Spin-flip excitations induced by dehydrogenation in a magnetic single-molecule junction

Recent scanning tunneling microscopy experiments on electron transport through iron(ii) phthalocyanine (FePc) molecules adsorbed on Au(111) surfaces have revealed that the measured differential conductance signals can be modulated through a selective dehydrogenation process [R. Li et al., Chem. Commun. 54, 9135 (2018)]. To understand the physical origin of the variation of line shapes in the measured dI/dV spectra, we employ a first-principles-based quantum transport simulation to calculate the electronic structures and transport properties in the dehydrogenated FePc/Au(111) composite system. Theoretical results indicate that the removal of eight outermost hydrogen atoms in the FePc molecule distorts the planar molecular conformation by increasing the distance between the Fe center and substrate and thus breaks the ligand field exerted on the Fe 3d orbitals. Such variations lead to a weaker coupling with the substrate along with a different local electronic configuration of the Fe center compared with that of the intact case, which is responsible for the suppression of Kondo resonance and the appearance of spin-flip excitation in the system. The simulated dI/dV plots are consistent with the experimental observations, showing the typical step features at finite bias associated with spin-flip excitations of a spin-1 quantum magnet. These findings provide important insights into the electron correlation effects modulated by the structural and chemical environment of the molecular spin center.