Photoredox reactions that run as radical chains
Quantum yields of 44, 77, and 18 show product-forming chains—not a closed photocatalytic cycle of Φ ≤ 1—in three visible-light reactions.
Source
Characterizing Chain Processes in Visible Light Photoredox Catalysis
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Photoredox reactions that run as radical chains
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The Diels–Alder photoredox reaction has Φ = 44.
from these data, we calculate a quantum yield value of Φ = 44.
A second reaction has Φ = 77.
we calculated a quantum yield of Φ = 77, again demonstrating that product formation is dominated by a chain process.
What they did
Authors measured 436 nm quantum yields against ferrioxalate (Φ = 1.01; flux 6.67×10−10 E s−1), combined them with luminescence quenching, and compared light/dark experiments on a radical-cation Diels–Alder, a radical-anion [2+2], and an α-alkylation.
What they found
Φ = 44 (Diels–Alder; chain length ~41), Φ = 77 for a [2+2] (chain length Φ/Q = 135), and Φ = 18 for α-alkylation (vs 0.49 in a prior aerobic report). Φ ≫ 1 requires chains; light/dark experiments cannot disprove them.
The limits
What it doesn't show
Not every photoredox reaction is a chain; the authors warn against over-generalizing, and aerobic vs N2 conditions change observed Φ.
Key terms
- Quantum yield Φ
- Moles of product per einstein of photons; Φ ≫ 1 implies chains.
- Chain length
- Turnovers per initiation; estimated as Φ or Φ/Q if quenching is incomplete.
- Light/dark experiment
- Turning the lamp off; cannot prove a closed photoredox loop.
- Ferrioxalate actinometry
- Φ = 1.01 at 436 nm used to get photon flux.
- Ru(bpz)3 2+
- Photocatalyst used for the cycloaddition quantum yields.
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Quiz yourself
Φ ≫ 1 in photoredox means:
Common questions
What Φ means a chain?
Φ much greater than 1.
Diels–Alder Φ?
44, chain length ~41.
Can light/dark rule out chains?
No.
Why was α-alkylation Φ 18 not 0.49?
N2 sparging removes O2 quenching of Ru*.
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