Pre-synthetic redox control of structure and properties in copper TTFtt coordination polymers

Abstract

Conductive coordination polymers (CPs) with sulfur-based ligands offer strong metal-ligand interactions and redox tunability, making them promising candidates for electronic applications. Tetrathiafulvalene-2,3,6,7-tetrathiolate (TTFtt) is a particularly attractive ligand. However, its strong metal-ligand covalency leads to rapid irreversible metal coordination, limiting control over structure and morphology. Here, we demonstrate structural control in Cu TTFtt CPs using a pre-synthetic redox control strategy. Two new copper-based CPs, CuTTFtt and Cu₂TTFtt, have been synthesized and thoroughly characterized from differentially oxidized TTFtt synthons. CuTTFtt forms a 1D chain, while Cu₂TTFtt adopts a 2D ribbon-like structure. Detailed spectroscopic studies confirm the structures of these materials as well as their ligand and metal oxidation states. Physical property measurements reveal that Cu₂TTFtt exhibits higher conductivity than CuTTFtt. Furthermore, Cu₂TTFtt also shows unusual diamagnetism which contrasts the paramagnetism observed in CuTTFtt and the related material NiTTFtt. Density functional theory (DFT) further elucidates the physical properties of these CPs and supports the observed conductivity trends. This study expands the structural landscape of TTFtt-based CPs and further establishes how redox-doping can tune CP structure and physical properties.

Fig. 1. Synthetic scheme for Cu₂TTFtt (left) and CuTTFtt (right).

Fig. 2. (A) Synchrotron (λ = 0.167 Å) PXRD patterns and of Cu₂TTFtt and CuTTFtt, (B) PDF of Cu₂TTFtt and CuTTFtt. Structural model of Cu₂TTFtt viewed along c-axis (C) and viewed from a-axis (D).

Fig. 3. Demonstration of proposed structural models of CuTTFtt (middle) and Cu₂TTFtt (bottom) based on PXRD, PDF and elemental analysis.

Fig. 4. Spectroscopic studies TTFtt ligand redox states. (A) Sulfur K-edge XAS, (B) Raman spectra, and (C) S 2p XPS spectra of Cu₂TTFtt and CuTTFtt.

Fig. 5. Charge transport and magnetic properties of CuTTFtt and Cu₂TTFtt. (A) Variable temperature resistance measurements. (B) Variable temperature magnetic susceptibility measurements of CuTTFtt at 0.1T. Curie–Weiss fit in pink. (C) Variable temperature magnetic susceptibility measurements of Cu₂TTFtt at 0.1T. Inset shows negative susceptibility, supporting diamagnetism.

Fig. 6. Band structure and DOS for Cu₂TTFtt and CuTTFtt. (A) Primitive structure and Brillouin zone of the Cu₂TTFtt unit cell. (B) Primitive structure and Brillouin zone of the CuTTFtt unit cell. (C) Projected band structure and DOS for Cu₂TTFtt with the Fermi level denoted by black dashed lines. (D) Projected band structure and DOS for CuTTFtt with the Fermi level denoted by black dashed lines.