Fine-Tuning the Molecular Design for High-Performance Molecular Diodes Based on Pyridyl Isomers

Better control of molecule-electrode coupling (Γ) to minimize leakage current is an effective method to optimize the functionality of molecular diodes. Herein we embedded 5 isomers of phenypyridyl derivatives, each with an N atom placed at a different position, in two electrodes to fine-tune Γ between self-assembled monolayers (SAMs) and the top electrode of EGaIn (eutectic Ga-In terminating in Ga₂ O₃ ). Combined with electrical tunnelling results, characterizations of electronic structures, single-level model fittings, and DFT calculations, we found that the values of Γ of SAMs formed by these isomers could be regulated by nearly 10 times, thereby contributing to the leakage current changing over about two orders of magnitude and switching the isomers from resistors to diodes with a rectification ratio (r⁺ =|J(+1.5 V)/J(-1.5 V)|) exceeding 200. We demonstrated that the N atom placement can be chemically engineered to tune the resistive and rectifying properties of the molecular junctions, making it possible to convert molecular resistors into rectifiers. Our study provides fundamental insights into the role of isomerism in molecular electronics and offers a new avenue for designing functional molecular devices.