Influence of aromatic π-extension on phenanthro[9,10-d]imidazole-based donor–acceptor systems for non-volatile resistive WORM memory device applications

Abstract

In this work, a series of donor–acceptor (D–A) small molecules was designed and synthesized to investigate the influence of aromatic π-extension on structure–property relationships and memory device performance. Phenanthroimidazole was employed as the electron-accepting core, while aromatic substituents of increasing π-extension - phenyl, naphthyl, anthracene, and pyrene were introduced as donor units. Photophysical and electrochemical analyses revealed a clear correlation between extended conjugation and enhanced intramolecular charge transfer (ICT), with the anthracene substituent yielding the lowest bandgap due to its linear, highly planar π-system. Thin-film studies confirmed uniform morphology and ordered molecular packing, enabling efficient charge transport throughout the film. All fabricated devices exhibited non-volatile, binary Write-Once-Read-Many (WORM) memory behavior, retaining data for over 1000 seconds and demonstrating endurance of up to 100 read cycles. Among them, the anthracene-based derivative showed the best performance, with the lowest switching threshold and the highest ON/OFF current ratio of 10⁶, attributed to its extended linear conjugation and superior ICT. Pyrene substitution also improved device characteristics but was partially limited by steric hindrance from its angular structure, whereas phenyl and naphthyl analogues demonstrated weaker ICT owing to their shorter conjugation lengths. Computational studies supported a synergistic mechanism of charge transfer and charge trapping, validating the experimental results. Overall, this study demonstrates how systematic tuning of aromatic π-extension can optimise the electronic and memory properties of D–A molecular systems, providing a valuable design strategy for next-generation organic memory device applications.