Elettra | NanoESCA Ligand conformation dictates function in 2D metal-organic frameworks Two-Dimensional Metal-Organic Frameworks (2D MOFs) are attracting increasing interest in the scientific community due to their exciting potential in next-generation technologies. These materials are built from metal atoms linked by organic molecules, and one of their key advantages lies in the ability to fine-tune their electronic and magnetic properties by selecting specific combinations of metals and linkers. Among a diverse range of metalorganic motifs, structures combining the 7,7,8,8-tetracyanoquinodimethane (TCNQ) ligands and 3d transition metals (TMs) have attracted recent attention as particularly promising candidates, especially in the emerging field of molecular spintronics, which aims to develop electronic devices based on the control of electron spin. However, in metal-supported 2D MOFs, interactions with the substrate can significantly influence the flexibility of the molecular system. As a result, the reported electronic and magnetic properties of 2D MOFs on metal substrates might not truly reflect the material’s intrinsic characteristics. Understanding and overcoming these challenges is crucial for unlocking the full potential of 2D MOFs in practical applications. To address this, we investigate a 2D MOF composed of Ni metal ions coordinated by TCNQ ligands on epitaxial graphene grown on an Ir(111) substrate (Fig. 1 a). To fully validate the formation of a 2D coordination network, we employed a combination of Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, conducted at the BOREAS beamline of the ALBA synchrotron, and valence band (VB) measurements, carried out at the NanoESCA beamline of the Elettra synchrotron. Fig. 1 (b) presents the N K-edge NEXAFS spectra measured with p- and s-polarized light for both the TCNQ self-assembled monolayer (SAM) and the Ni MOF. The changes observed in the Ni MOF spectra reveal charge redistribution and hybridization phenomena that occur during the formation of the MOF structure, offering clear spectroscopic evidence of the metal-organic coordination. Complementing this, the analysis of VB shows the momentumintegrated photoelectron spectra of graphene, the TCNQ SAM, and the Ni MOF (Fig. 1 c). The VB spectrum of the SAM exhibits a prominent feature, related to its highest occupied molecular orbital (HOMO), which is not observed in bare graphene. When Ni ions coordinate with TCNQ to form the MOF, three new features appear, linked to the Ni 3d states involved in bonding. Specifically, the lower binding energy peak (0.9 eV) can be mainly associated with the lowest unoccupied molecular orbital (LUMO), which is involved primarily in the metal-organic bonding in the MOF, becoming partially occupied due to charge redistribution between Ni 3d states and TCNQ molecular orbitals. Additionally, to further explore the electronic structure, we analyzed the photoemission momentum distributions using the photoelectron emission microscope (PEEM). Figs. 1 (d-f), then, show the momentum band maps along the high symmetry direction K – Γ − M of the surface Brillouin zone of graphene for three different systems: graphene, TCNQ SAM and Ni MOF. This comparison elucidates the impact of metal-organic coordination on the electronic structure, as reflected in the resulting band structure. We now shift the focus to the magnetic characteristics of the Ni ion in the MOF. To this end, we conducted NEXAFS and X-Ray Magnetic Circular Dichroism (XMCD) experiments at the Ni L3,2edges. The NEXAFS spectra acquired in normal incidence geometry with left and right circularly polarized light across the Ni L3,2-edges for the Ni MOF are depicted in Fig. 1 (g), together with the corresponding XMCD signal. A closer inspection of the XMCD spectrum reveals a strong dichroic behavior at lower photon energy at 851.4 eV (L3) and 868.6 eV (L2). In contrast, the resonances at higher photon energy at 853.4 eV (L3) and 870.4 eV (L2) do not show any XMCD signature. This observation supports the coexistence of two distinct configurations for the Ni ions within the Ni MOF, the Ni(I) (d9) with one unpaired electron and the low spin Ni(II) (d8) state with no unpaired electron. To better understand these observations, we performed density functional theory (DFT) calculations, identifying two locally stable configurations for the free-standing Ni MOF, labeled as "planar" and "twisted", characterized by two distinct Ni spin and oxidation states. In the former, the Ni ion is positioned within the TCNQ ligand in a planar coordination, maintaining a completely flat 2D structure. Conversely, in the latter, the cyano groups of the TCNQ ligand are twisted with respect to the XY plane. This alteration influences the alignment between the Ni and ligand orbitals, consequently inducing a diverse charge transfer mechanism in the metal-ligand bonding. To explain these differences, we examined the spin-resolved density of states (PDOS) projected onto the Ni 3d and the molecular TCNQ 2p states for both configurations (Fig. 1 h). Our results reveal that these two distinct conformations stabilize two Ni redox states with unique magnetic properties: the “planar” form associated with the Ni(I) state, and the “twisted” configuration, stabilized by the low spin Ni(II) state. In conclusion, our findings unveil how conformational changes in the ligands coordinated to the Ni ions influence the redox states and magnetism of the metal centers, leading to variations in their electronic structure. These insights are crucial for the future design of advanced 2D materials, where controlling the structure at the molecular level allows tuning of electronic and magnetic properties for specific technological applications. Elettra Highlights 2024–2025 22
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