Quadrupolar (A-pi-D-pi-A) Tetra-aryl 1,4-Dihydropyrrolo[3,2-b]pyrroles as single molecular resistive memory devices: substituent triggered amphoteric redox performance and electrical bistability

A series of quadrupolar (A-π-D-π-A) tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) derivatives synthesized are showcased as potential organic resistive memory (ORM) devices for the first time. The experimental observations coupled with density functional theory (DFT) calculations probe in detail the role of terminal substituent groups (p-NH2, p-Cl, p-CN, p-NO2, m-NO2) on the optical and electrical properties. Electrochemical studies reveal that the 3- and 4-dinitro derivatives form an unusual class of tetra-aryl DHPPs that exhibit intrinsic amphoteric redox behavior contrary to the literature reports. The bipolar nature within a single molecule was harnessed to design operational ORMs. Interestingly, the memory devices fabricated using the structural isomers exhibited dissimilar memory characteristics. While the p-NO2 derivative displays permanent Write Once Read Many times (WORM) memory, its meta-counterpart represents a behavior akin to rewriteable flash memory. The noticeably higher ON/OFF ratio (∼104) for the p-NO2 derivatives could be ascribed to their matched redox energy levels with the work function of active electrodes favoring better charge injection. Rational interpretation of these findings strongly suggests that the choice and strategic positioning of terminal substituents can significantly alter the nature of "charge traps" affecting the device outcome. These encouraging findings open up a relatively less chartered territory of air stable fused pyrrole systems that holds great promise for realizing next generation organic memory devices.