Photoredox · Reaction mechanism
Quantum yields of 44 and 77 say these photoredox reactions run radical chains
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Short answer
Quantum yields far above 1 in three visible-light reactions require product-forming radical chains, and light/dark experiments cannot exclude them.
What happened
Cismesia and Yoon measured 436 nm quantum yields against ferrioxalate actinometry and combined them with luminescence quenching. A radical-cation Diels–Alder gave Φ = 44 (chain length ~41), a radical-anion [2+2] gave Φ = 77, and an α-alkylation gave Φ = 18 — against Φ = 0.49 in an earlier report run under air, where oxygen quenches the excited photocatalyst.
Why it matters
Mechanistic claims in photoredox catalysis often rest on light/dark interruption experiments. This paper shows that test is not diagnostic: a chain can propagate on a timescale short enough to survive it. Quantum yield is the measurement that settles it, and it was rarely reported.
Evidence
- Study type
- Ferrioxalate-referenced quantum-yield measurements with luminescence quenching and light/dark controls
- Sample
- Three visible-light reactions; actinometry at 436 nm (ferrioxalate Φ = 1.01, flux 6.67×10⁻¹⁰ E s⁻¹)
- Journal
- Chemical Science · peer reviewed
- Replication
- Not assessed in this corpus; the α-alkylation value differs from a prior aerobic report because of oxygen quenching
- Limitations
- Three reactions, not a survey. Measured quantum yields depend on whether oxygen is rigorously excluded, so reported values are condition-specific.
What this connects to
Sources
The 2 studies this explanation is built from, by the role each plays. Every source links to PaperFren’s explanation of it and to the original paper.
Primary study
- 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.
What it does not showLimitations
Not every photoredox reaction is a chain; the authors warn against over-generalizing, and aerobic vs N2 conditions change observed Φ.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Supporting evidence
- Platinum photoredox catalysts for trifluoromethylation
Cyclometalated Pt(II) complexes trifluoromethylate unactivated alkenes in >82% yield under 450 nm LEDs by oxidatively quenching CF3I.
What it does not showLimitations
Heteroarene yields are only moderate; the paper does not invent a new CF3 reagent or run a process photochemistry scale-up.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Before
Visible-light photoredox reactions were typically drawn as closed catalytic cycles in which each absorbed photon turns over one substrate, supported by on/off irradiation tests.
Now
At least these three reactions propagate chains, and the community's standard control does not detect them. The authors are explicit that this does not make every photoredox reaction a chain process.