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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 devices3–5. 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 exceptions6–9. Robust metal–organic platforms, on the other hand, rely on external axial ligands to induce spin switching10–14. 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.

Rotors and switches are elementary building blocks of molecular machines. To achieve more advanced functions, these units have to be integrated into solid-state devices, which triggered interest in mounting these functional units in well-defined geometries onto surfaces. While vertically oriented switches and rotors have been obtained by various strategies, the design of surface-parallel switches and of altitudinal rotors with an in-plane oriented rotation axis has proven to be more difficult. We here demonstrate a molecular adlayer system with highly defined geometry and laterally oriented functional groups that combines facile photoswitching and rotation. We employ a custom-designed molecule with two platforms and pillars that span an azobenzene unit between them. The molecules form well-ordered monolayers on Au(111) with the azobenzene units parallel to and above the surface. Spectroscopic data and density functional calculations suggest that in the trans configuration, at room temperature, the azo unit is freely rotating. Upon irradiation with UV light, the azo unit switches to the bent cis configuration and rotation stops. Irradiation with 430 nm restores the rotating trans state. Notably, the photochemistry is not quenched by the metal surface. This approach offers a promising strategy to operate molecular machines on metal surfaces with light, which is still a major problem in molecular nanotechnology.