Thermodynamic and Kinetic Control of π-Stacked Dimer Conductance in Single-Molecule Junctions

Understanding how π-stacked dimers modulate charge transport is pivotal for molecular and supramolecular electronics. While extensive efforts have focused on transient, kinetically trapped dimers, thermodynamically stable π-dimers─equally essential for the evolution of molecular and supramolecular electronics─remain insufficiently investigated. Herein, we investigate distinct conductance features of these two dimeric states using donor-acceptor-donor diketopyrrolopyrrole molecular wires. Correlating scanning tunneling microscope break-junction measurements, flicker-noise analysis, concentration-dependent UV-vis spectroscopy, and DFT-NEGF calculations, we show that at 1.0 mM, DPP-1Th yields predominantly monomeric junctions, together with a low-conductance feature consistent with a kinetically generated π-dimer. Conversely, DPP-3Th shows evidence of preassembly into thermodynamically stable π-dimers at concentrations exceeding 0.5 mM. Notably, the thermodynamically equilibrated DPP-3Th dimer exhibits a longer junction displacement yet enhanced conductance relative to the DPP-1Th low-conductance state, indicative of superior π-orbital overlap and more efficient coupling pathways in the preorganized architecture. These results support distinct charge-transport characteristics for thermodynamically stable π-dimers and kinetically generated π-dimer. This work helps clarify an important ambiguity in supramolecular junction analysis and provides useful design guidelines for engineering aggregation-controlled charge transport in supramolecular electronic components.