Tuning the current rectification behavior of Rh₂-based molecular junctions by varying their supramolecular structures

Molecular junctions with similar backbones, tunable chemical structures and controllable length are critical for the systematic study of the structure-functionality relationships of their charge transport behavior. Taking advantage of the feasibility and tunability of stepwise fabrication, we built series of asymmetric supramolecular SAMs on gold using Rh₂(O₂CCR₃)₄ (Rh₂, R = CH₃, H, and F) as the building blocks and conjugated N,N'-bidentate ligands (pyrazine (L_S), 4,4'-bipyridine (L_M) and 1,2-bis(4-pyridyl)ethene (L_L)) as the bridges. By varying the Rh₂ units and bridging ligands, series of supramolecules with similar backbone and tunable chemical structures were assembled on gold. Their charge transport behavior was examined using conductive-probe atomic force microscopy. Notably, current rectification diminishes gradually as the degree of conjugation of the bridging ligands gets larger from L_S to L_L due to the decrease in the energy gap between the donor and the acceptor in π(Rh₂)-π(L) conjugated MO arrays. Additionally, current rectification can be enhanced when the charge transport mechanistic transits from tunneling in dimers to hopping in tetramers. Unlike charges hopping along the MO arrays in tetramers, charges tunnel through the frontier MOs in dimers. The occupied frontier MOs of dimers localize near the center of the supramolecules or delocalize on the donor and acceptor, which contributes to the weakening of the asymmetric charge tunneling. This work reveals that the frontier MO configurations of these supramolecules could be adjusted by varying their chemical structures, and consequently realize tuning of their charge transport behavior, which deepens the understanding of the charge transport behavior and benefits the establishment of the structure-functionality relationship of Rh₂-based molecular junctions.