Quantum interference and heteroaromaticity of para- and meta-linked bridged biphenyl units in single molecular conductance measurements

Markus Gantenbein,Colin J Lambert,Lin Wang,Alaa A Al-Jobory,Martin R Bryce,Wenjing Hong,Ali K Ismael

doi:10.1038/s41598-017-01903-0

Abstract

Is there a correlation between the (hetero)aromaticity of the core of a molecule and its conductance in a single molecular junction? To address this question, which is of fundamental interest in molecular electronics, oligo(arylene-ethynylene) (OAE) molecular wires have been synthesized with core units comprising dibenzothiophene, carbazole, dibenzofuran and fluorene. The biphenyl core has been studied for comparison. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions (para series 1–5) or meta-meta positions (meta series 6–10). A combined experimental and computational study, using mechanically controlled break junction measurements and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π–system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene ≈ dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. Our study establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junctions.

Highlights

The measurement and understanding of charge transport in single molecules is of fundamental interest and is relevant to the proposed future applications of molecules in electronic devices[1,2,3,4,5,6]
Oligo(arylene-ethynylene) (OAE)-type molecular wires have been widely explored in single molecular junctions[12,13,14,15,16]
From our experimental and theoretical data for the para series [1,2,3,4] and the meta series [6,7,8,9], that there is a clear correlation between aromaticity of the central ring when heteroatoms are present and the single-molecule conductance value in the para series

Summary

Introduction

The measurement and understanding of charge transport in single molecules is of fundamental interest and is relevant to the proposed future applications of molecules in electronic devices[1,2,3,4,5,6]. The motivation for the present work is to study for the first time the combined effects of two important molecular parameters on the single-molecule conductance of molecular wires: (i) heteroaromaticity in the core of the wire, and (ii) para versus meta conjugation through the core unit.

Results

Conclusion