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# Reversible coordination-induced spin-state switching in complexes on metal surfaces

## Abstract

Molecular spin switches are attractive candidates for controlling the spin polarization developing at the interface between molecules and magnetic metal surfaces1,2, which is relevant for molecular spintronics devices3,4,5. However, so far, intrinsic spin switches such as spin-crossover complexes have suffered from fragmentation or loss of functionality following adsorption on metal surfaces, with rare exceptions6,7,8,9. Robust metal–organic platforms, on the other hand, rely on external axial ligands to induce spin switching10,11,12,13,14. Here we integrate a spin switching functionality into robust complexes, relying on the mechanical movement of an axial ligand strapped to the porphyrin ring. Reversible interlocked switching of spin and coordination, induced by electron injection, is demonstrated on Ag(111) for this class of compounds. The stability of the two spin and coordination states of the molecules exceeds days at 4 K. The potential applications of this switching concept go beyond the spin functionality, and may turn out to be useful for controlling the catalytic activity of surfaces15.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## References

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## Acknowledgements

We are grateful to the SOLEIL staff for the smooth running of the facility and to F. Leduc for technical assistance. We acknowledge funding by the Deutsche Forschungsgemeinschaft (DFG) via SFB 677. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant no. 766726.

## Author information

Authors

### Contributions

F.G. and R.H. conceived and synthesized the molecules. M.G., S.R., T.J.-T., S.J., F.D., A.S., J.G. and M.S. carried out the NEXAFS measurements. M.G. analysed the corresponding data. F.G. and F.R. carried out the DFT calculations. A.K., M.G. and A.W. performed the STM measurements and analysed the data. All authors discussed the data and their interpretation. M.G. wrote the manuscript with input from R.B., A.K., F.G. and R.H.

### Corresponding authors

Correspondence to Rainer Herges or Manuel Gruber.

Peer Review Information Nature Nanotechnology thanks Mirko Cinchetti and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## Extended data

### Extended Data Figure 1 Description of the switching procedure of complex 2.

Examples of switching procedures ab from the LS to the HS state and cd from the HS to the LS state. The tip is positioned over the center of the complex experiencing the switching, and the current feedback loop is active while the voltage is gradually varied as a function of time. While the switching from the LS to the HS state (HS to LS) occurs at a sample voltage of 2.5 V ($$-1.7$$ V) for these examples, the voltage required to induce a transition varies from 2.1 to 2.7 V ($$-1.6$$ to $$-2.1$$ V), and the large voltage may need to be applied for several seconds to several minutes for transitions from the LS to the HS state. The large spread in the threshold voltages may be due to the exact positioning of the tip and details of the molecule’s environment. To increase the success rate of the switching from the LS to the HS state, we usually applied a sample voltage between 2.5 and 2.7 V with a current between 30 pA and 1 nA for approximately 5 min. While this procedure is very effective, it is at the cost of selectivity as neighboring molecules were usually switched as well. In contrast, HS to LS switching is relatively efficient. Sub-pA currents may be sufficient to induce the corresponding transition (see Extended Data Fig. 2). The arrows indicate the time at which the switching takes place.

### Extended Data Figure 2 Switching sequence of two complexes.

ae Constant-height STM topographs (1 V, 10 pA, 4.65 nm wide) describing the switching sequence of two complexes (compound 2) on Ag(111). In this example, a sample voltage of 2.7 V was applied for 30 s between the acquisition of topographs a and b, and between topographs b and c (tunneling current of 30 pA) to induce transitions from the LS to the HS state of the lower left and lower right complexes. The topographs a and b illustrate that the switching can be induced in neighboring molecules upon application of large voltages. Indeed, the tip was positioned on the top right molecule (white disk in a) while the lower right molecule is switched (lower right molecule in b). f, $$I-V$$ curve recorded atop the lower right molecule in c leading to the switching from the HS to the LS state. Note that a current of $$\approx 250$$ fA was sufficient to induce the transition highlighting the efficiency of the backward switching process.

## Supplementary information

### Supplementary Information

Supplementary Figs. 1–12, Tables 1–4 and refs. 31–56.

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Köbke, A., Gutzeit, F., Röhricht, F. et al. Reversible coordination-induced spin-state switching in complexes on metal surfaces. Nat. Nanotechnol. 15, 18–21 (2020). https://doi.org/10.1038/s41565-019-0594-8

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• DOI: https://doi.org/10.1038/s41565-019-0594-8