Studies on the spin-state switching characteristics of surface-bound thin films of spin-crossover (SCO) complexes are of interest to harness the device utility of the SCO complexes. Molecule–substrate interactions govern the SCO of surface-bound films in direct contact with the underlying substrates. In this study, we elucidate the role of molecule–substrate interactions on the thermal- and light-induced spin-state switching characteristics of a functional SCO-complex—[Fe(H2B(pz)2)2COOC12H25-bipy] (pz = pyrazole, C12-bpy = dodecyl[2,2′-bipyridine]-5-carboxylate) deposited at a submonolayer coverage on a highly oriented pyrolytic graphite (HOPG) substrate. A spin-state coexistence of 42% low-spin (LS) and 58% high-spin (HS) is observed for the 0.4 ML deposit of the complex at 40 K, in contrast to the complete spin-state switching observed in the bulk and in SiOx-bound 10 nm thick films. Cooling the sample to 10 K results in a decrease of the LS fraction to 36%, attributed to soft-X-ray-induced excited spin-state trapping (SOXIESST). Illumination of the sample with a green light (λ = 520 nm) at 10 K caused the LS-to-HS switching of the remaining (36%) LS complexes, by a process termed light-induced excited spin-state trapping (LIESST). The mixed spin-state in the submonolayer coverage of [Fe(H2B(pz)2)2COOC12H25-bipy] highlights the role of molecule–HOPG substrate interactions in tuning the thermal SCO characteristics of the complex. The 100% HS state obtained after light irradiation indicates the occurrence of efficient on-surface light-induced spin switching, encouraging the development of light-addressable molecular devices based on SCO complexes.
