Thin films of a dinuclear Fe2+ complex on HOPG: Spin-crossover studies using X-ray absorption spectroscopy

The spin-crossover (SCO) properties of the dinuclear complex [{Fe(H2B(pz)2)2}2μ−(ac(bipy)2)] were studied as (sub)-monolayer and thin film deposited by an ultra-high 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 approximately 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.