Excited-state proton-coupled electron transfer within ion pairs

Area of Science:

• Photochemistry
• Physical Chemistry
• Electron Transfer

Background:

• Proton-coupled electron transfer (PCET) reactions are crucial in chemistry and biology.
• Excited-state PCET (ES-PCET) offers new reaction pathways.
• Electrostatic ion pairs have aided electron transfer studies but not PCET.

Purpose of the Study:

• To demonstrate the utility of electrostatic ion pairs for studying ES-PCET mechanisms.
• To investigate the ES-PCET reaction between salicylate anions and ruthenium complexes.
• To measure PCET rate constants and understand the underlying electron transfer dynamics.

Main Methods:

• Formation of ion pairs between salicylate anions and tetracationic ruthenium complexes in acetonitrile.
• Photoexcitation of ruthenium complexes and monitoring of ES-PCET via transient absorption spectroscopy.
• Measurement of PCET rate constants using nanosecond photoluminescence spectroscopy.

Main Results:

• Ion pairs facilitated ES-PCET, leading to quenching of the ruthenium excited state.
• Transient absorption identified reduced ruthenium and oxidized salicylate as primary photoproducts.
• PCET rate constants saturated at higher driving forces, suggesting Marcus theory applicability.
• A salicylate tautomer was observed, enabling non-adiabatic electron transfer rate constant determination.

Conclusions:

• Electrostatic ion pairs provide a facile and general method to study ES-PCET.
• This approach bypasses the need for covalent linkage or competing hydrogen bonding sites.
• The study provides insights into Marcus theory predictions for non-adiabatic electron transfer in PCET.