because the energy back-transfer from the excited multiplet states of NdIII to the donating excited state of (L4)•+ is favored in these conditions. The striking feature of this system is that the luminescence is not quenched by the radical cation. TTF-based ligands are thus efficient antenna for the sensitization of lanthanide luminescence in both their neutral and oxidized forms and for a wide energetic range.

single crystal rotating magnetometry.43 From this experiment, the g-tensor was extracted (gx = 9.43, gy = 3.96, and gz = 14.22). The lack of axial anisotropy implies that 5 does not behave as a mononuclear SMM in the solid state. However, a pure molecular vision is not able to explain the observed solidstate magnetism. Indeed, relativistic ab initio calculations (SACASSCF/RASSI-SO) on the sole complex give an axial anisotropy. To reproduce the experimental magnetic properties, the theoretical interpretation was revisited by taking into account the intermolecular H-bond network, which organizes the threedimensional crystal packing in 5 (Figure 2). The dissolution of 5 in a frozen solution of dichloromethane destroys the hydrogen-bond network and promotes the SMM behavior. L5 was then alkylated with the methyl-2-pyridyl unit (L6) to chemically remove the secondary amine function source of the hydrogen bond. The complex [Dy(hfac)3(L6)] (6) (Figure 2) behaves now as a SMM with nearly the same barrier height (Δ ≈ 16 K) in solid state and frozen solution of dichloromethane.44 In this case, the crystal field and the nature of the ground state could be theoretically determined solely considering isolated molecules.43 The coordination polyhedron in 6 (N2O6) being adequate to observe SMM behavior, different methods were successfully tested to optimize its performances. First, a simple molecular engineering approach consisting in the substitution of the hfac− anions by tta− allowed stabilization of the molecular structure and reorganization of the charge density in the first coordination sphere of DyIII as suggested recently.45,46 In the mononuclear complex [Dy(tta)3(L6)] (7) (Figure 3), the easy

3. SINGLE MOLECULE MAGNET BEHAVIOR AND OPTIMIZATIONS The most popular lanthanide ion to build SMMs is probably DyIII due to its strong magnetic anisotropy and large magnetic moment. Moreover, it is crucial to isolate the magnetic objects (metallo-organic network) since intermolecular interactions (dipolar interaction, hydrogen bonds, etc.) may drastically influence the magnetic properties. In this respect, TTF derivatives are well-known to self-organize through short S···S contacts and π−π stacking to form organic networks. The latter can be seen as diamagnetic spacers between the paramagnetic centers and play the role of structural agent. The mononuclear complex [Dy(hfac)3(L5)] (5) (Figure 2) crystallizes in the P1̅ triclinic space group, which is adapted for

Figure 3. Molecular structure of 7 with temperature dependence of the relaxation time in solid state (full symbols) and a frozen CH2Cl2 solution (empty symbols) measured with Hdc = 0 Oe (circles) and Hdc = 1 kOe (squares). Red lines correspond to the best-fitted curves with Arrhenius or modified Arrhenius laws. Frequency dependencies of the magnetic susceptibility for the isotopically enriched 161Dy-(7) and 164 Dy-(7). Normalized hysteresis loop measured at 0.46 K for diluted 164 Dy-(7).

Figure 2. Molecular structure of 5 and 6. For 5, two molecules are represented to feature hydrogen bonds. 2837


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Accounts of Chemical Research magnetic axis orientation is experimentally determined with single crystal rotating magnetometry and rationalized with CASSCF/RASSI-SO calculations resulting in excellent agreement with the experiment from both qualitative (orientation) and quantitative (amplitude) points of view.47 The SMM behavior is preserved in both solid and frozen solution (Figure 3) but with a significant increase of the barrier height (Δ ≈ 40 K) with respect to 6 due to the increase of the negative charges on coordinated oxygen atoms in switching from hfac− to tta−. The relaxation time of 7 is slow enough to observe for the first time the opening of a butterfly hysteresis loop (closed at zero field and opened in field) up to 4 K both in solid state and in frozen solution. It is worth noticing that the use of the push− pull ligand L6 allows the observation of the hysteresis loop by MCD (magnetic circular dichroism) irradiating the HOMO − 2/HOMO − 5 → LUMO ILCT at 27400 cm−1. Observation of remnant magnetization (residual magnetization in the absence of an external field) is problematic in the case of mononuclear complexes of DyIII. The typical butterflyshaped hysteresis loop observed in 748,49,50 is a consequence of the hyperfine coupling and the internal field created in condensed phases by neighboring molecules.51 To optimize zero-field mononuclear DyIII-based SMMs, one may desire to remove these two perturbations. Consequently the second strategy was to isotopically enrich sample 7 with the isotope 164 DyIII (I = 0) (164Dy-(7)). The thermally activated regime is not affected by the isotopic enrichment. By contrast, the relaxation time in the thermally independent regime is slowed by a decade when a magnetically active nucleus is replaced by a magnetically inactive one (Figure 3). Finally the last strategy was to perform magnetic dilutions either through (i) a dissolution of the complex in a nondissociating amorphous matrix (CH2Cl2) or (ii) dilution in a diamagnetic isostructural matrix based on YIII in order to create a solid-solution (called diluted 164Dy-(7)).52 We found that the CH2Cl2 frozen solution of 164Dy-(7) and the diluted-164Dy-(7) ([164Dy0.04Y0.96(tta)3(L6)]) relax 50 and 1000 times slower than the crystalline 161Dy-(7) (161DyIII, I = 5/2) at 3 K, respectively. It thus results that the quantum tunneling of the magnetization was almost suppressed by combining DyIIIcentered isotopic enrichment and dilution strategies. Consequently a butterfly-shaped hysteresis loop at 0.46 K was observed for diluted 164Dy-(7) with a remnant magnetization at zero-field.52 Thus, isotopic enrichment and magnetic dilution are powerful tools to improve SMM performances.

Figure 4. Molecular structure of 8. (left) Splitting of the YbIII 2F7/2 multiplet ground state estimated from dc magnetic measurements. (right) Solid state 77 K emission spectrum is represented with an appropriate shift of the energy scale.

eight oxygen atoms (D2d symmetry). The χMT vs T as well as the first magnetization curves were reproduced using the Stevens technique with the following Hamiltonian H = ∑i 2= 1(B02Ô 02i + B04Ô 04i + B06Ô 06i + B44Ô 44i + B46Ô 46i) adapted to local site symmetry. The energy splitting of the ground state multiplet is represented on Figure 4. The emission spectrum of 8 at 77 K in solid-state obtained in exciting the HOMO → LUMO/(LUMO + 1) ILCT (λex = 20835 cm−1) (Figure 4) can be compared, after an appropriate shift of the energy scale, to the energy diagram from magnetic measurements. The first excited level MJ = ±3/2 matches exactly the second emission line (225 vs 234 cm−1), while the last two (405 and 408 cm−1) fall in the middle of the third (+362 cm−1) and fourth (+486 cm−1) lines. Such correlation between magnetism and luminescence allows the attribution of the energy levels in terms of MJ states. The ytterbium analogue (9) of complex 6 was also investigated.58 The characteristic luminescence of YbIII is observed upon irradiation at 21740 cm−1 in the HOMO → LUMO ILCT in solid-state at low temperature (Figure 5). At

4. MAGNETOSTRUCTURAL CORRELATIONS A well-resolved emission spectroscopy is a photograph of the MJ states splitting53 as shown in the first part of this Account. Interestingly this splitting is also responsible for the thermal variation of the magnetic susceptibility in absence of magnetic interaction. In other words, luminescence and magnetism are two magnifying glasses of the same phenomenon. Consequently these physical properties can be correlated, and a few examples of such magnetoluminescence correlations have been proposed for Tb,54 Dy,55 and Er.56 In this Account, we concentrate our efforts on YbIII derivatives because they present a single luminescence transition (2F5/2 → 2F7/2). Light orange single crystals of the dinuclear complex [Yb2(hfac)6(L7)2] (8) were selected as prototypes. Even if they are formally dinuclear entities, the distance between metal centers is larger than 10 Å, so they can be considered as magnetically isolated (Figure 4).57 The environment is made of

Figure 5. Energy splitting of the 2F7/2 multiplet in 9 estimated (left) with the Stevens technique (dc data) and (middle) with CASSCF/ MS-CASPT2/RASSI-SO calculations. (right) Solid state 77 K emission spectrum with an appropriate shift of the energy scale. 2838


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Figure 6. Molecular structure of 10. Energy splitting of the multiplet 2F7/2 estimated with the Stevens technique from the dc data (left) or from the ac data (middle). On the right, the solid state 77 K emission spectrum is represented with an appropriate shift of the energy scale.

agrees with the energy gap between the two most energetic emission lines (16 cm−1). Finally, Figure 6 illustrates the good agreement among the three sets of data (dc, ac, and luminescence) of the energy splitting for 2F7/2.

room temperature in CH2Cl2, the luminescence decay corresponds to a single exponential function of the time with a lifetime of the 2F5/2 state of 14.6 μs. The Rehm−Weller equation suggests that the electron-transfer process might be thermodynamically favored (ΔGET < 0) and that the YbIII luminescence sensitization mechanism could involve a photoinduced electron transfer (PET) with a charge-separated state24,59−61 (L6)•+/YbII interesting for photoconduction.62,63 The magnetic data for 9 were reproduced using the Stevens technique considering YbIII in D4d symmetry. The results of the fit give the following energy splitting for 2F7/2 ground multiplet state: MJ = ±5/2 (0 cm−1), MJ = ±3/2 (251 cm−1), MJ = ±7/2 (459 cm−1) and MJ = ±1/2 (544 cm−1). In order to give more insights into the magnetic and luminescent properties of 9, CASSCF/MS-CASPT2/RASSI-SO calculations were carried out. The calculated energy splitting is in excellent agreement with the experimental energy emission lines but also with the results extracted from the magnetism (Figure 5). The nature of the ground-state doublet was confirmed by the calculated gz value of 5.3 compared with 5.71 for a pure MJ = ±5/2 state in the effective spin 1/2 model. In 8 and 9, the ground state is not the pure Ising component MJ = ±7/2 of the ground state multiplet; therefore these complexes do not behave as SMMs. To stabilize the MJ = ±7/2 state, the Yb(tta)3·2H2O precursor was associated with the neutral L8 and anionic L9 ligands.64 The resulting complex [Yb(tta)2(L8)(L9)]2 (10) is a dinuclear entity in which two Yb(tta)2 units are bridged by two deprotonated L9 ligands while the coordination sphere of each YbIII center is completed by one terminal L8 ligand (Figure 6). The magnetic susceptibility is perfectly reproduced considering D4d symmetry with no interactions between magnetic moments. We found that the ground state is the pure MJ = ±7/2 component separated by 2.57 cm−1 from the first excited state MJ = ±1/2. The other two MJ states are located at +246 cm−1 (MJ = ±3/2) and +397 cm−1 (MJ = ±5/2) (Figure 6). The dynamic magnetic properties revealed the signature of a SMM operating in external applied field of 2000 Oe with Δ = 21.1(5) K and τ0 = 1.7(3) × 10−6 s. The YbIII NIR luminescence is sensitized by the irradiation in the HOMO → LUMO ILCT (18800 cm−1) (Figure 6). The emission spectrum gives a set of data for the energy splitting. The energy barrier value extracted from the ac data remarkably

5. CONCLUSIONS In this Account, we have reviewed coordination complexes of lanthanide involving TTF-based ligands. The latter plays the crucial roles of structural agent and organic chromophore, which guaranty both efficient magnetic isolation of the magnetic centers and sensitization of the lanthanide luminescence. Numerous examples show visible and NIR well-shaped lanthanide luminescence as well as SMM behavior. We have demonstrated that the mechanism of sensitization could involve an energy transfer from the excited singlet state or a photoinduced electron transfer. SMM behaviors have been observed for both oblate (DyIII) and prolate (YbIII) ions in solid and solution with the presence of memory effect in magnetic field up to helium-4 temperature. Three different strategies acting on the DyIII nucleus and on the environments of the molecules were successfully applied to slow the magnetic relaxation in zero field. The combination of isotope enrichment and magnetic dilution in a diamagnetic matrix led to the observation of memory effect even at zero field. We have also shown that luminescence is strongly correlated to magnetism and that these two properties can be investigated in parallel to elucidate the ground state splitting in a given environment. All these investigations were supported by quantum chemistry calculations to help understand and predict some unusual and unexpected results. For example, they allow explanation of why “weak” intermolecular interactions can destroy SMM behavior in condensed crystalline state. This Account has focused on the development and the study of the magnetic and photophysical properties of lanthanide− TTF based complexes in which TTF ligands are neutral. We are now exploring the possibility to modify or switch the physical properties by oxidizing or reducing our ligands. We already showed that the luminescence can be sensitized at low energy using the radical cation form of the TTF ligand. We decided to discuss herein only pure f compounds but heterobimetallic 3d4f coordination complexes can also be obtained with TTF-based ligands.65,66 This opens exciting opportunities to add new 2839


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resources. We gratefully acknowledge the contributions of coworkers and colleagues whose names are listed as coauthors in references section.

physical properties to the ones already investigated, such as thermal spin crossover or light-induced excited spin-state trapping (LIESST) effects.

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REFERENCES

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Fabrice Pointillart obtained his Ph.D. in physics and chemistry of materials from Pierre et Marie Curie University in 2005, supervised by Prof. Cyrille Train and Prof. Michel Verdaguer. He then entered a two year period of postdoctoral research at the Department of Chemistry “Ugo Schiff” and INSTM Research Unit, University of Florence, with Prof. Roberta Sessoli and Prof. Dante Gatteschi. He joined the University of Rennes 1 in 2007 as CNRS researcher. His current research interests are focused on multiproperty molecules based on tetrathiafulvalene ligands and lanthanide ions. He was awarded the CNRS bronze medal in 2014. Boris Le Guennic received his Ph.D. in Chemistry from the University of Rennes in 2002. He then successively moved to the Universities of Erlangen, Buffalo, and Bonn for postdoctoral stays in the groups of Jochen Autschbach and Markus Reiher. In 2005, he was appointed as CNRS researcher at ENS de Lyon (France). He is currently CNRS researcher at the University of Rennes where his research is devoted mainly to the modeling of magnetic and optical properties in lanthanide complexes and organic chromophores. Olivier Cador started his research activities in 1994 in Bordeaux (France), supervised by the late Professor Olivier Kahn. He was recruited by University of Rennes in 2003 after two postdoctoral research fellowships in Japan and Italy. His research is oriented toward multifunctional molecular-based materials. He recently oriented part of his activities on single molecule magnets (SMMs). Olivier Maury was graduated from École Nationale Supérieure de Chimie de Paris in 1993 and completed his Ph.D. in 1997 in uranium chemistry under the supervision of Dr. Michel Ephritikhine. After a postdoctoral stay with Dr. J.-M. Basset, he got a CNRS permanent position in 1999 at the University of Rennes in the group of H. Le Bozec. In 2004, he moved to École Normale Supérieure de Lyon to join the team of Dr. C. Andraud. His current research interests concern the synthesis and photophysical studies of lanthanides complexes and NIR dyes for nonlinear optics or bioimaging applications. Lahcène Ouahab received his Ph.D. from the University of Rennes in ̂ de conférences” at the University of Constantine 1985. He was “Maitre (Algeria) and then Associate Professor at the University of Rennes (1988) before getting a CNRS permanent position in 1989. He is presently a CNRS director of research and leads the molecular materials research group. He was awarded the 1998 prize of the Coordination Chemistry Division, the 2011 Claude Berthault Prize of the “Académie des Sciences”, and the 2012 “Grand Prix Pierre Süe” of the French Chemical Society.

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ACKNOWLEDGMENTS This work was supported by the CNRS, Rennes Métropole, Université de Rennes 1, Région Bretagne, FEDER, and ANR (No. ANR-13-BS07-0022-01). B.L.G. thanks the French GENCI-CINES/IDRIS center for high-performance computing 2840


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