Spin crossover (SCO) complexes possess a bistable spin state that reacts sensitively to changes in temperature or excitation with light. These effects have been well investigated in solids and solutions, while technological applications require the immobilization and contacting of the molecules at surfaces, which often results in the suppression of the SCO. We report on the thermal and light-induced SCO of [Fe(bpz)2phen] molecules in direct contact with a highly oriented pyrolytic graphite surface. We are able to switch on the magnetic moment of the molecules by illumination with green light at T = 6 K, and off by increasing the temperature to 65 K. The light-induced switching process is highly efficient leading to a complete spin conversion from the low-spin to the high-spin state within a submonolayer of molecules. [Fe(bpz)2phen] complexes immobilized on weakly interacting graphite substrates are thus promising candidates to realize the vision of an optically controlled molecular logic unit for spintronic devices.

Submono-, mono- and multilayers of the Fe(II) spin-crossover (SCO) complex [Fe(bpz)2 (phen)] (bpz=dihydrobis(pyrazolyl)borate, phen=1,10-phenanthroline) have beenprepared by vacuum deposition on Au(111) substrates and investigated with near edge X-ray absorption fine structure (NEXAFS) spectroscopy and scanning tunneling microscopy (STM). As evidenced by NEXAFS, molecules of the second layer exhibit a thermal spin crossover transition, although with a more gradual characteristics than in the bulk. For mono- and submonolayers of [Fe(bpz)2 (phen)] deposited on Au(111) substrates at room temperature both NEXAFS and STM indicate a dissociation of [Fe(bpz)2 (phen)] on Au(111) into four-coordinate complexes, [Fe(bpz)2 ], and phen molecules. Keeping the gold substrate at elevated temperatures ordered monolayers of intact molecules of [Fe(bpz)2 (phen)] are formed which can be spin-switched by electron-induced excited spin-state trapping (ELIESST).

Spin-state switching of transition-metal complexes (spin crossover) is sensitive to a variety of tiny perturbations. It is often found to be suppressed for molecules directly adsorbed on solid surfaces. We present X-ray absorption spectroscopy measurements of a submonolayer of [Fe(II)(NCS)2L] (L: 1-{6-[1,1-di(pyridin-2-yl)ethyl]-pyridin-2-yl}-N,N-dimethylmethanamine) deposited on a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. These molecules undergo a thermally induced, fully reversible, gradual spin crossover with a transition temperature of T1/2 = 235(6) K and a transition width of ΔT80 = 115(8) K. Our results show that by using a carbon-based substrate the spin-crossover behavior can be preserved even for molecules that are in direct contact with a solid surface.

The ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature and thus return to the parent closed form on short time scales. Here we report contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300 K, a thermally induced ring-opening reaction takes place on the gold surface, converting the initial highly ordered SP islands into MC dimer chains. We have found that the thermally induced ring-opening reaction has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn out to be the most stable phase. On the basis of the experimentally determined molecular structure of each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC form on the metal surface as a consequence of its conjugated structure and large electric dipole moment. The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability of the two isomers.

Graphene, a material with unique properties, [ 1 ] is expected to complement todays Si-based information technology with new and more effi cient functions. [2][3][4] It exhibits desirable properties for spin electronic applications such as high charge carrier mobility, low intrinsic spin-orbit interaction, as well as low hyperfi ne interaction. [5][6] In particular, magnetic molecules in contact with graphene constitute a tantalizing approach towards organic spin electronics because of the reduced conductivity mismatch at the interface. In such a system a bit is represented by a single molecular magnetic moment, which must be stabilized against thermal fl uctuations. [ 7 ] Here, we show in a combined experimental and theoretical study that the moments of paramagnetic Co-octaethylporphyrin (CoOEP) molecules on graphene can be aligned by a remarkable anti-ferromagnetic coupling to a Ni substrate underneath the graphene. This coupling is mediated via the π electronic system of graphene, while no covalent bonds between the molecule and the substrate are established, and the molecules sit at a distance of ≈ 3.3 Å above the graphene plane. The laterally extended π electron system of graphene exhibits metal-like electronic properties along the plane, and molecule-like properties perpendicular to the plane, which makes graphene highly relevant for the design of hybrid metal-organic spintronic materials. Several studies have focused on the spin-split electronic states of graphene or the injection of spin-polarized currents from metallic ferromagnetic electrodes. [8][9][10] Electronic spin transport and spin precession have been observed in graphene over micrometer distances even at room temperature. [ 11 , 12 ] Molecules adsorb on graphene mainly by Van der Waals interaction, thereby the molecular properties of the adsorbate, like its reactivity, are preserved. This opens the door to an optimization of the undisturbed molecular design of an adsorbate independently of its interaction with the substrate.We use X-ray magnetic circular dichroism (XMCD) in the absorption of soft X rays as an element-selective probe of the magnetization of CoOEP molecules in the submonolayer regime on a graphene-passivated Ni fi lm, [ 13 ] supported by a W(110) single crystal surface. In the upper and lower panel of Figure 1 a, we show Co L 2,3 X-ray absorption (XA) spectra and XMCD spectra of 0.7 molecular monolayers (ML) of CoOEP on top of graphene/Ni, respectively, measured in zero fi eld at remanence of the ferromagnetic Ni fi lm. The astonishing presence of a fi nite Co XMCD signal proves on the one hand a net 3 d magnetic moment localized on the Co ions and on the other hand an unexpected magnetic coupling between the magnetization of the Ni layer and these Co moments, stabilizing them against thermal fl uctuations. The XA spectrum as well as the XMCD spectra exhibit a particular fi nestructure at the Co L 3 edge, which, by comparison to CoPc bulk measurements, [ 14 ] is consistent with a d 7 low-spin state of Co. The XMCD difference cu...