Effective Transport Tunnels Achieved by 1,2,4,5-Tetrazine-Induced Intermolecular C-H...N Interaction and Anion Radicals for Stable ReRAM Performance

The D-A structured small-molecule-based resistive random-access memory (ReRAM) device has been well-researched in the last decade, and the switching mechanism was mainly induced by the intramolecular/intermolecular charge transfer processes from the donors to the acceptors. However, in the previous work, some small molecules with pristine electron acceptors in the backbone could still show the typical memory behaviors, of which the switching mechanism is still ambiguous. In this work, two 1,2,4,5-tetrazine based n-type small-molecular isomers, 2-DPTZ and 4-DPTZ, with the same electron acceptor, 1,2,4,5-tetrazine and pyridine, are chosen to investigate the isomeric effects on molecular packing, switching mechanism, and memory performance. Because of the abundant nitrogen atoms with a localized lone pair of electrons in the sp² orbital, 2-DPTZ and 4-DPTZ compounds could self-assemble into a long-range ordered molecular packing through intermolecular C-H...N interactions, affording effective transporting tunnels for charge-carrier transport. As expected, the sandwich-structured ITO/2-DPTZ or 4-DPTZ/Al memory devices both showed binary memory characteristics, with 2-DPTZ based memory devices showing the write once read many times (WORM) memory behavior and 4-DPTZ based memory devices having the negative differential resistance (NDR) memory performance. These distinct ReRAM properties arose from the different morphologies of 2-DPTZ and 4-DPTZ films that were induced by the different packing styles between the adjacent molecules, as confirmed by X-ray diffraction (XRD) and tapping-mode atomic force microscopy (AFM) height images. Most importantly, the switching mechanism was thought to be attributed to the injected electrons that reduced the neutral molecules of 2-DPTZ and 4-DPTZ to their corresponding anion radicals. Thus, this present work helps us better understand the conducting mechanism of small molecules with pristine electron acceptors in the backbone and provides a supplementary guideline for designing multilevel small molecules to match the structure-stacking-property relationship.