Detecting Mechanochemical Atropisomerization within an STM Break Junction

Published on 2018-02-13T05:29:55Z (GMT) by
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 <i>ortho</i> position of the phenyl rings (S2 compounds). These <i>ortho</i> substituents lead to two atropisomers, <i>cis</i> and <i>trans</i>, 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 C<sub>2</sub>Cl<sub>4</sub>D<sub>2</sub>). 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 <i>cis</i>-to-<i>trans</i> conversion takes place in the junction for the <i>cis</i> 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.

Cite this collection

Leary, Edmund; Roche, Cécile; Jiang, Hua-Wei; Grace, Iain; González, M. Teresa; Rubio-Bollinger, Gabino; et al. (2018): Detecting

Mechanochemical Atropisomerization within

an STM Break Junction. ACS Publications. Collection.