Tuning the cooperativity in spin-state switching of molecules in contact to surfaces

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Due to the possibility to switch their molecular magnetic moment on and off by external means, spin-crossover (SCO) molecules are highly interesting candidates for programmable building blocks in molecule-based spin electronics. For that purpose, however, the molecules need to be immobilized and contacted, which implies that they are supported by a solid surface. Correspondingly, mono- and submonolayers of spin-crossover molecules on surfaces have been prepared by, e.g., deposition from the gas phase, and thermal and light-induced spin-state switching could be detected for these systems in recent years by our groups and others. However, the cooperativity, which usually accompanies spin transitions of SCO molecules in solids, is in most cases absent. In order to improve the switching properties of spin-crossover molecules on surfaces, it would be highly desirable to re-establish cooperativity for these systems. This goal, which relates to the general question regarding the ultimate scale limit at which cooperativity becomes effective, is addressed in the present proposal by the stepwise assembly of mononuclear spin-crossover molecules to dimers, trimers, and (ultimately) infinite chains and the deposition as well as characterization of these systems on surfaces. For the multimers, intramolecular cooperativity will emerge, which is studied for the surface-adsorbed molecules. Chain-like, polymeric spin-crossover compounds are in part known to exhibit highly cooperative spin transitions in the solid state, and we want to investigate whether these properties can be retained when depositing monolayers of these polymers on surfaces. To this end, a range of chain-like spin-crossover compounds will be synthesized and investigated regarding their deposition capability on surfaces. Different techniques such as deposition from solids or solution in vacuum as well as by dip or spin coating from solution under ambient pressure, possibly followed by on-surface coupling strategies, are examined to generate such systems. Moreover, horizontal as well as vertical deposition geometries will be explored. The presence of (potentially ordered) monolayers of polymeric spin-crossover compounds on surfaces is examined by atomic force microscopy and scanning tunneling microscopy. The cooperativity in the thermal and light-induced spin transitions of these systems is investigated by x-ray absorption and x-ray photoelectron spectroscopy. The cooperativity is evaluated from the steepness of the thermal spin transition as well as from the detailed analysis of the thermal back-relaxation behavior after an optically induced spin switching at low temperatures. We expect to gain comprehensive knowledge about the factors establishing cooperative spin switching of molecules supported on solid surfaces.