Ultrafast voltammetry for probing interfacial electron transfer in molecular wires

Area of Science:

• Molecular electronics
• Electrochemistry
• Supramolecular chemistry

Background:

• Self-assembled monolayers (SAMs) are crucial for molecular electronics.
• Oligophenylenevinylene (OPV) molecular wires facilitate electron transfer.
• Understanding electron transfer mechanisms in SAMs is key to device performance.

Purpose of the Study:

• To investigate the influence of counterion accessibility on electron transfer rates in OPV molecular wires.
• To differentiate between superexchange and electron hopping mechanisms based on environmental factors.
• To quantify electron transfer rate constants in complex SAMs.

Main Methods:

• Ultrafast cyclic voltammetry was employed to measure electron transfer kinetics.
• Systematic variation of molecular wire structure to control redox center accessibility.
• Analysis of rate-limiting steps influenced by electrolyte counterion mobility.

Main Results:

• Electron transfer rate constants exceeding 10^6 s^-1 were observed when redox centers were accessible to counterions.
• Counterion movement was identified as essential for charge compensation during electron transfer.
• Buried redox centers led to counterion movement becoming rate-limiting, significantly reducing and altering the measured rates.

Conclusions:

• The accessibility of electroactive moieties to electrolyte counterions critically dictates electron transfer rates in OPV molecular wires.
• Charge compensation by counterions is a primary factor governing electron transfer efficiency.
• Environmental factors, such as hydrophobicity, can switch the rate-limiting step from intramolecular processes to ion transport, impacting electron transfer mechanisms.