Multilevel Resistive Memory Behavior in Quinoline–Pyrazole-Based Donor–Acceptor Systems

A series of pyrazole-functionalized quinolines with donor–acceptor (D–A) and donor–acceptor–donor (D–A–D) architectures was synthesized for application in multilevel organic memory devices. The donor substituents, such as tert-butyl and methoxyphenyl groups, as well as the functionalization of the pyrazole moiety on the quinoline, enable the transition from binary to ternary and quaternary memory states, thereby enhancing charge storage density. Notably, quaternary memory devices, which are rarely reported, have three ON states and one OFF state, enabling multilevel data storage and allowing two bits to be stored in a single memory cell. These devices exhibit low threshold voltages of –0.58 to –3.09 V and ON/OFF ratios of 102 to 105, thereby improving memory device performance. Photophysical and electrochemical analyses indicate enhanced charge-transfer capabilities, as evidenced by a red shift in the absorption spectra and reduced band gaps (3.02–3.15 eV). All compounds demonstrated nonvolatile write-once-read-many memory behavior, in which charge transport is primarily influenced by intramolecular charge transfer and trap-controlled space-charge-limited current mechanisms. The devices also exhibited significant stability over 100 reading cycles and 2000 s under constant stress at 0.5 V. The observed memory performance is strongly correlated with molecular structure, showing that the nature of the donor substituents and molecular architecture influence charge-transfer efficiency, trap distribution, and, ultimately, multilevel memory performance. These findings represent a significant advancement in the development of organic high-density storage memory devices.