Detecting Mechanochemical Atropisomerization within an STM Break Junction.

Area of Science:

• Molecular electronics
• Supramolecular chemistry
• Organic chemistry

Background:

• Investigating single-molecule conductance is crucial for molecular electronics.
• Diaryl porphyrins with specific binding groups offer tunable electronic properties.
• Atropisomerism in molecules can influence their self-assembly and electronic behavior.

Purpose of the Study:

• To measure and compare the single-molecule conductance of diaryl porphyrins with thioacetyl (SAc) and methylsulfide (SMe) binding groups.
• To understand the role of atropisomerism (cis and trans isomers) in conductance measurements.
• To elucidate the charge transport pathways and junction mechanics.

Main Methods:

• Scanning tunneling microscope break-junction (STM-BJ) technique.
• Synthesis of 5,15-diaryl porphyrins with ortho-substituted SAc or SMe groups.
• Analysis of conductance histograms and junction length distributions.

Main Results:

• All S2 compounds showed two conductance groups, attributed to direct Au-porphyrin and S-S pathways.
• Methylsulfide (SMe) isomers exhibited distinct junction lengths reflecting their S-S distances.
• Thioacetyl (SAc) isomers showed no length difference, indicating cis-to-trans isomerization within the junction.

Conclusions:

• The barrier to isomerization is lower than Au-S/Au-Au bonds but higher than Au-SMe bonds.
• Strain in the junction induces isomerization in cis-SAc porphyrins, but not in SMe porphyrins.
• Single-molecule conductance measurements can probe conformational dynamics and isomerization barriers.