Tuning Single-Molecule Conductance by Controlled Electric Field-Induced trans-to-cis Isomerisation

C.S Quintans,Katrin F Domke,Albert C Aragonès,Denis Andrienko,Daniel Aravena,Ismael Díez-Pérez,Sangho Koo

doi:10.3390/app11083317

Abstract

External electric fields (EEFs) have proven to be very efficient in catalysing chemical reactions, even those inaccessible via wet-chemical synthesis. At the single-molecule level, oriented EEFs have been successfully used to promote in situ single-molecule reactions in the absence of chemical catalysts. Here, we elucidate the effect of an EEFs on the structure and conductance of a molecular junction. Employing scanning tunnelling microscopy break junction (STM-BJ) experiments, we form and electrically characterize single-molecule junctions of two tetramethyl carotene isomers. Two discrete conductance signatures show up more prominently at low and high applied voltages which are univocally ascribed to the trans and cis isomers of the carotenoid, respectively. The difference in conductance between both cis-/trans- isomers is in concordance with previous predictions considering π-quantum interference due to the presence of a single gauche defect in the trans isomer. Electronic structure calculations suggest that the electric field polarizes the molecule and mixes the excited states. The mixed states have a (spectroscopically) allowed transition and, therefore, can both promote the cis-isomerization of the molecule and participate in electron transport. Our work opens new routes for the in situ control of isomerisation reactions in single-molecule contacts.

Highlights

The development of novel, more efficient ways to control molecular reactions has been a restless quest for synthetic chemists
TMC isomers conductance and to study the effect of the electric fields (EEFs) generated by the STM bias voltage, on the conductance of the TMC single-molecule junction
Trans-TMC is ascribed to the molecular junction displaying average conductance values of ca. 3 × 10−4 Go, and the in situ generated cisTMC is ascribed to molecular junctions average conductance values of ca. 1.6 × 10−4 Go

Summary

Introduction

The development of novel, more efficient ways to control molecular reactions has been a restless quest for synthetic chemists Many different triggers, such as light, heat or external electric fields (EEF) are being used to promote chemical reactions [1,2]. To enable new transition states [4,5] in a theoretically predictable manner These field-induced chemical reactivity experiments, which are well reported in single-molecule devices [3,5], represent an exciting alternative to traditional bulk chemical catalyst approaches since EEF provide a cost-, material-efficient methodology to precisely control molecular reactions in a cleaner, more sustainable way. (4) The difference in junction’s lifetime between cis-TMC and trans-TMC isomers is attributed to a relatively high stability and more constrained configuration of the former in the molecular junction. Spontaneous formation of a molecular junction (with a finite lifetime τ), (3) current drop to It upon spontaneous molecular junction breakdown

Experiments

Simulations

Results and Discussion

Conclusions