An orbital rule for electron transport in molecules

Area of Science:

• Molecular electronics
• Quantum chemistry
• Materials science

Background:

• Electron transfer is crucial for biological systems and electronic devices.
• Single-molecule electronic devices offer unique applications like molecular wires and memory.
• Theoretical models like Landauer formula and Green's function are used for electron transport calculations.

Purpose of the Study:

• To develop a chemical understanding of electron transport in molecules using frontier orbital theory.
• To derive an orbital rule for predicting electron transport properties in metal-molecule-metal junctions.
• To experimentally validate the derived orbital rule.

Main Methods:

• Developed a theoretical framework based on frontier orbital theory (HOMO and LUMO).
• Derived an orbital rule relating orbital coefficients and connectivity.
• Utilized Green's function techniques for theoretical analysis.
• Experimentally measured single-molecule conductance using mechanically controllable break junctions.

Main Results:

• An orbital rule was derived, predicting electron transmission based on the phase and amplitude of frontier orbitals.
• Symmetry-allowed connections (e.g., 1,4-naphthalene dithiol) showed significantly higher conductance than symmetry-forbidden ones (e.g., 2,7-naphthalene dithiol).
• Experimental results confirmed theoretical predictions, with a two-order-of-magnitude difference in conductance.

Conclusions:

• The phase, amplitude, and spatial distribution of frontier orbitals are key determinants of molecular electron transport.
• The derived orbital rule provides a powerful tool for rationally designing molecular electronic devices.
• This work bridges theoretical chemistry and experimental molecular electronics.