Spin in a Closed-Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction

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Spin in a Closed-Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction, Angew. Chem. Int. Ed. 2019, 58, 821

 

 

It is desirable to design sustainable pure organic magnets to replace metals. Potential candidates are spin 1/2 molecular radicals. Despite recent advances in depositing multilayers of these species, the open-shell structure of radicals renders them difficult to work with. To date a few classical radicals, such as nitric oxide, 1,3,5-triphenyl-6-oxoverdazyl (TOV), polychloro triphenyl methyl, and bis-diphenylene-phenyl allyl (BDPA) radicals have been reported to be stable on metal surfaces. Typically, intact radicals can be deposited on metals that show low reactivity, such as the relatively inert Au(111) surface. Besides depositing the radicals, detecting them requires spin-polarized spectroscopy techniques. Photon-emission techniques are well adapted for the study of overlayers of radical molecules. Furthermore, creating local spins with eventual patterning for application is of great interest for the study of the inorganic–organic spinterface. To this end, scanning probes can locally produce molecular radicals.

 

 

In conclusion, DFT calculations show that ReA on a Au surface may be converted into a long-lived organic radical by an STM-induced reaction. When the molecule is activated through suitable voltage/current pulses, allylic hydrogen from the endocyclic bond is likely to migrate and eventually dissociate, leading to a stable radical with the spin entirely localized to the cyclohexene group. The proposed mechanism is consistent with all experimental observations. In recent STM manipulation experiments, cholesterol, a molecule with a cyclohexene group embedded into several cyclohexane groups, has likewise been switched into a state displaying a Kondo effect. On some other molecules with a cyclohexene unit like shikimic acid related attempts remained without success. We speculate that a keto-enol tautomerism involving the endocyclic bond may be a competing reaction path in the unsuccessful cases. A search for further molecules that may be switched on surfaces to a state with a localized spin is in progress. The use of local probes to induce reactions on organic layers allows us to achieve an enhanced control of charges and spins at the interface between a molecular layer and a solid substrate. The conversion of a single molecule into a stable radical in an otherwise closed-shell molecular layer, provides new possibilities to create hierarchical structures in a spinterface that combine radicals and closed shell molecules.

 

N'hésitez pas à consulter le communiqué de presse associé à cet article : Quand des molécules non-magnétiques organiques se transforment en aimant !

 

Également un communiqué de presse (en anglais) provenant de Mapping Ignorance : Spin in a closed-shell organic molecule stabilized on a metallic surface

 

Résumé: 

Angew. Chem. Int. Ed., 2019, 58, 821

 

Inert metal surfaces present more chances of hosting organic intact radicals than other substrates, but large amounts of delocalized electronic states favor charge transfer and thus spin quenching. Lowering the molecule–substrate interaction is a usual strategy to stabilize radicals on surfaces. In some works, thin insulating layers were introduced to provide a controllable degree of electronic decoupling. Recently, retinoid molecules adsorbed on gold have been manipulated with a scanning tunneling microscope (STM) to exhibit a localized spin, but calculations failed to find a radical derivative of the molecule on the surface. Now the formation of a neutral radical spatially localized in a tilted and lifted cyclic end of the molecule is presented. An allene moiety provokes a perpendicular tilt of the cyclic end relative to the rest of the conjugated chain, thus localizing the spin of the dehydrogenated allene in its lifted subpart. DFT calculations and STM manipulations give support to the proposed mechanism.

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Physico-chimie Théorique
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Spin in a Closed-Shell Organic Molecule on a Metal Substrate Generated by a Sigmatropic Reaction

 

Marie-Laure Bocquet,  Nicolas Lorente, Richard Berndt, and Manuel Gruber

 

Angew. Chem. Int. Ed. 2019, 58, 821

 

DOI: 10.1002/anie.201812121