Reducing undesired solubility of squarephaneic tetraimide for use as an organic battery electrode material

Abstract

Conjugated macrocycles can offer excellent redox properties for use as battery electrode materials, if they feature Type I concealed antiaromaticity (Type I-CA). Macrocycles with Type I-CA have a formal macrocyclic conjugated system with 4n π-electrons (Scheme, left; bold bonds), but despite this 4n π-electron system they are not antiaromatic due to the presence of locally aromatic subunits (Scheme, left; grey shadings) that dominate the overall structure and conceal the antiaromaticity. This increases the stability of the macrocycles in the neutral state while preserving their ability to become globally aromatic in the doubly charged states (Scheme, right), enabling very stable charged states and highly reversible redox reactions. Paracyclophanetetraene and its derivatives are prime examples of macrocycles exhibiting such behaviour, including excellent redox properties in solution. However, although they have also shown great promise in the solid state when used as n-type organic battery electrode materials, partially due to the porosity that results from the macrocyclic geometry of the molecules, the relatively high solubility of the charged macrocycles in commonly used battery electrolytes affects their performance. To solve this issue, we have synthesized unsubstituted squarephaneic tetraimide (Scheme, SqTI-H), which features four imide groups capable of intermolecular hydrogen bonding. As expected, these intermolecular interactions drastically reduced the solubility in the electrolyte, enabling excellent performance in the solid state. The underlying molecular design ideas, different synthetic routes to SqTI-H, as well as a comprehensive characterisation of SqTI-H and other, structurally related macrocycles will be presented. Focus will be given to the challenges (reducing the solubility, increasing the number of electrons stored per molecule without increasing the molecular weight, electronic and ionic conductivity etc.) and prospects of conjugated macrocycles for organic battery electrodes.