Host–Guest Interactions Enhance Charge Transport across Single Cyclodextrin/Azobenzene Complex Junction

While azobenzene has been studied extensively for its single-molecule charge transport properties, its complexation with guest ring molecules may significantly influence charge transport that is not yet well understood. In this work, we study the influence of host-guest interactions between α-cyclodextrin (α-CD) and azobenzene on the single-molecule conductance of azobenzene in an aqueous solution. Hydrophobicity of azobenzene drives its formation of an α-CD/azobenzene host-guest complex with α-CD in water, which is indicated in our nuclear magnetic resonance and ultraviolet-visible spectroscopy experiments. We see a modest ∼3.5-fold conductance increase for amine-terminated azobenzene upon host-guest complex formation. Notably, this enhancement displays progressive conductance attenuation over time, finally down to the conductance value of the azobenzene junction, which we attribute to the declining number of formed complexes in the aqueous solution as α-CD aggregates with time. In contrast, for amine-terminated stilbene (backbone modification) and for thiomethyl-terminated azobenzene (linker modification), no conductance change is seen with the addition of α-CD. First-principles simulations suggest that the lowest unoccupied molecular orbital (LUMO) of the α-CD/amine-azobenzene complex junction is at a lower energy than that of amine-azobenzene, thereby suggesting a possible conductance increase, agreeing with our experimental observations. Taken together, this study provides valuable perspectives on the intricate roles that the host-guest interactions play in regulating the molecular electronic properties.