In our first article since moving to TU Wien, we report the development of unsubstituted squarephaneic tetraimide (SqTI-H) for use as an organic battery electrode material. This collaborative work with the groups of Birgit Esser (Ulm University), Felix Plasser (Loughborough University) and Martin Heeney (KAUST, Imperial College London) has been accepted for publication in the Faraday Discussions journal and will be discussed at the Faraday Discussions conference on Challenges and prospects in organic photonics and electronics later this year.
Squarephaneic tetraimide (SqTI) macrocycles are four-electron reducible, owing to the global aromaticity of the dianion and presumed global Baird aromaticity of the tetraanion states. 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.