Exploring the Coexistence of Spin States in [Fe-(tpy-ph)$_2$]$^{2+}$ Complexes on Au(111) using ab initio calculations

TL;DR

This study uses ab initio calculations to analyze the spin states of a specific iron complex on gold, revealing how substrate interaction and computational parameters influence spin stability and suggesting coexistence of spin states during X-ray excitation.

Contribution

It provides a detailed computational analysis of spin state stability in [Fe-(tpy-ph)$_2$]$^{2+}$ complexes on Au(111), highlighting the effects of the Hubbard U parameter and substrate interaction.

Findings

Low-spin state favored at U(Fe) ≤ 3 eV in gas phase

High-spin state stabilized at U(Fe) > 3 eV in gas phase

Substrate interaction raises critical U to 3.5 eV for spin transition

Abstract

In this work, we systematically study the electronic structure and stability of spin states of the [Fe-(tpy-ph)$_2$]$^{2+}$ molecule in both gas phase and on a Au(111) substrate using density functional theory +U (DFT+U) calculations. We find that the stability of the Fe$^{2+}$ ion's spin states is significantly influenced by the Hubbard U parameter. In the gas phase, the low-spin (LS, S=0) state is found to be energetically favorable for U(Fe) $\leq$ 3 eV, whereas the high-spin (HS, S=2) state is stabilized for U(Fe) > 3 eV. Interaction with the Au(111) substrate is found to elevate the critical U for the spin-state transition to 3.5 eV. Additionally, we perform L-edge X-ray absorption spectroscopy (XAS) calculations based on time-dependent DFT (TD-DFT) for both HS and LS states. The calculated XAS suggests that the HS state more closely aligns with the experimental observations, indicating the potential coexistence of the HS state as the initial state during the X-ray excitation process. These findings enrich our understanding of spin-state dynamics in [Fe-(tpy-ph)$_2$]$^{2+}$.