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A Peierls transition in long polymethine molecular wires: evolution of molecular geometry and single-molecule conductance

DOI: 10.1021/jacs.1c10747 DOI Help

Authors: Wenjun Xu (University of Oxford) , Edmund Leary (Universitario de Cantoblanco) , Sara Sangtarash (University of Warwick) , Michael Jirasek (University of Oxford) , M. Teresa González (Campus Universitario de Cantoblanco) , Kirsten E. Christensen (University of Oxford) , Lydia Abellán Vicente (Campus Universitario de Cantoblanco) , Nicolás Agraït (Campus Universitario de Cantoblanco; Universidad Autónoma de Madrid) , Simon J. Higgins (University of Liverpool) , Richard J. Nichols (University of Liverpool) , Colin J. Lambert (Lancaster University) , Harry L. Anderson (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Journal Of The American Chemical Society , VOL 23

State: Published (Approved)
Published: November 2021
Diamond Proposal Number(s): 20876

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 (Cy3+–Cy11+). 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 Cy9·PF6 and Cy11·B(C6F5)4 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 Cy3+–Cy11+ with the large B(C6F5)4– counterion, but with the PF6– counterion, the conductance decreases for the longer molecules, Cy7+–Cy11+, 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, Cy3+–Cy7+, increases with length (negative attenuation factor, β = −1.6 nm–1), but the conductance still drops in Cy9+ and Cy11+ with the small polarizing PF6– counteranion.

Journal Keywords: Salts; Dyes and pigments; Crystal structure; Molecules; Electrical conductivity

Subject Areas: Chemistry

Instruments: I19-Small Molecule Single Crystal Diffraction

Added On: 25/11/2021 14:52

Discipline Tags:

Organic Chemistry Physical Chemistry Chemistry

Technical Tags:

Diffraction Single Crystal X-ray Diffraction (SXRD)