Spin-State and Reorganization Energy Considerations for Metal-Centered Photoredox Catalysis

Area of Science:

• Photochemistry
• Organometallic Chemistry
• Catalysis

Background:

• Metal-centered excited states offer distinct photochemistry, including photoredox catalysis.
• Cobalt(III) complexes utilize the ³T₁ state for efficient photoinduced electron transfer.
• Photoreactivity from the ⁵T₂ state in iron(II) polypyridyl complexes is limited.

Purpose of the Study:

• To investigate the photoreactivity of iron(II) polypyridyl complexes from the ⁵T₂ excited state.
• To enhance excited-state energies and reactivity by introducing stronger-field ligands.
• To re-evaluate mechanistic considerations for electron transfer from Fe(II) excited states.

Main Methods:

• Synthesis of iron(II) polypyridyl complexes with stronger-field ligands.
• Characterization of excited-state lifetimes and thermodynamic driving forces.
• Reinvestigation of ground-state interactions and photoreactivity.

Main Results:

• Despite favorable conditions, no photoreactivity was observed with modified Fe(II) complexes.
• Ground-state interactions between Fe(II) complexes and substrates mimicked dynamic quenching.
• Electron transfer from the ⁵T₂ state of low-spin d⁶ metals faces significant reorganization and spin conservation barriers.

Conclusions:

• Leveraging reductive chemistry from the ⁵T₂ excited state of Fe(II) is hindered by intrinsic barriers.
• Ligand fields stabilizing the ³T₁ state provide more accessible pathways for excited-state electron transfer.
• Spin-state changes and reorganization energies are critical factors in metal-centered photoredox catalysis.