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A two-dimensional network of spin crossover molecules controlled by light

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Controlling the spin state of a two-dimensional molecular network with light, it’s what just demonstrated teams of Laboratoire de Matériaux et Phénomènes Quantiques, of Institut de Chimie Moléculaire et des Matériaux d’Orsay, and of Service de Physique de l’Etat Condensé (CEA Saclay). In the work published in Nature Communications, the researchers observed under light irradiation and by means of a scanning tunneling microscope (STM) the individual switching of molecules from a non-magnetic state to a paramagnetic state in the molecular monolayer. This experimental and theoretical work opens promising prospects in the spin-state control at the single molecule scale.

Spin cross-over molecules present the unique property of being able to switch under light excitation from a stable low-spin electronic state to another high-spin state (bistability) giving them a memory effect . Therefore, these objects are of major interest in the design of new components for molecular spintronics. Changing the molecule spin-state involves structural distortions influenced by the intermolecular interactions inside the monolayer as well as by the contact with the metallic substrate. Therefore, the observation under light irradiation of individual molecular switch within an organized monolayer of spin cross-over molecules is a fundamental issue in the field of molecular electronics. It is the common work realized by three teams of physicists, chemists and theoreticians belonging to MPQ, ICMMO and SPEC respectively.
They have studied by STM the structure of a spin cross-over molecular monolayer deposited on a gold surface and its evolution under light irradiation. Calculations made by Density functional theory were then used to characterize unambiguously the spin state of the molecules seen on the surface, especially by the description of inelastic process observed only in the scanning tunnelling spectra of the magnetic state. They thus showed the formation of a long-range order superstructure alternating one molecule in the magnetic state and two molecules in the non-magnetic state. This order corresponds to a new thermodynamic phase of spin cross-over molecules in low dimensionality. In addition, blue light irradiation induces the molecular switching from the low spin state to high-spin in the molecular islands, revealing propagating effects in the layer, as well as fluctuations of the photoexcited phase at low temperature (4.6 K).
This study demonstrates for the first time the switching by light of individual molecules organized in a monolayer and the propagation of the magnetic phase thus revealing the importance of the collective effects in the molecular layers. It thus opens the way to control the bistability of spin cross-over molecules in two-dimensional ordered layers, with promising prospects for molecular spintronics.

Figure :

Scanning tunneling microscopy image (50x50nm2) showing the excited phase of high-spin molecules induced by light irradiation. The whole phenomenon of excitation (laser turned on) and thermal relaxation (laser turned off and then turned on again) can be viewed on the attached film that consists of a series of 386 images.

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Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer, K. Bairagi, O. Iasco, A. Bellec, A. Kartsev, D. Li, J. Lagoute, C. Chacon, Y. Girard, S. Rousset, F. Miserque, Y. J. Dappe, A. Smogunov, C. Barreteau, M-L. Boillot, T. Mallah and V. Repain, Nature Communications 7, 12212 (2016).