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

The spin crossover (SCO) behavior of molecules in (sub)monolayers can differ from that of the bulk: in particular, it can be quenched or "pinned" by direct contact between the SCO molecules and the substrate, causing problems for potential device applications. Moreover, processing on sublimated SCO films is limited by reactivity to solvents. Here, it is shown that the preparation from solution of self-assembled monolayers (SAMs) of the Fe(II) complex [Fe(Tp(4-NHCOC₁₀H₂₀SCOCH₃))(Tp)], where Tp = tris(1H-pyrazol-1-yl)borohydride, yields reversible, nonpinned SCO on gold substrates. The formation of chemisorbed SAMs is confirmed by atomic force microscopy, time-of-flight secondary ion mass spectrometry, polarization modulation infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy. Cyclic voltammetry demonstrates the redox activity of the SAMs, with clear evidence of the Fe(II)/Fe(III) redox couple, and corresponding calculations confirm a dense surface coverage. Variable temperature X-ray photoelectron and X-ray absorption spectroscopies at the Fe L_2,3 edges indicate a reversible SCO in the monolayers identical to the bulk behavior, with a gradual conversion and $T_{\frac{1}{2}}$ of 367 K. These findings show that monolayers of SCO compounds can be assembled from solution while retaining a reversible SCO, making them suitable for use in switchable molecular electronic systems.