Engineering chemisorption of Fe4 single-molecule magnets on gold

Gaining control over the grafting geometry is critically important for any application of surface-supported single-molecule magnets (SMMs) in data storage, spintronics and quantum information science. Tetrairon(III) SMMs with a propeller-like structure are functionalized with thioacetyl-terminated alkyl chains (CH2)nSAc of different lengths (n = 3, 4, and 5) to promote chemisorption on gold surfaces and to evaluate differences in adsorption geometry and magnetic properties as a function of n. The prepared monolayers are investigated using X-ray absorption techniques with linearly and circularly polarized light to extract geometrical and magnetic information, respectively. All derivatives remain intact and form partially oriented monolayers on the gold surface. According to the results of ligand field multiplet analysis, the three-fold molecular axis is invariably biased towards the surface normal (z). Assuming that its deviation from z follows an angular step distribution with maximum angle ξMAX, the three derivatives have ξMAX = 10±5°, 48±2°, and 51±2°, respectively. The five-fold smaller angle spanned by the shortest-chain derivative (n = 3) is in gross agreement with the results of ab initio calculations. Monolayers of the derivative with n = 3 are further studied with an ultra-low temperature X-ray magnetic circular dichroism setup operating down to 350 mK. The isothermal field sweeps with the magnetic field at normal incidence show an open hysteresis loop below 1 K, while measurements at different incidence angles clearly prove the magnetic anisotropy of the monolayers.