Detecting Mechanochemical Atropisomerization within an STM Break Junction

Leary, Edmund and Roche, Cecile and Jiang, Hua-Wei and Grace, Iain and Teresa Gonzalez, M. and Rubio-Bollinger, Gabino and Romero-Muniz, Carlos and Xiong, Yaoyao and Al-Galiby, Qusiy and Noori, Mohammed and Lebedeva, Maria A. and Porfyrakis, Kyriakos and Agrait, Nicolas and Hodgson, Andrew and Higgins, Simon J. and Lambert, Colin J. and Anderson, Harry L. and Nichols, Richard J. (2018) Detecting Mechanochemical Atropisomerization within an STM Break Junction. Journal of the American Chemical Society, 140 (2). pp. 710-718. ISSN 0002-7863

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 degrees 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.