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

## TL;DR

This study shows how host-guest interactions between cyclodextrin and azobenzene can temporarily boost charge transport in single-molecule junctions.

## Contribution

The novel finding is the time-dependent conductance enhancement due to α-CD/azobenzene complexation in aqueous solutions.

## Key findings

- Amine-terminated azobenzene shows a 3.5-fold conductance increase when complexed with α-CD.
- Conductance enhancement decreases over time as α-CD aggregates.
- Modifications to azobenzene or using stilbene show no conductance change with α-CD.

## Abstract

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.