Spectroscopic Fingerprinting of Coordination-driven Spin States in Metal-organic Architectures

Determining the local geometry of metal-organic architecture on substrates is challenging, as substrate interactions can alter the metal coordination relative to the free-standing structure. Here, combining density functional theory (DFT) and restricted open-shell configuration interaction with singles (ROCIS) calculations on isolated cobalt-7,7,8,8-tetracyanoquinodimethane (Co-TCNQ) complexes, together with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD), directly reveals the coordination motifs of Co centers in a 2D Co-TCNQ framework on graphene. The calculated Co L_3,2-edges spectroscopic fingerprints for nearly planar (Co²⁺, S = 1/2) and distorted tetrahedral (Co²⁺, S = 3/2) structures exhibit distinct features, allowing unambiguous assignment of spin and oxidation states of the metal centers, as well as confirmation of the local geometry. Comparison with experimental spectra confirms that the high-spin tetrahedral geometry is realized in the supported framework, demonstrating how spectroscopic fingerprints can directly link coordination geometry to spin and oxidation states in low-dimensional metal-organic systems.