Detecting Mechanochemical Atropisomerization within an STM Break Junction

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.