Abstract
Using cryogenic scanning tunneling microscopy, we measured the electronic states and Kondo resonance of single $\mathrm{Fe}(\mathrm{II})$-porphyrin molecules adsorbed on a $\mathrm{Au}(111)$ surface with intramolecular resolution. We found that the $\mathrm{Fe}(\mathrm{II})$ ion introduces a spin-polarized molecular state near the Fermi level. Tunneling spectroscopy revealed that this state gives rise to Kondo resonance exhibiting characteristics different from those of the $\mathrm{Fe}(\mathrm{II})$ spin state. Spin-polarized density functional theory calculations revealed that the molecule was weakly adsorbed on the surface, yet still switches its spin configuration from $S=1\phantom{\rule{0.28em}{0ex}}\text{to}\phantom{\rule{0.28em}{0ex}}2$. The spin switching was found to be driven by three effects: a structural distortion of the macrocyclic ring from planar to saddle shaped, a weak chemical bonding between the Fe and the Au surface atom underneath, and weakened Fe-N bonds due to $\mathrm{Au}(111)$-molecule charge transfer.
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