Mono- and sub-monolayer films of a high1/2 spin-crossover molecule on HOPG: temperature- and light-driven spin-state transition.

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 in the temperature range from 10 to 350 Kfor 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 (HS) state () and the temperature at which a thermal spin transition occurs (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 betweenand the high1/2 from the different film thicknesses of the tridentate[Fe{H2B(pz)(pypz)}2]spin-crossover molecule, with thinner samples exhibiting lower1/2 values. Finally, the highest HS 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 Kand constant light irradiation.