Terminal Substitution-Driven Non-Volatile WORM Memory Behavior in Functionalized Fluorenes

The effect of terminal substituents on resistive memory characteristics was studied in a series of Donor-π-Acceptor/Donor (D-π-A/D) molecules with fluorene as the core donor, incorporating different terminal substitutions using a styryl π-spacer, which were synthesized via Knoevenagel condensation followed by Suzuki cross-coupling. The photophysical and electrochemical studies showed that all the synthesized compounds have a narrow band gap of 3.02-3.09 eV and irreversible oxidation peaks. All the memory devices fabricated from these compounds exhibited non-volatile binary Write-Once-Read-Many (WORM) memory characteristics, with an I_ON/I_OFF ratio of 10³, and the trifluoromethyl terminal group exhibited a lower threshold voltage of -1.34 V under negative bias. All the fabricated devices showed improved reliability and cell-to-cell reproducibility, as evidenced by statistical analyses of threshold voltages and ON/OFF current ratios. Density Functional Theory (DFT) calculations and electrostatic potential analysis support a combined charge-transfer and charge-trapping resistive memory mechanism, consistent with the experimental data. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analyses confirm good film coverage and moderate roughness. Thus, the potential of the terminal substitution strategy in the D-π-A/D system with fluorene as the core donor has unlocked the development of efficient, solution-processable resistive memory devices for data storage.