Spin‐State Patterns in Surface‐Grafted Beads of Iron(II) Complexes

Abstract

Novel strategies for the design of functional materials are in increasing demand, as the down-scaling of lithographic processes (the top-down approach) will soon encounter the fundamental physical limits of miniaturization. One of the fascinating perspectives of molecular electronics is information storage at the single-molecule level, on the basis of arrays of molecular switches. Spin-crossover (SCO) compounds hold considerable potential in this context. SCO can occur in octahedral transition-metal complexes in which the metal ion has a d to d electron configuration. The transition may be stimulated externally, by a change in temperature or pressure, or by irradiation. SCO is entropydriven and, in the solid state, is influenced strongly by intermolecular interactions, such as hydrogen bonding or p–p stacking. Such interactions give rise to cooperativity between SCO complexes within the ensemble. High cooperativity can cause the change in spin state to be accompanied by hysteresis, which confers bistability on the system and thus a memory effect. A viable reading/writing procedure, that is, a means of reproducible actuation on the single-molecule level, is a formidable challenge that has yet to be met, but in this way SCO compounds could serve in devices of unsurpassable storage density. In principle, reliable information storage could be achieved even in the absence of hysteresis, provided the energy difference between low-spin state and high-spin state of the complexes within the SCO ensemble is sufficiently large (on the order of several kT). A large number of spin-crossover systems are known, with complexes of iron(II) the most numerous, both in solution and in the solid state. Usually, ferrous iron is in a quasi-octahedral N6 coordination environment, and switching occurs between a low-spin (LS, A1g/t2g , S= 0) and a high-spin state (HS, T2g/ t2g eg , S= 2). SCO systems have been characterized by physical techniques including M ssbauer and UV/Vis spectroscopy, magnetic susceptibility measurements, and diffraction methods, applied to the bulk solids. Many attempts have been made to obtain SCO materials in the form of thin films, multilayers, or nanocrystals. Recent strategies include the sequential assembly of coordination polymers on metal or biopolymer supports (such as gold or chitosan) and the preparation in polymeric matrices, in surface-grown multilayer thin films incorporating iron(II) coordination polymers, and in nanoparticulate iron(II) complexes. Our approach is to use spin-switchable iron(II) complexes of bis(pyrazolyl)pyridine ligands, with a variety of substituents that can serve as surface anchors depending on the kind of substrate. We studied the spin state of adsorbates at the single-molecule level with scanning tunneling microscopy (STM) techniques at room temperature (298 K). For the present study, we chose [Fe(L)2](BF4)2 (1; L= ligand), whose synthesis, solid-state structure, and spin behavior have been reported in detail. The solid-state structure of the dication in 1 is shown in Figure 1a. The magnetic susceptibility of 1