Spiro-Conjugated π-σ-π Architectures Enable Single-Molecule Insulators via Destructive Quantum Interference.

Area of Science:

• Molecular electronics
• Quantum interference phenomena
• Organic electronics

Background:

• Controlling molecular conductance is crucial for single-molecule electronics.
• Highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO) interference is a known factor.
• Destructive quantum interference (DQI) offers a route to lower conductance.

Purpose of the Study:

• To investigate DQI in pi-sigma-pi molecular frameworks.
• To explore the role of bilateral interference in molecular conductance.
• To establish a molecular design strategy for single-molecule devices.

Main Methods:

• Utilized linear diphenylmethane and orthogonal 9,9'-spirobifluorene frameworks.
• Designed four model molecules (DM22-MT, DM44-MT, SF33-MT, SF44-MT) with -SMe anchoring groups.
• Employed scanning tunneling microscope-break junction (STM-BJ) measurements and theoretical simulations.

Main Results:

• All examined molecules exhibited intrinsically low conductance, consistent with DQI.
• Site-dependent effects in spiro systems disrupted conjugation and altered coupling.
• Observed counterintuitive conductance trends in spiro systems.

Conclusions:

• Provided the first experimental evidence for bilateral DQI in pi-sigma-pi systems.
• Established a molecular design strategy for insulating and functionalized single-molecule devices.
• Demonstrated control over molecular conductance beyond HOMO-LUMO interference.