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Locking and unlocking the spin crossover transition in Fe(II) complexes
Spin crossover (SCO) molecules form a significant class of materials for which the magnetic structure can be altered at the atomic level by an external stimulus from low a spin diamagnetic state to paramagnetic high spin state. We find, however, that the transition is quite complex and the SCO molecular transition can be mediated by interactions at the substrate interface. We have investigated the occupied and unoccupied electronic structure of thin films of the SCO [Fe(H2B(pz)2)2(bipy)] complex. The thermally induced spin crossover transition for molecules deposited on the organic ferroelectric copolymer PVDF-TrFE is suppressed. The choice of high spin or low state favored, over a wide of temperature, is seemingly influenced by the ferroelectric polarization direction of ferroelectric PVDF-TrFE substrate. We have also found that the molecule was locked in the low spin state also when deposited on a SiO2/Si substrate, but can be excited to high spin state by X-ray irradiation in a temperature range from 15 K to 345 K. The locking of the spin state implies there is an activation barrier to the spin crossover transition, which is confirmed by time dependent X-ray absorption spectroscopy. The X-ray induced spin crossover transition is likely due to the surface charge that builds up at the interface during the X-ray photoemission process. The isothermal spin state switching at room temperature, has been observed in SCO thin films grown on both magnetic NiCo2O4 and La0.65Sr0.35MnO3 thin film substrate surfaces. The X-ray excited high spin state then can be isothermally relaxed back into low spin state by oscillating the direction of substrate magnetic moment. All of these effects point to a strong influence, by the interface, on the spin state changes on SCO molecules. The mixture of SCO and dipolar zwitterion molecule is found suppressed in low spin state but can be excited into high spin state with X-ray radiation from 79 K to 340 K, leading to a possible new chemical path about spin states manipulation at the single molecule scale, and providing an exciting possibility for single molecule devices.
Physical chemistry|Physics|Condensed matter physics
Zhang, Xin, "Locking and unlocking the spin crossover transition in Fe(II) complexes" (2016). ETD collection for University of Nebraska-Lincoln. AAI10151397.