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.

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What Φ would a closed photoredox loop without chains max out at?

A maximum theoretical quantum yield of 1.

What Φ value proves a product-forming chain?

Φ ≫ 1.

What Φ was measured for the radical-cation cycloaddition?

Φ = 44 (44 product molecules per photon absorbed, as a lower bound).

How does that compare to estimated chain length?

Average chain length 41, within error of Φ = 44.

What Φ did the radical-anion [2+2] give?

Φ = 77; chain length Φ/Q = 135 after quenching correction.

What Φ did the α-alkylation give under N2?

Φ = 18, vs König/Riedle Φ = 0.49 for the same transformation.

Why was prior Φ so much lower?

Triplet O2 quenches Ru*; N2 sparging removes that inefficiency.

Can light/dark experiments rule out chains?

No: they cannot be used to conclusively rule them out.

What actinometry settings were used?

436 nm; ferrioxalate Φ = 1.01; flux 6.67×10−10 E s−1.

Should every photoredox paper assume chains?

No: these three cases do not mean all photoredox reactions are chains, but chains should be considered.

How can you estimate a lower limit on chain length if quenching is incomplete?

Divide Φ by the quenching fraction Q.