A Peierls Transition in Long Polymethine Molecular Wires: Evolution of Molecular Geometry and Single-Molecule Conductance

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Xu, Wenjun, Leary, Edmund, Sangtarash, Sara, Jirasek, Michael, Teresa Gonzalez, M, Christensen, Kirsten E, Abellan Vicente, Lydia, Agrait, Nicolas, Higgins, Simon J ORCID: 0000-0003-3518-9061, Nichols, Richard J ORCID: 0000-0002-1446-8275 et al (show 2 more authors) , Lambert, Colin J and Anderson, Harry L (2021) A Peierls Transition in Long Polymethine Molecular Wires: Evolution of Molecular Geometry and Single-Molecule Conductance. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 143 (48). pp. 20472-20481.

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Abstract

Molecules capable of mediating charge transport over several nanometers with minimal decay in conductance have fundamental and technological implications. Polymethine cyanine dyes are fascinating molecular wires because up to a critical length, they have no bond-length alternation (BLA) and their electronic structure resembles a one-dimensional free-electron gas. Beyond this threshold, they undergo a symmetry-breaking Peierls transition, which increases the HOMO-LUMO gap. We have investigated cationic cyanines with central polymethine chains of 5-13 carbon atoms (<b>Cy3</b>^<b>+</b><b>-Cy11</b>^<b>+</b>). The absorption spectra and crystal structures show that symmetry breaking is sensitive to the polarity of the medium and the size of the counterion. X-ray crystallography reveals that <b>Cy9·PF</b>_<b>6</b> and <b>Cy11·B(C</b>_<b>6</b><b>F</b>_<b>5</b><b>)</b>_<b>4</b> are Peierls distorted, with high BLA at one end of the π-system, away from the partially delocalized positive charge. This pattern of BLA distribution resembles that of solitons in polyacetylene. The single-molecule conductance is essentially independent of molecular length for the polymethine salts of <b>Cy3</b>^<b>+</b><b>-Cy11</b>^<b>+</b> with the large B(C₆F₅)₄^- counterion, but with the PF₆^- counterion, the conductance decreases for the longer molecules, <b>Cy7</b>^<b>+</b><b>-Cy11</b>^<b>+</b>, because this smaller anion polarizes the π-system, inducing a symmetry-breaking transition. At higher bias (0.9 V), the conductance of the shorter chains, <b>Cy3</b>^<b>+</b><b>-Cy7</b>^<b>+</b>, increases with length (negative attenuation factor, β = -1.6 nm^-1), but the conductance still drops in <b>Cy9</b>^<b>+</b> and <b>Cy11</b>^<b>+</b> with the small polarizing PF₆^- counteranion.

Item Type:	Article
Divisions:	Faculty of Science and Engineering > School of Physical Sciences
Depositing User:	Symplectic Admin
Date Deposited:	31 Jan 2022 08:48
Last Modified:	18 Jan 2023 21:14
DOI:	10.1021/jacs.1c10747
Related URLs:	Author Publisher
URI:	https://livrepository.liverpool.ac.uk/id/eprint/3147860