Viologen-based conjugated ionic polymer for nonvolatile rewritable memory device

A novel soluble viologen-based conjugated ionic copolymer, PTPA-V, was synthesized by using Suzuki coupling reaction in the presence of the tetrakis(triphenylphosphine)palladium(0) as the catalyst. Its HOMO, LUMO, EA, IP and Eg values derived from the CV data are −5.44, −3.10, 3.07, 5.70 and 2.34 eV, respectively. The maximum absorption peak observed in the UV/Vis absorption spectrum of the PTPA-V thin film was shifted to the red by about 10 nm, suggesting stronger aggregation effect occurred in the thin film. The fluorescence decay profiles of the singlet excited state of PTPA-V in different deaerated organic solvents revealed a mono-exponential decay with a lifetime of 8.9 ns (toluene), 6.4 ns (THF) and 5.2 ns (DMF). The as-prepared electronic device with a configuration of Al/PTPA-V/ITO exhibited two accessible conductivity states, i.e., the high-conductivity (ON) state and the low-conductivity (OFF) state. A typical nonvolatile rewritable memory behavior was observed, with turn-on and turn-off voltages of −1.92 and 2.67 V, and a large ON/OFF current ratio of more than 10⁵. Unlike poly(triphenylamine) without any electron paramagnetic resonance (EPR) signal, PTPA-V shows an EPR signal with a g factor of 2.004, suggesting existence of unpaired electrons or free radicals in this polymer due to the charge transfer between the triphenylamine (TPA) moieties and the viologen (V) moieties, which resulted in the formation of TPA⁺ and V^-.