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MULtiple PROperties Single Molecule Magnets

Periodic Reporting for period 2 - MULTIPROSMM (MULtiple PROperties Single Molecule Magnets)

Reporting period: 2019-02-01 to 2020-07-31

SMMs are molecules that show slow relaxation of the magnetisation, which can lead to the observation of magnetic hysteresis of molecular origin. SMMs may exhibit a memory effect at the molecular scale and thus present potential applications in high density data storage, spintronic and quantum computing. However the use of molecular magnets is currently locked by one key parameter: the temperatures at which the magnets operate are still far too low since the magnetic poles of the molecules cannot be trapped in one direction. To overcome these limitations, my project proposes to cancel the perturbations on the magnetic moment and the internal magnetic field by isotopic enrichment in free nuclear spin lanthanide and magnetic dilutions.
Lanthanides possess also specific luminescent properties with an emission ranging from the visible to the near infrared spectral range and also a μs-ms luminescence lifetime with large Stoke-shifts (difference between the excitation and emission wavelengths). These unique characteristics generate new applications in material science or bioimaging, in the conception of OLED, in time-resolved luminescent immunoassays, or mono- or biphotonic imaging microscopy. The fundamental point is that both lanthanide magnetism and luminescence have the same origin i.e. the energy splitting of the ground multiplet state. An actual challenge is to combine in the same molecule the SMM property to one or more subsequent physical properties in order to obtain a multifunctional SMM. In my project the lanthanide luminescence will be used as a spectroscopic tool to control and understand the magnetic properties.
The Spin Crossover (SCO) phenomenon occurs in some metal complexes of the first raw. The spin state changes in varying the temperature, the pressure, the magnetic field or in stimulating complexes with light irradiation. While the spin crossover phenomenon occurs at the molecular level, the thermally induced spin transition in the bulk can be accompanied by thermal hysteresis with long-range cooperative interactions between complexes. The change of spin state induced by light irradiation (LIESST: Light Induced Excited Spin State Trapping) is essentially from the Low Spin (LS) to a metastable High Spin (HS) state and is essentially detected at low temperature when one absorbed photon can convert the spin of one metal complex. In my project both thermal (SCO) and light induced (LIESST) magnetic bistabilities will be combined with the memory effect of the SMM behaviour.
Circularly polarized luminescence (CPL) measures the differential spontaneous emission of right-circularly vs. left-circularly polarized light by chiral molecular systems and can be viewed as the emission analogue of circular dichroism (CD). To date, CPL has been used mainly to investigate configurational and conformational changes in chemical and biological edifices because it combines the sensitivity of luminescence measurements and the specificity of the signal for the chiral environment.

MULTIPROSMM, MULtiple PROperties Single Molecule Magnets, aims to combine several physical properties to the single-molecule magnet behaviour to enhance and better understand the magnetic properties to step forward towards potential applications. My project allows the design of original molecular systems able to present magnetic bistabilities under several stimuli (temperature, magnetic field and light) on an unprecedented wide temperature range (very low temperature with Single Molecule Magnet (SMM), intermediate temperature with Light Induced Excited Spin State Trapping (LIESST) and high temperature with Spin Crossover (SCO)) using the luminescence and Circularly Polarised Luminescence (CPL) for the optimal understanding of the magnetic properties. Both isotopic enrichment and shaping will allow the enhancement of the magnetic properties and a step forward towards applications.
Main results achieved so far:
1) Redox magnetic switching in Single-Molecule Magnet
2) Heterobimetallic 3d4f and 4f4f’ complexes with modulation of the SMM and emission behavior of the 4f ion depending of the nature of the associated 3d or 4f’ ions.
3) Trinuclear, hexanuclear and polymers were designed by assembly of SMM and SCO building blocks.
4) Redox-active chiral SMM
5) NIR CPL and field-induced SMM in ytterbium binaphtyl derived biphosphate polymers series
6) Complete and restricted active space self-consistent field (CAS-/RAS-SCF) wave function methods was applied to calculate circular dichroism (CD) and CPL for europium complex.
7) Role of the hyperfine coupling constants on the slow magnetic relaxation of a Dysprosium SMM
8) Detection of the isotopic enrichment effect on the slow magnetic relaxation by Muon spin spectroscopy
9) Unprecedented observation of the Isotopic enrichment effect on the slow magnetic relaxation of an Yb(III) SMM.
10) Grafting of redox-active SMM and chiral SMM on gold surface with development of computational approaches to rationalize their structural and physical properties.
The listed 10 results given to the previous section already illustrated the progress beyond the state of the art but other results are expected until the end of the project such as:
From the WP1, the first molecular system displaying both (thermal and photo-induced) spin crossover and Single-Molecule Magnet (SMM) is expected in order to extend the temperature range on which a magnetic bistability is observed.
From the WP2, the first observation of both Single-Molecule Magnet behavior and Circularly Polarized Luminescence at the same temperature for an unique molecular system.
Calculs of CPL for anisotropic Ln Complexes and coordination complexes grafted on surface.