Effect of Light Irradiation on Spin Crossover Single Ion Magnets Involving Redox-Active Ligands – SCOSIMLIGHT

The 4f elements are ideal candidates for the elaboration of molecules with memory effect due to their high magnetic moment and strong magnetic anisotropy. Nevertheless these magnetic properties are complex and difficult to understand. The 4f elements are also well known for their specific emission properties which can be seen like a photography of the energy states responsible for the magnetic properties. Thus is fundamental to obtain well resolved and intense luminescence optimizing its sensitization by antenna effect using adequate organic chromophore. Then magnetism and luminescence are correlated. Understand the magnetic properties of such objects is crucial but the molecule mustn’t lose its magnetic memory in absence of applied magnetic field which limit the applications. To keep the memory effect, even without magnetic field, the system can be isolated from all the magnetic perturbations isolating the molecules one from the others in the single crystal. Thus the molecule is not under the influence of the magnetic field coming from the neighboring molecules: that is the magnetic dilution. Another method consists in playing with the magnetism of the nucleus changing the isotope of the metal center.

The major results of this project are:
- The level of understanding of lanthanide-based single molecule magnet behavior is increased thank to the correlation between magnetic and optical properties.
- The memory effect at the molecular scale can be observed with the magnetic dilution putting the system in solution or doping a diamagnetic isomorphous matrix with our active system.
- Observation of the molecular memory effect at zero magnetic field by isotopic enrichment.
- Thermal and photo-induced spin crossover for a Fe(II) complex involving an electro-active ligand.
- 11 new research collaborations including 7 formalized projects and attribution of an ERC Consolidator project.

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Among the 42 months of this project, 22 scientific articles have been published in high impact factor journals. 55 scientific communications have been given with 13 invited seminar, 42 oral communications in national (14) and international (28) conferences. The CNRS Bronze Medal and the SCF/SFP chemistry-physic division and SCF coordination chemistry division young researcher price have awarded some researchers of the consortium of the project.

Recently, it appeared more and more clearly that molecules will serve as the fundamental component of future electronic or spintronic devices. Indeed, since isolated molecules represent the smallest conceivable stable structures, they constitute the ultimate goal toward the miniaturization of “electronic circuits”. Chemists are now able to design selectively target molecules with a specific property for a peculiar application. One may cite single molecule magnets (SMM) where the magnetic information is stored on a 1 nm scale bit or spin crossover molecules that can change of spin-state under an external stimulus.
The aim of this proposal of fundamental issue is to confer multiple properties to a single isolated complex which may display simultaneously Single Ion Magnet (SIM), Spin Crossover (SCO) behaviours and Near Infrared luminescence thanks to the controlled insertion of both Ln(III) and Fe(II) ions in the molecule. The resulting 3d4f heterobimetallic complexes will be submitted to light irradiation to modulate these properties through the possible appearance of “Light Induced Excited Spin State Trapping” (LIESST) and Photo-induced Electronic Transfer (PET) phenomena.
To achieve such multi-properties 3d4f complexes, our strategy leans, on one hand, on lanthanide ions (Ln = Tb, Dy, Er and Yb) which are ideal candidates to produce SIM due to their large magnetic moment and anisotropy, and which possess specific luminescent properties. On the other hand, the SCO phenomenon occurs in some metal complexes of the first raw in which the spin state changes due to external stimuli such as a variation of temperature, pressure, magnetic field or light irradiation. Among the 3d ions, Fe(II) in an octahedral environment is certainly the most studied because the spin changes from a diamagnetic low spin (LS) state to a paramagnetic high spin (HS) state. In both SIM and SCO complexes, the ligands are non innocent as they essentially drive the phenomena. Indeed, in SIM, the coordination polyhedron around the 4f element is essential to achieve Ising type anisotropy. Furthermore, sensitization of the emission directly depends on the ability for the ligand to play the role of antenna. In SCO complexes, the ligand field created around the Fe(II) ion must be finely tuned to balance the system between the HS and LS spin state by small external stimulus. The involved partners have already demonstrated that the TTF-based ligands can play the double role of structural agent for SIM behaviour and organic chromophore for antenna effect. Thus, this proposal involves TTF core functionalized with two sites of coordination specifically suited for Fe(II) and Ln(III) ions: i) the benzoimidazole-2-pyridine acceptor (A) which could give SIM behaviours when it is associated to Ln(III), ii) the 2,6-di(pyrazol-1-yl)pyridine which is well known for the elaboration of SCO Fe(II) complexes displaying LIESST effect. The target physical properties (SCO, SIM and luminescence) will be separately studied thank to the fine choice of metal ions to design TTF-A-[Fe]-[Y] (for SCO) and TTF-A-[Zn]-[Ln] (Ln = Nd, Tb, Dy, Er and Yb for SIM and luminescence) complexes. Finally the complexes TTF-A-[Fe]-[Ln] will be irradiated in the charge transfer bands to evaluate the modifications on the initial properties. The characterization of the compounds will be carried out by a combination of methods including X-ray diffraction, absorption and emission measurements, magnetometry. Coupled to quantum chemical calculations these studies will afford information on their charge transfers (absorption), electronic structure (luminescence), magnetic susceptibility (SCO/LIESST), magnetic anisotropy axis orientation (SIM).
This proposal is innovative and exciting, it constitutes a real breakthrough in the field. It is targeting for the first time novel and unknown molecules involving both SIM and SCO behaviours. These systems are therefore excellent candidates for light irradiation effects.