Tuned by the ligand field, some single metal atoms in coordination complexes can offer two redox processes with a large potential difference, a key characteristic of symmetrical redox flow batteries (RFBs). We have designed a new anionic tridentate ligand N-(quinolin-8-yl)pyridine-2-sulfonamide (psq⁻) with the aim of tuning the Co^III/Co^II and Co^II/Co^I couples compared to those for analogous bis-homoleptic cobalt complexes of 2,2′:6′,2′′-terpyridine (terpy) and derivatives which have been tested as bipolar energy storage molecules. With similar structural parameters to these bis-terpy analogues, the single crystal X-ray structures of [M(psq)₂] (M = Co^II and Zn^II) and [Co^III(psq)₂](PF₆) show a planar ligand geometry. However the cyclic voltammogram of Co^II(psq)₂ reveals negative shifts of 0.840 V and 0.175 V for the reversible Co^II/Co^I and Co^III/Co^II redox processes compared to the bis-terpy analogue. Therefore, with a peak to peak separation between the Co^II/Co^I and Co^III/Co^II couples that is 0.68 V larger, bipolar RFBs employing Co^II(psq)₂ as the energy storage material will operate at higher cell potentials than those offered by [Co^II(terpy)₂]²⁺. In an H-cell set-up, Co^II(psq)₂ delivers an initial effective charge to 83%. A charge capacity fade of 3% per cycle observed over 30 cycles is partly ascribed to ligand reduction. This work shows that psq⁻ is a promising scaffold for bipolar energy storage molecules and illustrates the opportunities for innovation in RFB design through the development of fit-for-purpose redox-active coordination complexes.

The unique redox properties of monopyrrolotetrathiafulvalene can be used to induce directional movement in interlocked molecules. In this study, the kinetics for the directional movement of cyclobis(paraquat-p-phenylene) across the dioxidised monopyrrolotetrathiafulvalene in a [2]rotaxane is quantified by time-resolved ¹H NMR spectroscopy.

Studies of isomeric [2]rotaxanes revealed that two different di-oxidised tetrathiafulvalene units (green) constitute different barriers for cyclobis(paraquat-p-phenylene) (blue) which can be used to induce directional movement with 90% efficiency.