JCJC SIMI 7 - JCJC - SIMI 7 - Chimie moléculaire, organique, de coordination, catalyse et chimie biologique

Synthesis, characterization and Optical measurements of Chiral Spin-Transition compounds – CHIROTS

Switchable chiral objects : synthesis and characterization.

Spin crossover complexes are switchable objects that are prospected as molecular bits, or coloured indicators sensitive e.g. to light. Incorporating optical activity to this objects can constitute an innovative approach for reading/writing these materials at the molecular scale.

Spin crossover and optical activity : a new read/write tool ?

Molecular magnetism has drawn much interest these last 20 years particularly because of the possibility of using designed molecular precursors to obtain controlled geometries and properties. Spin crossover is a much studied phenomenon given its potential applications in data storage or thermo- or piezochrome indicators. In this context, the CHIROTS project aims to introduce a new variable, optical activity, in the complex landscape of spin crossover compounds, with complex physics underlying the phase transitions seen. The difficulty in combining spin crossover and chirality is met through the use of chiral counteranions and ligands, for complexes either mononuclear, with generally helicoidal conformations, or polynuclear, where the Fe(II) coordinative lability will be blocked by the supramolecular architecture. Complexes thus obtained open the way towards in-depth optical studies: time-resolved light sources, polarized light, non-linear optical effects. A better understanding of the photophysics of such objects can open the way towards the use of light as a read/write tool for discrete molecular objects, through playing on both spin crossover and optical activity. Indeed the use of such objects for data storage is in part limited by the necessity of having such a read/write tool at molecular level.

The project focussed on the synthesis of optically active counteranions, and of chiral and achiral ligands. The purchase and implementation of an analytical and semi-preparative High-Pressure Liquid Chromatography chain (HPLC), thanks to a combination of ANR and Région Aquitaine funding, must improve the productivity of this step. The next step consists in synthesizing Fe(II) complexes starting from those ligands and counteranions. Once these complexes obtained, they are characterized both in the solid state and in solution. The recent purchase of a single-crystal APEXII diffractometer, and of a potentiostat, complete the extended range of physico-chemical analysis methods available at ICMCB. Once those characterizations performed, more in-depth optical studies will be undertaken within the local collaborations with Laboratoire Optique et Matériaux d’Aquitaine (time-resolved studies on the COLA platform, Centre Optique et Laser Aquitain, non-linear optical spectroscopy) and with the Spectroscopy group at Institut de Sciences Moléculaires (Vibrational Circular Dichroism and Raman Optical Activity). Recent results on the deposition of spin crossover complexes on surfaces and their study by Scanning Tunneling Microscopy and X-ray Absorptions Spectroscopy using synchrotron light open the way towards the modification of the non-centrosymmetric molecules used in those studies and the realization of evaporations where this non-centrosymmetry will be taken advantage of, which will allow the use of polarized light on perfectly defined objects.

The recent start of the project and the undergoing recruitment of a graduate student have made it so that there is still no major result to be published. Recent results on a related subject pursued by the coordinator deal with the deposition of spin crossover complexes on surfaces and the study of those coverings by Scanning Tunneling Microscopy and X-ray Absorption with synchrotron light, and a paper is currently being written. An adsorption model has been published recently by competitors,1 but this model doesn’t take into account the non-centrosymmetry of the complexes. In the context of synthesizing chiral ligands as planned within the CHIROTS project, we intend to work on modified complexes in order to take profit of this non-centrosymmetry, which could potentially lead towards chiral spin-crossover complexes perfectly isolated on a surface. Those objects would be ideal for studying the influence of light polarisation on the photophysics of the light-induced spin conversion.
1 T. G. Gopakumar et al., Angew. Chem. Int. Ed., 2012, 51, 6262.

Spin crossover is a phenomenon with potential applications which depend on the chemist’s capacity of designing and synthesizing new objects in a controlled way. The introduction of chirality doesn’t only imply the availability of new optical properties, which can be a step towards a better control of the photophysics underlying the phenomeno. It also means a new molecular engineering tool, more particularly concerning polynuclear complexes and cation and chiral counteranions interactions. We can thus hope an energy discrimination between iso- and heterochiral interactions which could lead to a modification of the spin crossover behavior.

Results predating the start of the project have not yet been published (publication submitted to Chem. Eur. J. et Dalton Trans. with refusals due to too long manuscripts). The graduate student preparing his Ph. D. is completing those results and rewriting two shorter manuscripts derived from the earlier manuscript.
Results acquired on the spin crossover molecules evaporation on a surface have resulted in a Letter: B. Warner et coll., J. Phys. Chem. Lett., 2013, 4, 1546.

The CHIROTS project aims at exploring fully a new variable, chirality, in an already complex subject, spin crossover compounds. Chiral spin crossover complexes are indeed very rare, and up to now they were usually obtained by serendipity.
The first goal of the CHIROTS project will be pursuing further the on-going synthesis of chiral spin transition compounds at ICMCB. New species will be obtained through controlled and rational design. Since spin crossover is known to be quite an elusive property, the synthesis effort to achieve novel species must not be underestimated: 1) synthesis of ligands which are either chiral or to be used with chiral counteranions, and if necessary their subsequent resolution as enantiopure species; 2) synthesis and resolution of chiral anions; 3) combination of the synthesized ligands and counteranions with a chosen metallic precursor, with optimisation of the reaction conditions, in order to obtain single crystals of respectable dimensions of the targeted complex if possible.
Two subordinated, and more difficult, objectives will be
1) to check the possibility of using chiral interactions in the crystal to guide or even control elastic intermolecular interactions which mediate cooperativity between metallic centres, that is to use chirality as a toolbox for supramolecular and crystalline engineering;
2) to aim at objects of higher dimensionality, which can prove to be “more chiral” than mononuclear complexes.
The second goal will be the complete characterization of the synthesized species with the whole panel of physical techniques locally available. Very important points here will be the elucidation of possible differences in physical properties for racemic and enantiopure species.
The third goal of the CHIROTS project will be to extend optical studies of preselected complexes through external collaborations (ISM and CPMOH). They will consist in dichroic vibrational studies (Vibrational Circular Dichroism and Raman Optical Activity) of complexes in solution, to characterize conformations and chiral interactions, then in time-resolved spectroscopy of photoexcited states of spin crossover species, for both racemic and chiral configurations. For the latter the effect of using circularly polarized light will be investigated. Complexes in solution will have to be studied first as simpler models, without anisotropy or free of intermolecular interactions. The study will then be extended to the solid state when convenient single crystals will be available.
The CHIROTS project, structured in 4 self-consistent and inter-dependent scientific tasks, require the recruitment of a PhD student for the synthesis and the physical characterization of chiral spin crossover complexes. This PhD student will integrate a group of researchers experimented in coordination chemistry, chemical, structural and physical characterization, and photophysics, completing the relevant knowledge and skills with the necessary manpower. CHIROTS takes advantage of the geographical closeness of collaborating groups which will induce a maximal reactivity and interactivity in the study of some selected chiral compounds synthesized, reinforcing thus the network of local collaborations available in the context of the interdisciplinary GIS network “Advanced Materials in Aquitaine”. The novel approach of CHIROTS is expected to produce enough interesting results to draw the attention of the community and arouse new collaborations.

Project coordination

Patrick ROSA (CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN) – rosa@icmcb-bordeaux.cnrs.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ICMCB CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE - DELEGATION AQUITAINE LIMOUSIN

Help of the ANR 203,999 euros
Beginning and duration of the scientific project: February 2012 - 42 Months

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