Thermal- and Light-Induced Spin-Crossover Characteristics of a Functional Iron(II) Complex at Submonolayer Coverage on HOPG

Abstract

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 SiO x -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.

Substrate driven Spin Crossover in a Fe(II) scorpionate complex.

“…It has been reported in some recent works that the Soft X-ray Induced Electron Spin State Trapping (SOXIESST) [32][33][34] effect can be activated by the interaction with SCO molecules and the surface. 14,66,75,76 In our case, at cryogenic temperatures, SOXIESST cannot be observed in either sub-monolayer samples. Accordingly, even under green (516 nm) and red (660 nm) light irradiation, no Light Induced Spin State Trapping (LIESST) 77 has been detected in the 0.7ML film on HOPG (see Figure S28 in Supplementary Information).…”

Spin Crossover in a Cobalt Complex on Ag(111)

“…Although several examples of functional Fe‐based SCO complexes in direct contact with surfaces have been reported,[ 12 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ] the weakness of the coordination bonds has often been an issue. [ 14 , 29 , 34 , 35 , 36 ] Coordination bonding in Co complexes is expected to be even weaker owing to the larger population of the antibonding e g orbitals.…”

Observation of Exchange Interaction in Iron(II) Spin Crossover Molecules in Contact with Passivated Ferromagnetic Surface of Co/Au(111)

“…[19,20] Therefore, to maintain SCO, either intrinsically inert and robust molecules and/or weakly interacting surfaces have been selected. [19,[21][22][23][24] Indeed, the demonstrated ability to reversibly switch the spin state between LS and HS states at nanometric scale, for example, by external stimuli such as light, [25,26] X-ray, [27] voltage, [28] and temperature, [29] makes Fe(II) SCO complexes ideal candidates for molecular spintronics and promising application as building blocks for classical bits, sensors, and actuator devices. [9,19,30,31] Extensive research has focused on the SCO phenomenon regarding the practical properties of the SCO molecules itself, and combine systems like spin valves, heterostructures, etc.…”