Jahn-Teller-like Distortion in a One-dimensional {\pi}-Conjugated Polymer

TL;DR

This study reveals that a one-dimensional conjugated polymer undergoes a spontaneous symmetry-breaking distortion driven by electron-lattice interactions, significantly affecting its electronic properties and stability.

Contribution

It demonstrates a Jahn-Teller-like distortion in an extended { extpi}-conjugated polymer, combining experimental synthesis, microscopy, and theoretical calculations to elucidate the mechanism.

Findings

Polymer exhibits out-of-plane backbone distortion.

Distortion stabilizes non-planar isomers via a Jahn-Teller-like mechanism.

Distortion increases the bandgap and lowers the total energy.

Abstract

Structurally distorting low-dimensional {\pi}-conjugated systems can profoundly influence their electronic properties, but controlling such behavior in extended-width systems remains challenging. Here we demonstrate that a one-dimensional conjugated polymer, poly-(difluorenoheptalene-ethynylene) (PDFHE), undergoes a pronounced out-of-plane backbone distortion, equivalent to a spontaneous symmetry breaking (SSB) of its mirror symmetry. We synthesized PDFHE on noble metal surfaces and characterized its structure and electronic states using low-temperature scanning tunneling microscopy. Rather than adopting a planar, high-symmetry conformation, PDFHE relaxes into non-planar isomers stabilized by a Jahn-Teller-like mechanism that relieves an electronic instability relative to the gapped planar structure. Density functional theory calculations corroborate these findings, revealing that distortion lowers the total polymer energy and enlarges the bandgap, providing a microscopic explanation for the SSB. Our results show that even in mechanically robust extended {\pi}-systems, subtle electron-lattice coupling can spontaneously drive significant structural rearrangements.