Spin‐Crossover in a Dinuclear Iron(II) Complex on Highly Oriented Pyrolytic Graphite: An X‐Ray Absorption Spectroscopy Study

This study investigates a spin-crossover dinuclear Fe(II) complex deposited as a thin film under ultrahigh-vacuum conditions by a pulsed layer injection technique. Its temperature-dependent switching behavior, light-induced excited spin-state trapping, soft-X-ray-induced excited spin-state trapping, and temperature- and light-induced spin crossover transitions are examined using X-ray absorption spectroscopy. These characteristics are compared to the bulk material.

The spin-crossover (SCO) properties of the dinuclear complex [{Fe(H2B(pz)2)2}2μ−(ac(bipy)2)]$\left[\right. \left{\right. \text{Fe} \left(\left(\right. \left(\text{H}\right)_{2} \text{B} \left(\left(\right. \text{pz} \left.\right)\right)_{2} \left.\right)\right)_{2} \left(\left.\right}\right)_{2} \mu - \left(\right. \left(\text{ac(bipy)}\right)_{2} \left.\right) \left]\right.$ are studied as (sub)-monolayer and thin film deposited by an ultrahigh vacuum liquid-jet deposition technique on highly oriented pyrolytic graphite (HOPG) by X-ray absorption spectroscopy. A comparison of the SCO properties of thin films and a dropcast sample indicates that the spin-switching probability of the thin films is limited due to substrate–molecule interactions. The maximum percentage of molecules in the low-spin (LS) state observed for 0.7 and 1.8 monolayers (ML) is ≈43% at a temperature of 80 K in comparison to the dropcast sample where ≈66% of the complex is in the LS state. The similar switching properties of the dropcast sample as of a bulk powder sample confirm that the SCO properties are not affected by the presence of solvent necessary for deposition. The soft-X-ray-induced excited spin-state trapping (SOXIESST) effect is pronounced in all samples, although the light-induced high-spin (HS) fraction of the dropcast and the thin-film samples on HOPG is higher as compared to the HS fraction attained by SOXIESST, which confirms the sensitivity of the complex to light.

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