Exchange Coupling Inversion in a High-Spin Organic Triradical Molecule

The magnetic properties of a nanoscale system are inextricably linked to its local environment. In adatoms on surfaces and inorganic layered structures, the exchange interactions result from the relative lattice positions, layer thicknesses, and other environmental parameters. Here, we report on a sample-dependent sign inversion of the magnetic exchange coupling between the three unpaired spins of an organic triradical molecule embedded in a three-terminal device. This ferro-to-antiferromagnetic transition is due to structural distortions and results in a high-to-low spin ground-state change in a molecule traditionally considered to be a robust high-spin quartet. Moreover, the flexibility of the molecule yields an in situ electric tunability of the exchange coupling via the gate electrode. These findings open a route to the controlled reversal of the magnetic states in organic molecule-based nanodevices by mechanical means, electrical gating, or chemical tailoring.This work was supported by an advanced ERC grant (Mols@Mols). We also acknowledge financial support by the Dutch Organization for Fundamental research (NWO/FOM). JV, CR, IPRM, DRM, SB thank funds from Networking Research Center on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN) DGI and MAT2012-30924 (Spain) and from MINECO (BEWELL CTQ2013-40480-R, PRI-PIBIN-2011-1028 and CTQ2012-30751) and Generalitat de Catalunya (2014-SGR-17, 2014-SGR-97, 2014-SGR-97, XRQTC). EB thanks funds from the EU FP7 program through project 618082 ACMOL and the Dutch funding organization NWO (VENI). We acknowledge C. Franco and V. Lloveras for checking the sample purity and V. Lloveras (ICMAB) for ESR spectra.Peer reviewe