Sub-nanoscale, single-molecule, magnetic–electronic switching from externally perturbedspin states in iron (III)-based complexes

Abstract

Both the temperature and pressure dependent spin state transition behaviour of two Fe(III)complexes will be exemplified. Such compounds are considered attractive as potential molecularswitch devices for low power, low weight, high density nanomemory arrays in computerapplications, or as sensors. Spin state configurations are readily investigated by 57Fe Mössbauer spectroscopy in the range 300–4 K, or at high pressures up to 20 GPa indiamond anvil cells.An iron coordination compound with the chemical formula {[FeIIIL2] [ClO4]}2·EtOH, where L− is a uni-negative ligand, HL is N-(pyridin-2-ylmethyl)salicylideneamine and EtOH is ethanol, is in the high spin (HS), 6A1g, state at room temperature. The onset of low spin (LS) population occurs at K and this increases monotonically as the temperature is lowered to 100 K.At 70 K the sample is predominantly in the LS state although a remnant HS population persists to the lowest recorded temperature of K. Thermodynamic parameters associated with the energy barrier between the twospin states are deduced from the temperature dependence of the equilibrium constantK.Pressure induced spin switching from the high moment HS state tothe low moment LS electronic state in a haem-porphyrin [FeIII(TPP)(NCS)] will also be shown. Spin pairing onset occurs at a moderate pressure of GPa. HS and LS isomers exist in equilibrium up to GPa at room temperature. The sample is fully LS populated at P>12 GPa. Temperature and pressure dependent spin–spin relaxation is used to account forthe pressure evolution of the (asymmetric) quadrupole split resonance profile. Densityfunctional theory calculations, in conjunction with crystallographic data, areused to consider the structural response to such a spin state variation, perhapssuggesting the possibility of a mechanically operated single-molecule magnetic switch.