Embedding a mobile ligand in a complex to control its spin state on a surface Molecular spin switches for which the THE HAIRCLIP CONCEPT magnitude of the magnetic moment can be The targeted compound consists of pyridine axial ligand rigidly toggled between two values usually do not strapped to a Ni porphyrin. The rigid strap provides a sort of work on metal surfaces. We have designed a molecular feedback mechanism to couple the change of the porphyrin shape (planar or flat), which depends on the Ni new molecular switch that may be reversibly spin state (high spin or low spin), to the position of the axial switched on Ag(111). It relies on the motion of ligand and thereby the coordination number of the Ni (5 and 4, a ligand that is strapped above the central Figure 1). The molecules are locked either in one or the other state and an action on any of the three properties can trigger metal ion of the complex. As a consequence, switching. The mechanism bears some similarity to that of a the position and binding of the ligand as well spring-loaded hairclip. as the spin and are interlocked. IMPLEMENTATION & VERIFICATION Spin crossover complexes may be switched between states Three derivatives implementing the hairclip concept were with a low and a high spin. This makes them appealing for synthesized. Subtle changes in the geometry between the applications such as data storage [1]. Metallic substrates arederivatives allow tuning the ground state as inferred from x-ray desirable to electrically address the molecules. However, thesediffraction, nuclear magnetic resonance, x-ray absorption (XA) compounds tend to fragment or lose their function on metallicspectroscopy and x-ray magnetic circular dichroism measurements. surfaces [2]. On the other hand, molecules like porphyrins We performed further XA measurements on submonolayer are fairly insensitive to the interaction with the surface but docoverages of the different derivatives on a Ag(111) surface not exhibit spin-state bistability themselves although gaseous(Figure 2). The interlocked spin and coordination states are molecules such as ammonia can change their coordination and directly inferred from the line shape of the Ni L edge, where we spin state [3]. We present a concept where a small molecule 3 observe that the ground state of these derivatives is essentially is fixed as an axial ligand to a complex. The bond length canpreserved on the surface. Derivative 1 and 2 have a low-spin be changed and thus the spin state of an individual molecule (coordination number 4) ground state while derivative 3 has a can be controlled. high spin (coordination number 5) ground state. FIGURE 1 22