ETSMM · New photoswitchable materials based on electron transfer process

New photoswitchable materials based on electron transfer process

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.

Data: CORDIS, © European Union