Molecular Switch Cobalt Redox Shuttle with a Tunable Hexadentate Ligand

Area of Science:

• Coordination Chemistry
• Materials Science
• Electrochemistry

Background:

• Cobalt complexes are widely studied for redox applications.
• Tuning redox potentials is crucial for optimizing electrochemical devices.
• Dye-sensitized solar cells (DSSCs) require efficient redox mediators for regeneration.

Purpose of the Study:

• To synthesize and characterize novel strong-field hexadentate cobalt complexes.
• To investigate the redox properties and spin-state behavior of these complexes.
• To evaluate their performance as redox shuttles in DSSCs.

Main Methods:

• Synthesis of strong-field hexadentate ligands and their coordination to cobalt.
• Electrochemical characterization (redox potential tuning via ligand modification).
• Electron self-exchange rate measurements.
• Fabrication and testing of DSSCs using the synthesized complexes as redox shuttles.

Main Results:

• Three new low-spin to low-spin Co(III/II) redox couples were synthesized.
• Redox potentials were tunable from -200 to -430 mV versus Fc+/0.
• A reversible molecular switch from five-coordinate Co(II) to six-coordinate Co(III) was observed.
• Faster electron self-exchange rates (2.2-4.2 M-1 s-1) were achieved compared to [Co(bpy)3]3+/2+.
• Improved photocurrents (19.8 mA/cm2) were demonstrated in DSSCs with AP25 + D35 dyes.

Conclusions:

• The synthesized cobalt complexes offer tunable redox potentials and unique coordination switching behavior.
• These complexes serve as effective redox shuttles in DSSCs, outperforming traditional cobalt complexes.
• Future work should focus on pairing these complexes with near-IR absorbing dyes for enhanced solar energy conversion.