Inorganic — single-electron photocatalysis
Photoredox catalysis: visible-light single-electron transfer
How the Photoredox catalysis works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: [Ru(bpy)₃]²⁺ (or Ir(ppy)₃), substrate (aryl diazonium shown), visible light. Conditions: Blue LED or sunlight, MeCN, 25 °C, degassed. Bonds formed: radical (Ph•); new C–C after addition; bonds broken: C–N (diazonium); Ru(II)*. Stereochemistry: Radical intermediates; stereocontrol needs a second catalyst. Rate and selectivity: Excited-state lifetime ≈ 1 µs; quantum yield 0.1–1+ for chain pathways.
At a glance
Step 1: Light absorption: excited-state catalyst
A photocatalyst (often a Ru or Ir polypyridyl complex) absorbs visible light, promoting an electron to a higher-energy orbital and generating a long-lived excited state that is simultaneously a much stronger oxidant and reductant than the ground-state complex.
Step 2: Single-electron transfer to the substrate
This excited catalyst transfers a single electron to or from a substrate — outer-sphere electron transfer, the same mechanism covered earlier for simple metal complexes — generating a reactive open-shell radical (or radical ion) on the substrate.
Step 3: Radical chemistry, then catalyst regeneration
That radical goes on to do further chemistry (addition, fragmentation, cyclization), and a second electron transfer regenerates the ground-state photocatalyst, ready to absorb another photon — this strategy lets ordinary visible light drive genuinely difficult single-electron organic transformations.
Abbreviations and terms
- Ph
- Phenyl. –C₆H₅.
- MeCN
- Acetonitrile. Polar aprotic solvent.
- bipy
- 2,2′-Bipyridine. Bidentate N,N-ligand.
- SET
- Single-electron transfer. Movement of one electron (not a pair), generating radicals/radical ions.
- hν
- Light. Photochemical activation.
How it compares with related reactions
| Photoredox catalysis | Inner-sphere electron transfer | Outer-sphere electron transfer | |
|---|---|---|---|
| Reagents | [Ru(bpy)₃]²⁺ (or Ir(ppy)₃), substrate (aryl diazonium shown), visible light | Cr²⁺(aq) + [Co(NH₃)₅Cl]²⁺ | [Fe(H₂O)₆]²⁺ and [Fe(H₂O)₆]³⁺ |
| Conditions | Blue LED or sunlight, MeCN, 25 °C, degassed | Aqueous acid, 25 °C | Aqueous solution, 25 °C |
| Stereochemistry | Radical intermediates; stereocontrol needs a second catalyst | Chloride transferred from Co to Cr (Taube) | Fe–O bonds contract in Fe(III), lengthen in Fe(II) |
| Rate / selectivity | Excited-state lifetime ≈ 1 µs; quantum yield 0.1–1+ for chain pathways | k ≈ 6×10⁵ M⁻¹s⁻¹; bridging ligand needed | k ≈ 4 M⁻¹s⁻¹; Marcus reorganization energy |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Photoredox catalysis?
[Ru(bpy)₃]²⁺ (or Ir(ppy)₃), substrate (aryl diazonium shown), visible light. Conditions: Blue LED or sunlight, MeCN, 25 °C, degassed.
Which bonds are formed and which are broken?
Formed: radical (Ph•); new C–C after addition. Broken: C–N (diazonium); Ru(II)*.
What is the stereochemical outcome?
Radical intermediates; stereocontrol needs a second catalyst
What controls the rate or the selectivity?
Excited-state lifetime ≈ 1 µs; quantum yield 0.1–1+ for chain pathways
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam