Photoredox Learn

Redox potentials

A redox potential belongs to a couple, the oxidised and reduced forms of one species: Ox + e⁻ ⇌ Red. It is always written as a reduction potential, even when the reaction of interest is an oxidation, so an amine's card reads E(R₃N•⁺/R₃N). The more positive E, the more strongly Ox pulls in an electron (a strong oxidant); the more negative E, the more readily Red gives one away (a strong reductant).

E½

the half-wave potential of a reversible couple in cyclic voltammetry, the mean of the two peak potentials. It is close to the formal potential, the thermodynamic value.

Eₚ and Eₚ/₂

for an irreversible wave, when the radical ion falls apart before it can be turned back, only the peak potential Eₚ can be read, and it overshoots. Roth, Romero and Nicewicz (2016) measured the half-peak potential Eₚ/₂ for about 180 substrates under one set of conditions: for mesitylene Eₚ/₂ = +2.07 V, close to the true +2.05 V, whereas Eₚ is +2.3 V.

E°

the standard potential, tabulated for couples in water against the standard hydrogen electrode.

Reference electrodes

Potentials are differences, so each needs its reference. In water it is the standard hydrogen electrode (SHE, 0 V by definition); the saturated calomel electrode (SCE) sits at +0.241 V against it. Most photoredox papers report V vs SCE in acetonitrile. For organic solvents IUPAC recommends the ferrocenium/ferrocene couple measured in the same solution, Fc⁺/Fc, whose potential against SCE depends on the solvent:

The Potentials vs option converts with these offsets (Connelly and Geiger 1996): E vs Fc⁺/Fc = E vs SCE − offset. Aqueous potentials stay against SHE.

How far to trust the numbers

• Reversible couples, such as the photocatalysts' ground states, are reproducible to a few hundredths of a volt; the same catalyst can still differ by about 0.1 V between papers (Ru(bpy)₃³⁺/²⁺: +1.29 V in MacMillan's table, Eₚ/₂ = +1.39 V in Nicewicz's).
• Irreversible reductions of alkyl halides depend strongly on how they are measured: CF₃I is quoted at −1.22 and −1.52 V vs SCE, α-bromoacetophenone at −0.49 and −1.46 V. Treat such values as rough guides.
• Comparing potentials measured in different solvents adds about 0.1 V of uncertainty.

Excited states

Light gives the catalyst its excitation energy E₀,₀ (in eV), which makes it both a better donor and a better acceptor (the Rehm–Weller approximation, ignoring small Coulomb terms):

E(PC⁺/PC*) ≈ E(PC⁺/PC) − E₀,₀the excited state as a reductant

E(PC*/PC⁻) ≈ E(PC/PC⁻) + E₀,₀the excited state as an oxidant

For Ru(bpy)₃²⁺: +1.29 − 2.10 = −0.81 V and −1.33 + 2.10 = +0.77 V, with E₀,₀ ≈ 2.1 eV, close to the energy of its 615 nm emission (2.0 eV).

Will the electron move?

For an electron going from a donor D to an acceptor A, in eV:

ΔG_ET ≈ E(D•⁺/D) − E(A/A•⁻)negative: downhill, the transfer can happen

That is the comparison behind the Electron transfer and Cycles tabs. A downhill step is necessary but not sufficient: the partners must also meet within the excited state's lifetime (about 1 µs for *Ru(bpy)₃²⁺).

Potentials from calculations

A reduction potential measures a free-energy difference: E = −ΔG_red/(nF) − E_abs(ref), where ΔG_red is the free energy of Ox + e⁻ → Red in solution and E_abs the absolute potential of the reference electrode (about 4.4 V for the SHE; values from 4.28 to 4.44 V are in use). It is related to, but not the same as, the ionisation energy and electron affinity: those are for the isolated molecule, while the potential includes the very different solvation of the neutral and the ion. Roth, Romero and Nicewicz computed their whole data set with B3LYP and M06-2X/6-31+G(d,p) and CPCM acetonitrile, from the free energies of the two forms; the calculated values followed the measured ones with R² = 0.96–0.97.

Photoredox cycles

A photocatalyst absorbs visible light and, in its long-lived excited state, gives or accepts one electron. Two cycles return it to the ground state (Prier, Rankic and MacMillan 2013):

Oxidative quenching

the excited catalyst gives an electron to an acceptor A, which needs E(PC⁺/PC*) below E(A/A•⁻). The oxidised catalyst PC⁺ then accepts an electron from a donor D, which needs E(PC⁺/PC) above E(D•⁺/D). Typical oxidative quenchers: viologens, aryldiazonium salts, polyhalomethanes, electron-poor arenes, persulfate.

Reductive quenching

the excited catalyst accepts an electron from a donor D, which needs E(PC*/PC⁻) above E(D•⁺/D). The reduced catalyst PC⁻ then gives it to an acceptor A, which needs E(PC/PC⁻) below E(A/A•⁻). Typical reductive quenchers: tertiary amines, Hantzsch esters, carboxylates, borates.

Each card in the two photocatalyst sets gives one of these four potentials. Changing the ligands moves them together: electron-poor ligands (bpz, dF(CF₃)ppy) shift every potential positive and give strong excited-state oxidants; electron-rich cyclometalated ligands (ppy) give strong reductants, as in fac-Ir(ppy)₃ (−1.73 V).

Thermodynamics says which cycles are possible; when both are, the partner that meets the excited state first decides. Stern–Volmer experiments measure this: the emission fades as an added quencher takes the excited state away, and only the partners that quench it can start the cycle. Some catalysts do neither: they pass on their excitation energy to a partner with a lower triplet energy (energy transfer), and no electron moves.

The Cycles tab asks three kinds of question: which cycle a published reaction uses, which catalyst can do both steps for a given donor and acceptor, and which partner can bring the catalyst back after the first step. The last two are generated from the potentials on the cards; each step counts as possible when ΔG_ET is at least 0.1 eV downhill and impossible when it is at least 0.1 eV uphill.

References

1. J. K. Prier, D. A. Rankic, D. W. C. MacMillan, Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis. Chem. Rev. 2013, 113, 5322–5363. open copy on PMC
2. H. G. Roth, N. A. Romero, D. A. Nicewicz, Experimental and calculated electrochemical potentials of common organic molecules for applications to single-electron redox chemistry. Synlett 2016, 27, 714–723. doi:10.1055/s-0035-1561297
3. N. G. Connelly, W. E. Geiger, Chemical redox agents for organometallic chemistry. Chem. Rev. 1996, 96, 877–910. doi:10.1021/cr940053x
4. Z. Zuo, D. T. Ahneman, L. Chu, J. A. Terrett, A. G. Doyle, D. W. C. MacMillan, Merging photoredox with nickel catalysis: coupling of α-carboxyl sp³-carbons with aryl halides. Science 2014, 345, 437–440. open copy on PMC
5. D. A. Nagib, D. W. C. MacMillan, Trifluoromethylation of arenes and heteroarenes by means of photoredox catalysis. Nature 2011, 480, 224–228. open copy on PMC
6. A. McNally, C. K. Prier, D. W. C. MacMillan, Discovery of an α-amino C–H arylation reaction using the strategy of accelerated serendipity. Science 2011, 334, 1114–1117. open copy on PMC
7. D. S. Hamilton, D. A. Nicewicz, Direct catalytic anti-Markovnikov hydroetherification of alkenols. J. Am. Chem. Soc. 2012, 134, 18577–18580. open copy on PMC
8. S. Lin, M. A. Ischay, C. G. Fry, T. P. Yoon, Radical cation Diels–Alder cycloadditions by visible light photocatalysis. J. Am. Chem. Soc. 2011, 133, 19350–19353. open copy on PMC
9. J. C. Tellis, D. N. Primer, G. A. Molander, Single-electron transmetalation in organoboron cross-coupling by photoredox/nickel dual catalysis. Science 2014, 345, 433–436. open copy on PMC
10. D. N. Primer, I. Karakaya, J. C. Tellis, G. A. Molander, Single-electron transmetalation: an enabling technology for secondary alkylboron cross-coupling. J. Am. Chem. Soc. 2015, 137, 2195–2198. open copy on PMC

Each card names the paper or table its potential comes from, and each Cycles question the paper or catalyst it is built on.