Highly Efficient and Bidirectional Photochromism of Spirooxazine on Au 111

Controlling molecules in direct contact with surfaces is central to molecular electronics. Photochromic molecules immobilized and contacted by a surface promise to provide remote control on the molecular level using light. Combining X-ray absorption spectroscopy, differential reflectance spectroscopy, and density functional theory, we demonstrate highly efficient and bidirectional photochromism of a spirooxazine molecular switch in direct contact with a Au(111) surface. The ring-opening reaction by UV light is 2 orders of magnitude more efficient than previously reported for other surface-adsorbed systems, and even more importantly, the red-light-induced ring-closing is accessible even in contact with a metal surface. This opens new prospects for applications by utilizing the gold surface with directly adsorbed functional units consisting of molecular photochromic switches

Vacuum evaporable spin crossover complexes physicochemical properties in the crystalline bulk and in thin films deposited from the gas phase

Vacuum-evaporable Fe(ii) complexes exhibit greatly different spin-crossover behaviour in thin films deposited from the gas phase and the crystalline bulk.</p

Vacuum Evaporable Spin Crossover Complexes in Direct Contact with a Solid Surface Bismuth versus Gold

To investigate the ability for spin-state switching of spin-crossover (SCO) complexes adsorbed to solid substrates, the SCO complex [Fe­(H₂B­(pz)₂)₂(phenme₄)] (pz = pyrazole, phenme₄ = 3,4,7,8-tetramethyl-1,10-phenanthroline) is prepared. The new complex is investigated by magnetic susceptibility measurements and Mößbauer spectroscopy in the solid state and by temperature-dependent UV/vis spectroscopy in a thin film deposited by physical vapor deposition (PVD) on quartz glass. Thermal- and light-induced SCO is observed in the bulk and the film on glass. Submonolayers of this complex obtained by PVD are studied by temperature-dependent near-edge X-ray absorption fine structure (NEXAFS) on Au(111) as well as Bi(111) and by scanning tunneling microscopy (STM) on Au(111). NEXAFS shows thermal- and light-induced spin-state switching of the complex on Bi(111), however, with a large temperature-independent high-spin fraction (∼50%). On the other hand, combined evidence from NEXAFS and STM indicates that on Au(111) the complex dissociates into [Fe­(H₂B­(pz)₂)₂] and phenme₄. Similar observations are made with the parent complex [Fe­(H₂B­(pz)₂)₂(phen)], which on Bi(111) stays intact and exhibits thermal-induced as well as light-induced SCO, but on Au(111) dissociates into [Fe­(H₂B­(pz)₂)₂] and phen

Highly Efficient Thermal and Light Induced Spin State Switching of an Fe II Complex in Direct Contact with a Solid Surface

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 device

Surface orientation and ligand dependent quenching of the spin magnetic moment of Co porphyrins adsorbed on Cu substrates

Porphyrin molecules are particularly interesting candidates for spintronic applications due to their bonding flexibility, which allows to modify their properties substantially by the addition or transformation of ligands. Here, we investigate the electronic and magnetic properties of cobalt octaethylporphyrin (CoOEP), deposited on copper substrates with two distinct crystallographic surface orientations, Cu(100) and Cu(111), with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). A significant magnetic moment is present in the Co ions of the molecules deposited on Cu(100), but it is completely quenched on Cu(111). Heating the molecules on both substrates to 500 K induces a ring-closure reaction with cobalt tetrabenzoporphyrin (CoTBP) as reaction product. In these molecules, the magnetic moment is quenched on both surfaces. Our XMCD and XAS measurements suggest that the filling of the dz2 orbital leads to a non-integer valence state and causes the quench of the spin moments on all samples except CoOEP/Cu(100), where the molecular conformation induces variations to the ligand field that lift the quench. We further employ density functional theory calculations, supplemented with on-site Coulomb correlations (DFT+U), to study the adsorption of these spin-bearing molecules on the Cu substrates. Our calculations show that charge transfer from the Cu substrates as well as charge redistribution within the Co 3d orbitals lead to the filling of the Co minority spin dz2 orbital, causing a ‘turning off’ of the exchange splitting and quenching of the spin moment at the Co magnetic centers. Our investigations suggest that, by this mechanism, molecule–substrate interactions can be used to control the quenching of the magnetic moments of the adsorbed molecules

Thermal and Light Induced Spin Crossover Characteristics of a Functional Iron II Complex at Submonolayer Coverage on HOPG

Studies on the spin state switching characteristics of surface bound thin films of spin crossover SCO complexes are of interest to harness the device utility of the SCO complexes. Molecule substrate interactions govern the SCO of surface bound films in direct contact with the underlying substrates. In this study, we elucidate the role of molecule substrate interactions on the thermal and light induced spin state switching characteristics of a functional SCO complex [Fe H2B pz 2 2COOC12H25 bipy] pz pyrazole, C12 bpy dodecyl[2,2 amp; 8242; bipyridine] 5 carboxylate deposited at a submonolayer coverage on a highly oriented pyrolytic graphite HOPG substrate. A spin state coexistence of 42 low spin LS and 58 high spin HS is observed for the 0.4 ML deposit of the complex at 40 K, in contrast to the complete spin state switching observed in the bulk and in SiOx bound 10 nm thick films. Cooling the sample to 10 K results in a decrease of the LS fraction to 36 , attributed to soft X ray induced excited spin state trapping SOXIESST . Illumination of the sample with a green light amp; 955; 520 nm at 10 K caused the LS to HS switching of the remaining 36 LS complexes, by a process termed light induced excited spin state trapping LIESST . The mixed spin state in the submonolayer coverage of [Fe H2B pz 2 2COOC12H25 bipy] highlights the role of molecule HOPG substrate interactions in tuning the thermal SCO characteristics of the complex. The 100 HS state obtained after light irradiation indicates the occurrence of efficient on surface light induced spin switching, encouraging the development of light addressable molecular devices based on SCO complexe