Inverse spin crossover in fluorinated Fe(1,10-phenanthroline)$_2$(NCS)$_2$ adsorbed on Cu (001) surface

TL;DR

This study uses advanced DFT calculations to show that fluorination induces an inverse spin crossover in Fe(phen)2(NCS)2 molecules, both in gas phase and on Cu(001), by electron doping that stabilizes the high-spin state.

Contribution

It demonstrates that fluorination causes an inverse spin crossover in Fe(phen)2(NCS)2, a phenomenon not previously observed, using a combination of DFT+U and electron density analysis.

Findings

Fluorinated Fe(phen)2(NCS)2) switches from low to high spin state.

Electron doping increases Fe-N bond lengths, reducing ligand-field splitting.

Distinct STM images predicted for fluorinated vs. unfluorinated molecules.

Abstract

Density functional theory (DFT) including van der Waals weak interaction in conjunction with the so called rotational invariant DFT+U, where $U$ is the Hubbard interaction of the iron site, is used to show that the fluorinated spin crossover Fe(phen$)_{2}$(NCS$)_{2}$ molecule whether in the gas phase or adsorbed on Cu(001) surface switches from the original low spin state to the high spin state. Using Bader electron density analysis, this inversion of the spin-crossover is explained in terms of electron doping of the Fe-octahedron cage which led to an increase of the Fe-N bond lengths. Consequently, the ligand-field splitting is drastically reduced, making the high-spin ground state more stable than the low-spin state. The calculated scanning tunneling microscopy (STM) images in the Tersoff-Hamann approximation show a clear distinction between the fluorinated and the unfluorinated molecule. This theoretical prediction is awaiting future STM experimental confirmation.