Non-Pinned, Reversible Spin Crossover in Self-Assembled Monolayers of a Functionalized Fe(II) Scorpionate Complex

The spin crossover (SCO) behaviour of molecules in (sub)monolayers is typically significantly altered from that of the bulk: in particular, the SCO can be quenched by direct contact between the SCO molecules and the substrate, known as pinning, which causes problems for potential device applications. Here, an Fe (II) complex is presented that exhibits fully reversible, non-pinned SCO in self-assembled monolayers (SAMs) on template stripped gold substrates. The complex, [Fe(Tp(4-NHCOC10H20SCOCH3))(Tp)] where Tp = tris(1H-pyrazol-1-yl borohydride), has a broad SCO with a T½ of 366 K. The SAMs are uniform and homogeneous, as indicated by Atomic Force Microscopy, and contain the target molecules in a well-oriented layer with the expected thickness for a monolayer of the complex, as revealed by polarization modulation infrared reflection-absorption spectroscopy, time-of-flight secondary ion mass spectrometry and cyclic voltammetry. Variable temperature X-ray photoelectron spectroscopy, as well as X-ray absorption spectroscopy at the Fe L2,3 edges, indicates a reversible SCO in the monolayers that is identical to the bulk behaviour.