Mono- and sub-monolayer films of a high T_1/2 spin-crossover molecule on HOPG

We investigated the properties of the [Fe{H2B(pz)(pypz)}2] spin-crossover complex deposited on highly oriented pyrolytic graphite (HOPG) by x-ray absorption spectroscopy (XAS) in the temperature range from 10 to 350 K for sample thicknesses from 0.9(1) to 1.3(2) ML. The highest temperature at which light can excite the system to a long-lived metastable high-spin state (TLIESST) and the temperature at which a thermal spin transition occurs (T_1/2) for the thin film samples are found as about 45 and 325 K, respectively, in agreement with the bulk values. The bulk phase has previously been studied by magnetic susceptibility measurements where it was found that two different polymorphic modifications exist. These findings suggest a potential correlation between TLIESST and the high T_1/2 from the different film thicknesses of the tridentate [Fe{H2B(pz)(pypz)}2] spin-crossover molecule, with thinner samples exhibiting lower T_1/2 values. Finally, the highest high-spin fraction achieved for [Fe{H2B(pz)(pypz)}2] is 0.77 for samples of 0.9(1) and 1.1(1) ML on HOPG at 10 K and constant light irradiation.