Orbital-Symmetry-Dependent Electron Transfer through Molecules Assembled on Metal Substrates

Area of Science:

• Surface science
• Physical chemistry
• Materials science

Background:

• Investigating electron transfer dynamics at interfaces is crucial for understanding chemical reactions and developing electronic devices.
• Self-assembled monolayers (SAMs) on metal substrates provide a model system for studying interfacial electron transfer.
• Cyano-terminated molecules are relevant in molecular electronics due to their electron-withdrawing nature.

Purpose of the Study:

• To investigate the femtosecond charge-transfer dynamics in cyano-terminated ethanethiolate self-assembled monolayers on gold.
• To understand the role of orbital symmetry in governing electron transfer rates.
• To correlate theoretical predictions with experimental observations of electron transfer.

Main Methods:

• Utilized the core hole clock method with state-selective excitation of nitrogen K-shell electrons.
• Employed X-ray photons with well-defined polarization to exploit symmetry selection rules.
• Performed theoretical calculations to analyze orbital extension and electron transfer pathways.

Main Results:

• Observed significantly different charge-transfer times from two symmetry-split π* orbitals of the cyano group to the gold substrate.
• Demonstrated that orbital symmetry, not just energy, governs electron transfer rates.
• Theoretical calculations confirmed that differential orbital extension onto the alkane backbone and sulfur atom influences transfer dynamics.

Conclusions:

• The symmetry of occupied orbitals plays a critical role in determining electron transfer rates at molecular-metal interfaces.
• Femtosecond core hole clock spectroscopy is a powerful tool for probing ultrafast interfacial charge-transfer dynamics.
• Understanding orbital-dependent electron transfer is essential for designing functional molecular electronic devices.