Tracking molecular resonance forms of donor-acceptor push-pull molecules by single-molecule conductance experiments.

Henriette Lissau,Anders Kadziola,Mogens Brøndsted Nielsen,Kurt V Mikkelsen,Riccardo Frisenda,Herre S J Van Der Zant,Thorsten Hansen,Stine T Olsen,Martyn Jevric,Christian R Parker

doi:10.1038/ncomms10233

Henriette Lissau, Anders Kadziola + Show 8 more

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https://doi.org/10.1038/ncomms10233

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Abstract

The ability of molecules to change colour on account of changes in solvent polarity is known as solvatochromism and used spectroscopically to characterize charge-transfer transitions in donor–acceptor molecules. Here we report that donor–acceptor-substituted molecular wires also exhibit distinct properties in single-molecule electronics under the influence of a bias voltage, but in absence of solvent. Two oligo(phenyleneethynylene) wires with donor–acceptor substitution on the central ring (cruciform-like) exhibit remarkably broad conductance peaks measured by the mechanically controlled break-junction technique with gold contacts, in contrast to the sharp peak of simpler molecules. From a theoretical analysis, we explain this by different degrees of charge delocalization and hence cross-conjugation at the central ring. Thus, small variations in the local environment promote the quinoid resonance form (off), the linearly conjugated (on) or any form in between. This shows how the conductance of donor–acceptor cruciforms is tuned by small changes in the environment.

Highlights

The ability of molecules to change colour on account of changes in solvent polarity is known as solvatochromism and used spectroscopically to characterize charge-transfer transitions in donor–acceptor molecules
While the Aviram–Ratner diode has the donor and acceptor units as parts of the actual wire, we became interested in placing D–A units orthogonally to an OPE3 molecular wire, incorporating thioacetate end groups to allow anchoring to gold electrodes
The question we set out to answer is the following: How sensitive is the conductance of a D–A molecule to the local environment, which may promote electron delocalization of the molecule towards one or the other of the two resonance forms shown in Fig. 1? As the environment of a molecule in a break-junction at a certain bias may change from one opening–closure experiment to the other and from one electrodes position to the other along the same opening experiment, this technique was chosen to track variations in conductance

Summary

Results

(a þ b À 2c)/2, where a, b and c are the lengths of the three C À C bonds in the ring between the DTF and CHO units; due to asymmetry of the molecule, two values are calculated) This corresponds to roughly 15% of the quinoid character of p-benzoquinone, which has a quinoid character of 0.16 Å Molecule 2 exhibits the lowest energy of the charge-transfer transition, which signals both the stronger acceptor strength of the indan-1,3-dione unit and a weaker electronic coupling in the ground state between donor and acceptor units24—as reflected by a smaller ground-state dipole moment of 2 than of 1 (3.04 D versus 5.48 D, see calculations below).

A A ðx À 2s21

Discussion

Methods