Bimolecular Excited-State Proton-Coupled Electron Transfer within Encounter Complexes.

Area of Science:

• Photochemistry
• Inorganic Chemistry
• Supramolecular Chemistry

Background:

• Ruthenium polypyridyl complexes are crucial in photochemistry.
• Proton-coupled electron transfer (PCET) is vital in biological and chemical systems.
• Understanding excited-state reactions is key to designing new functional materials.

Purpose of the Study:

• To investigate bimolecular excited-state proton-coupled electron transfer (PCET*) reactions.
• To elucidate the reaction mechanism of a specific ruthenium complex with pyridinium acceptors.
• To compare PCET* behavior with related systems involving electron transfer (ET*) and proton transfer (PT*).

Main Methods:

• Spectroscopic analysis (visible absorption) to identify reaction products.
• Utilizing ruthenium complexes with modified bipyridine ligands, specifically [(dpab)2Ru(4,4'-dhbpy)]2+.
• Studying reactions in dry acetonitrile solutions with N-methyl-4,4'-bipyridinium (MQ+) and N-benzyl-4,4'-bipyridinium (BMQ+).

Main Results:

• Observed bimolecular PCET* for the [(dpab)2Ru(4,4'-dhbpy)]2+ complex.
• Distinguished PCET* products from ET* and PT* products via distinct visible absorption spectra.
• Demonstrated a different reaction pathway compared to the analogous complex with unsubstituted bipyridine (bpy).

Conclusions:

• The substitution of bpy with dpab significantly alters the thermodynamics of ET* and PT*.
• The observed PCET* pathway is rationalized by changes in free energy for competing ET* and PT* processes.
• This study provides a deeper understanding of excited-state reaction mechanisms in metal-organic complexes.