A tris-azo anion radical ligand-wrapped singlet Co(ii) complex with multiple redox for an efficient molecular memristor towards neuromorphic computing

Abstract

Achieving multilevel conduction in a cost-effective transition metal complex of a redox-active ligand will be an efficient way for designing a molecular memristor. This study presents a rare example of a tris-azo anion radical ligand-wrapped singlet five-coordinate low-spin Co(II) complex, [(L)˙⁻Co(II)]PF₆, [1]PF₆, that showed solution-processible resistive switching memory with synaptic functionality. The single-crystal X-ray structure, variable-temperature magnetic studies, and DFT calculations of [1]PF₆ showed that it has a one-electron-reduced ligand, [L]˙⁻, which is antiferromagnetically coupled with the low-spin Co(II) center that results in an s = 0 ground state. [1]PF₆ showed multiple reversible and quasi-reversible redox events, indicating that it can be explored as a molecular memristor. The indium tin oxide/[1]PF₆/Ag memristor demonstrated excellent switching performance with a large ON/OFF ratio (>10³), excellent endurance (>500 cycles), long retention time (>10⁴ s), and long-term stability at an elevated temperature (100 °C). Moreover, upon applying pulsed electrical stimuli, the memristor exhibited potentiation and depression behaviors, a key feature for synaptic plasticity. The device was SET at a cathodic potential, and thus, the facile ligand-based reductions in [1]PF₆ played a decisive role in the device.