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

Locally aromatic alkyl-N-substituted squarephaneic tetraimide (SqTI) conjugated macrocycles are four-electron reducible, owing to global aromaticity and presumed global Baird aromaticity of the dianion and tetraanion states, respectively. However, their good solubility inhibits their application as a battery electrode material. By applying sidechain removal as a strategy to reduce SqTI solubility, we report the development of its unsubstituted derivative SqTI-H, which was obtained directly from squarephaneic tetraanhydride by facile treatment with hexamethyldisilazane and MeOH. Compared to alkyl-N-substituted SqTI-Rs, SqTI-H exhibited further improved thermal stability and low neutral state solubility in most common organic solvents, owing to computationally demonstrated hydrogen-bonding capabilities emanating from each imide position on SqTI-H. Reversible solid state electrochemical reduction of SqTI-H to the globally aromatic dianion state was also observed at -1.25 V vs. Fc/Fc+, which could be further reduced in two stages. Preliminary testing of SqTI-H in composite electrodes for lithium-organic half cells uncovered imperfect cycling performance, which may be explained by persistent solubility of reduced states, necessitating further optimisation of electrode fabrication procedures to attain maximum performance. Unsubstituted squarephaneic tetraimide with reduced solubility due to H-bonding interactions is synthesised, characterised, and tested as a battery electrode material.

Automated summary

This study reports the development of an unsubstituted derivative, SqTI-H, of locally aromatic alkyl-N-substituted squarephaneic tetraimide (SqTI) conjugated macrocycles. By removing sidechains, the solubility of SqTI-H is reduced, resulting in improved thermal stability and low solubility. The unsubstituted SqTI-H also exhibits reversible solid state electrochemical reduction to a globally aromatic dianion state. However, further optimization of electrode fabrication procedures is needed to overcome the solubility issues and improve the cycling performance.