Ladder-Type Polyaniline Derivatives as a Highly Robust Multi-State Switch in Single-Molecule Junction

Polyaniline (PANI) is of considerable interest due to its versatile chemical and electrochemical doping states. However, they suffer from chemical instabilities at acidic and oxidative conditions, which limits their applications in higher oxidative and protonated states. Herein, we reported a series of ladder-type PANI derivatives flanked with methylthiol groups on both ends as the model molecule for single-molecule conductance studies. The implementation of the ladder-type constitution prevents undesired hydrolysis and isomerization, rendering high stability of these molecules at different protonation and oxidation states. We studied the single-molecule charge transport properties of ladder-type PANI derivatives using the scanning tunneling microscope-break junction (STM-BJ) method. Our results demonstrate a 10-fold conductance increase from the reduced benzenoid structure to the oxidized quinonoid structure. Furthermore, leveraging the chemical stability that ladder-type constitution provides, acid/lithium salt addition renders two-stage protonation/lithiation of the quinonoid molecule to a dicationic form which shows open-shell diradical character in ambient conditions. The fully protonated/lithiated state displays a conductance level 15 times higher than the neutral quinonoid state. Surprisingly, intermolecular charge transport is also observed via the formation of stacked dimers assisted by pi-pi interaction. The experimental results are complemented by DFT calculations, which reveal smaller energy gaps and extended molecular conformations for higher protonated and lithiated states. Overall, this work highlights a highly robust, multi-state, single-molecule switch where a ladder-type constitution is incorporated into the PANI system to advance the stability, programmability, and efficiency of the single-molecule devices.