Sulfur-rich spirofluorene-bridged N-heterotriangulene redox-active polymers

Sulfur-rich spirofluorene-bridged N -heterotriangulene (FTN) polymers featuring covalently linked oligosulfide units and a terthiophene-based analogue were synthesized via nucleophilic aromatic substitution and Stille cross-coupling polymerization. The resulting materials are amorphous, insoluble solids with high thermal stability and sulfur contents up to 25 wt%. Structural and compositional analyses by combustion analysis, ToF-SIMS, FT-IR, XPS, and solid-state NMR confirm the efficient incorporation of short oligosulfide to disulfide linkages and well-defined terthiophene units in the respective polymers. Electrochemical characterization in lithium half-cells indicates a predominantly FTN-centered high-voltage redox process at 3.8–4.0 V (vs. Li/Li⁺), accompanied by sulfur-related low-voltage redox processes in the sulfur-linked polymers and a transient linker-associated low-voltage activity in the terthiophene-linked polymer between 1.5–2.5 V (vs. Li/Li + ). Sulfur incorporation markedly increases the theoretical and initial discharge capacities (up to 129 mA h g⁻¹), while the sulfide conversion processes exhibit rapid fading and poor reversibility due to sulfide dissolution. In contrast, the terthiophene-linked polymer shows only transient linker-associated low-voltage activity, while the sustained electrochemical response is governed by the persistent FTN backbone redox event, with Coulombic efficiencies approaching 99.7%. Our results highlight how different sulfur-containing linkers influence the electrochemical behavior of FTN-based polymers and provide insights into the design of functional organic cathode materials featuring multi-redox processes.