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New photoswitchable materials based on electron transfer process

Periodic Reporting for period 1 - ETSMM (New photoswitchable materials based on electron transfer process)

Reporting period: 2017-04-01 to 2019-03-31

Digital technology has intimately invaded our modern life for the last three decades. A recent study reports that in 2018, the number of data created by humankind reached huge values of 2.5·1030 octets per day (for a chemist this is equivalent to 4.1 millions of moles of data per day). Researchers and engineers face now the challenge of creating and optimizing new devices based on functional magnetic or optical materials to respond to this market demand.

Interestingly, certain molecular compounds show promising features for data storage. For example, the discovery of slow magnetization relaxation in polynuclear compounds (named SMM for Single-Molecule Magnets) created the hope to store magnetic information on a single molecule. Another example concerns molecules that present two configurations that are close in energy and that can be optically (or thermally) interconverted. These phenomena can be also used to store data if the switching is accompanied with a memory effect when the input (i.e. magnetic field, light or temperature) is switched off. Under this condition, each state can be coded in the universal binary system: “0” for one state and “1” for the other state.

Most of the switchable coordination compounds are based on spin crossover (SCO, rearrangement of unpaired electrons located on one metal ion) processes or an electron transfer (ET) between two metal ions. The SCO phenomenon has been extensively studied during the last decades but the ET materials have been described more recently, and are therefore less explored. Moreover, the bimetallic nature of the ET materials gives extended possibilities to optimize their physical properties compared to SCO materials.

The first example of photo-induced ET in a molecule-based magnetic compound was reported by the group of Prof. Hashimoto in 1996. His team discovered that a cubic three-dimensional Fe-Co Prussian Blue analogue (PBA) is transformed by red light from a diamagnetic state (a non magnetic state) to a magnet with a Curie temperature of 16 K. This effect was attributed to a metal-to-metal electron transfer in the constitutive Fe-CN-Co units converted from the diamagnetic Fe(II)Co(III) (no unpaired electron) to the paramagnetic Fe(III)Co(II) (with unpaired electrons responsible of the magnetic properties) states. This result motivated several groups to obtain a molecular equivalent of this network, which would be easier to manipulate and shape into the devices for the future. The employed strategy was to mimic the structure of the network using blocking ligands (organic molecules that coordinate to metal ions limiting further coordination) to isolate a fragment of the network. The hosting group at Centre de Recherche Paul Pascal (CRPP, Pessac, France), in collaboration with Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB, Pessac, France) was successful in this approach and reported the first molecular analogues (Fe4Co4 cube, Fe2Co2 squares and FeCo pairs), which show a thermal- and light-induced ET at the molecular level.

However, before developing molecule-based materials at an industrial scale, several requirements need to be fulfilled first: high operating temperature, low fatigue, thermal stability, fast writing, non-destructive read-out capabilities, etc. Since bistable molecular materials are still quite limited in quantity, it is important to focus research efforts towards discovery of new switchable molecular systems fulfilling these requirements. Thus, the objective of this fellowship was to prepare novel ET pairs and squares that would be further used to build new functional polynuclear and extended compounds with interesting photoswitchable properties.
The fellow realized soon that the proposed approach, which consisted on the use of flexible ligands in ET pairs and squares to allow further coordination, had some drawbacks. Thus, the control on the dimensionality of the final compounds was lost due to the flexibility of the ligands as, instead of obtaining pairs, the different building blocks assembled in a trinuclear and a mononuclear molecular objects. By studying the newly synthesized compounds and the compounds known in the host groups, it was concluded that to get the desired properties, the redox potential difference between the donor (Co) and acceptor (Fe) units needs to be close to 1 V. If the difference is larger, the ET process will never be observed while if it is lower, diamagnetic entities will be obtained.

On the other hand, the fellow managed to synthesize a novel square with an interesting property: multistep ET process. In this compound, the ET occurs at different temperatures for the two FeCo halves of the square, leading to a two-step decrease on the magnetic susceptibility upon cooling. This compound can be then photoexcited to give the initial paramagnetic state. It should be noted that this is only the second FeCo molecular square displaying such behaviour, while a three-step process has also been observed in one molecular system.

Finally, the fellow worked on the preparation of polynuclear and extended coordination compounds using different ET building blocks. In parallel, extended systems with interesting properties were also targeted by using a second synthetic approach: the use of redox non-innocent ligands. In these systems, an ET process from the metal to the ligand can lead to radical ligands and to interesting magnetic and conductive properties. The work carried out in this regard was published in Polyhedron 2018, 153, pp 248-253 and an overview article was written for a broader audience: Redox-Active Approach Towards New Magnetic And Conductive Two-Dimensional Materials (published in the Science Trends platform).

The results on FeCo Prussian Blue Analogues have been presented in the following conferences: GdR MCM-2 (Dourdan, France, 2017 and 2018), 68th CJSCC (Sendai, Japan, 2018) and 43rd ICCC (Sendai, Japan, 2018) and 16th ICMM (Rio de Janeiro, Brasil, 2018). The results have been presented in two formal group meetings to the teams involved in the project and several visiting researchers. The fellow attended to 3rd BOOK-D (Pessac, France, 2017), 6th ECMM (Bucharest, Rumania, 2017) and a meeting about Emergent Physical Phenomena in Condensed-Matter Systems (Bordeaux, France, 2019). The fellow participated in two outreach activities to promote gender equality in the scientific community: Journée Filles et Maths (Bordeaux, France, 2018) and La Science Taille XX Elles (Toulouse, France, 2018). Finally, during one week the fellow showed her job and her activities in the host laboratory to three motivated French students of 14-15 years.
"A careful examination of the already known compounds and the compounds synthesized during this project revealed the required redox potential difference between the building blocks to observe the targeted ET properties. Thus, by studying the different building blocks we are now able to predict the properties of the final materials, and therefore, to find a good match between the building blocks without wasting time in the laboratory trying all the possible combinations. In addition, we know how to prepare ET squares with more than one ET process, which could be good candidates for ""intelligent"" molecular devices of the future. However, it should be noted that the ETSMM project deals with fundamental research and that there are no short term potential impacts envisioned for the moment."