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Home > Research Groups > Multifunctional Inorganic Materials

Multi-Properties Coordination Complexes: Luminescence, Single Molecule Magnet, Spin Crossover and Electronic Conductivity

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Dr. Lahcène Ouahab, Dr. Stéphane Golhen, Dr. Olivier Cador, Dr. Fabrice Pointillart
& Mr. Bertrand Lefeuvre

This five-membered group is hardly involved in the research of coordination complexes that exhibit multiple properties such as magnetism, luminescence and electronic conductivity. The common denominator is the intensive use of Tetrathiafulvalene derivatives as ligand to promote additional properties as well as chiral ligands.

Their skills cover the synthesis of the precursors and the targeted complexes, their characterizations and the investigations of their physical properties. In this frame they developed local, national and international collaborations:

  • ENS Lyon (O. Maury)
  • Japan (TIT, Kyoto University, University of Osaka)
  • Russia (G. A. Razuvaev organometallic chemistry Institute of Russian Sciences Academy)
  • Ukraine (L. V. Pisarzhevskii Institute of Physical Chemistry of the National Academy of Sciences of the Ukraine)
  • Italy (University of Florence)
  • Brazil (Universidade Federal Minas Gerais - Belo Horizonte, Universidade Federal Fluminense - Niterói)
  • Algeria (University of Setif, University of Constantine, University of Tebessa)
  • Switzerland (University of Bern, ETH Zurich)
  • India (Amritsar, NISER - Bhubaneswar)

Sensitization of the luminescence of Lanthanide ions by antenna effect

(collaboration with Dr. Olivier Maury and Dr. Yannick Guyot)

Since the pioneering studies of Weissman in the 1940s, the sensitization of lanthanide luminescence via the antenna effect is becoming extremely classical. The photophysical sensitization process generally involves an energy transfer from the triplet excited state of an organic or a transition metal containing an antenna chromophore. Recently, the direct lanthanide sensitization from charge transfer excited state appears as an alternative process resulting in a red-shift of the excitation wavelength far in the visible thanks to the design of ligands with appropriate donor–acceptor charge transfer transitions. In the last five years we have developed TTF-based donor-acceptor ligands to sensitize the luminescence of lanthanide ions such as EuIII in the visible range or YbIII in the near infra-red. (see for example Coord. Chem. Rev. 2012, 256, 1604)

Single Molecule Magnet behaviour in tetrathiafulvalene-based complexes of lanthanides and chiral magnets (Collaboration with Dr. Boris Le Guennic, Dr. Kevin Bernot, Dr. Jeanne Crassous, Dr. Claudia Lalli, Dr. C. L. M. Pereira, Dr. F. Pineider, Dr. G. Campo)

SMMs are objects that are able to store a magnetic information at molecular scale. In the last ten years, lanthanides have proved their efficiency to produce new magnets with higher and higher operating temperatures. In this frame, a part of our research activity is devoted to the elaboration of Single-Molecule Magnets (SMMs) based on TTF ligands and chiral SMMs.

SMMs in all phases
One of our main goals was to elaborate SMMs which behave so not only in crystalline state but also in solution (J. Am. Chem. Soc. (2013) 135, 16332-16335).

This compound has been obtained from the functionalization of TTF with a nitrogenated bis-chelating benzimidazole-pyridyl group and a DyIII β-diketonate precursor. The magnet behaviour measured in crystalline state persists when the molecule is dispersed in dichloromethane solution. It demonstrates that this complex phenomenon originates from the molecule itself and not from the crystal packing.
This work has been highlighted by the CNRS:
http://www.cnrs.fr/inc/communication/direct_labos/leguennic.htm

Exotics SMMs

In the course to improve the efficiency of SMMs, in other words to raise the temperatures at which the magnet can operate, we opted for a new strategy based on isotopic enrichment. The low temperature behaviour of SMMs is known to be governed by quantum mechanics such as tunnelling effect. This tunnelling effect can be viewed as an advantage for quantum computation but limits the operability of the magnet as a storage unit. Our idea was to cancel, at least partially, the tunnel in minimizing perturbations on the electronic magnetic moment. To do so, we carefully selected dysprosium isotopes with and without nuclear spins (161Dy and 164Dy). The enrichment with a nuclear spin free isotope opens the hysteresis loop at zero field (here below). This work also has been highlighted on the CNRS website and has been covered by Angewandte Chemie (Angew. Chem. Int. Ed. 2015, 54, pp. 1504–1507).

http://www.cnrs.fr/inc/communicatio...

Chiral SMMs

In our quest of multifunctional SMMs we have made a break at chirality stop. Combination, within the same material, of several electronic properties that may either simply coexist or strongly interact is an important issue with several applications. In this line, the synergy between chirality and magnetism is very promising. We have decided to tackle this problem in synthesizing configurationally stable ortho-fused aromatic rings ([6]helicenes) functionalized with 2,2’-bipyridine moieties to coordinate DyIII. Racemic and optically pure forms have been synthesized. They magnetically behave differently with the enantiomerically pure derivatives being better magnets than the racemic one (Chem. Commun. (2016) 52, 14474-14477).

Crystal packings of the racemic form (a) and the (M) and (P) helicoidal arrangements. The dash line represents the mirror between both enantiomers.

Magnetic hysteresis loops recorded at 500 mK and measured at a sweep rate of 16 Oe s-1 for optically pure (green lines) derivatives and the racemic form (red line).

Luminescence and magnetic correlation in redox-active luminescent single molecule magnet

(collaboration with Dr. Boris Le Guennic and Dr. Olivier Maury)

Coordination chemistry contributed to the development of new functional molecules through, for instance, 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 behaviours. 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. Over the last five years we have demonstrated that TTF-based derivatives act as organic chromophores for the sensitization of visible and near-infrared luminescence of lanthanides. 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. We recently accounted for our contributions to this field (Acc. Chem. Res. (2015) 48, 2834-2842).

Spin crossover systems based on TTF

(collaboration with S. Decurtins, S.-X. Liu, B. Le Guennic and M. Kepenekian)

Octahedral Fe(II) complexes can be either in their Low Spin state (LS, S=0) or in their High Spin state (HS, S=2) depending on the strength of the ligand field. For intermediate field an external stimulus such as temperature, pressure or light irradiation can change the spin state. We have developed a TTF-based ligand to produce such a spin-transition. A mononuclear Fe(II) complex involving a tetrathiafulvalene-based ligand exhibits thermal spin-crossover (around 143 K) with a large thermal hysteresis (48 K). The chromophoric and π-extended ligand allows a Near-Infrared (NIR) sensitization for the Light-Induced Excited Spin-State Trapping (LIESST) with the photoinduced spin conversion (LS→HS) at temperatures as high as 90 K (Dalton Trans. (2016) 45, 11267-11271).