State-Specific Nonresonant and Resonant Plasmon-Driven Electron Transfer into Single Molecules

Area of Science:

• Plasmonics
• Photocatalysis
• Surface Science
• Molecular Optoelectronics

Background:

• Plasmonic energy drives selective chemical transformations, crucial for photocatalysis and solar energy conversion.
• Distinguishing between nonresonant hot electron transfer and resonant charge transfer pathways in plasmon-driven chemistry remains challenging, particularly in complex molecular systems.

Purpose of the Study:

• To directly unveil and differentiate nonresonant and resonant plasmon-driven charge transfer channels with submolecular resolution.
• To correlate plasmon-induced current with specific molecular orbitals.
• To investigate the effect of different charge transfer pathways on chemical bond breaking.

Main Methods:

• Utilized a scanning tunneling microscopy (STM) junction with a silver (Ag) tip and individual pentacene molecules on an Ag(110) surface to generate nanocavity plasmons.
• Employed chopper-modulated laser excitation combined with lock-in detection for high-resolution plasmon-induced current mapping (0.3 nm resolution).
• Tuned photon energy and bias voltage to selectively probe different electronic states and transfer mechanisms.

Main Results:

• Successfully mapped plasmon-induced current with submolecular resolution, correlating it with the frontier molecular orbitals of pentacene.
• Identified nonresonant hot-electron transfer occurring via the lowest unoccupied molecular orbital (LUMO).
• Identified more efficient resonant charge transfer occurring via the LUMO + 1 state, which significantly accelerated C-H bond breaking.

Conclusions:

• Directly demonstrated and distinguished between nonresonant and resonant charge transfer pathways in a model plasmonic system.
• Established that resonant charge transfer via the LUMO + 1 orbital is a highly effective pathway for driving chemical reactions, specifically C-H bond activation.
• Provides a high-resolution methodology for studying plasmon-driven chemistry at the molecular level, applicable to catalysis and optoelectronics.