Detecting Mechanochemical Atropisomerization within an STM Break Junction.

Edmund Leary,Gabino Rubio-Bollinger,Maria A Lebedeva,Iain Grace,Hua-Wei Jiang,Nicolás Agrait,Carlos Romero-Muñiz,Qusiy Al-Galiby,Yaoyao Xiong,Cécile Roche,M Teresa González,Andrew Hodgson,Simon J Higgins,Harry L Anderson,Colin J Lambert,Richard J Nichols,Mohammed Noori,Kyriakos Porfyrakis

doi:10.1021/jacs.7b10542

Abstract

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in C2Cl4D2). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au-porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S-S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au-S and Au-Au bonds, but higher than that of the Au-SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound.

Highlights

Molecular electronics is an extremely exciting area of nanotechnology, which aims to develop new electronic devices operating at the single molecule level.[1−4] Understanding and controlling the conformation of molecules in single molecule junctions (SMJs) is a key challenge on the road to functional electronic devices based on individual molecules
For all S2 compounds (SAc and SMe) we find this percentage falls between 7−14%, with
Article no obvious bias toward one termination or isomer. These rates are generally lower than typically found for oligo(phenylene ethynylene) (OPE)-based compounds measured under solvent-free conditions

Summary

Introduction

Molecular electronics is an extremely exciting area of nanotechnology, which aims to develop new electronic devices operating at the single molecule level.[1−4] Understanding and controlling the conformation of molecules in single molecule junctions (SMJs) is a key challenge on the road to functional electronic devices based on individual molecules.

Methods

Results

Conclusion