Understanding Single-Molecule Charge Transport in Multi-state, Ladder-type Conjugated Molecules

Development of stimuli-responsive materials is critical for new advances in molecular electronics. Our work focuses on the ability to precisely control the switching of electronic signals in single molecules using triggered changes in pH, mechanical force, optical or electric fields. Polyaniline (PANI) serves as a popular functional material due to its versatile chemical and electrochemical doping states arising from controlled changes in the conjugation and net charge change along the backbone. However, aniline cores suffer from chemical instabilities and a flexible backbone for facile bond rotation, which can lead to elusive electronic properties and hinder charge transport efficiency. To overcome these challenges, we synthesized and studied the charge transport properties of a new class of ladder-type redox-active molecules. In particular, pernigraniline molecules were modified with a ladder-type strategy to lock the molecular conformation, thereby restricting the free rotation of backbone bonds. We further studied the single molecule charge transport properties of ladder-type pernigraniline molecules using the scanning tunneling microscope-break junction (STM-BJ) method. Our results show a 100-fold increase in molecular conductance for pernigraniline molecules upon full protonation and lithiation. Intermolecular charge transport is also observed via the formation of stacked dimers assisted by π-π interaction. Experimental results are complemented by DFT calculations, which reveals smaller energy gaps and extended molecular conformations for higher protonated and lithiated states. Overall, our work shows that ladder-type molecules with functional aniline moieties provide a promising class of materials with well-defined electronic properties at different prontonation and oxidation states.