The Rehm-Weller equation predicts whether photoinduced electron transfer (PET) between an excited photocatalyst PC* and a ground-state quencher is thermodynamically downhill:
ΔG_PET = e[E_ox(D) − E_red(A)] − E_0,0 − w_p
w_p = e²/(4πε₀·ε_s·r) (Coulombic stabilization of the ion pair)
In the oxidative cycle the excited photocatalyst PC* gives up an electron to the quencher A, so D = PC and A = the quencher: ΔG = E_ox(PC) − E_red(A) − E_0,0 − w_p. In the reductive cycle PC* instead accepts an electron from a donor D, so the slider potential is read as E_ox of the donor and the photocatalyst supplies E_red: ΔG = E_ox(D) − E_red(PC) − E_0,0 − w_p.
The excited-state energy E0,0 is subtracted because photon absorption makes PC* a far stronger oxidant/reductant than its ground state — this is the whole reason photoredox catalysis can drive electron transfers a ground-state catalyst never could. Unlike the fixed 6 Å / acetonitrile case, here you can drag the quencher marker (or use the sliders) to change the encounter distance r and the solvent, so w_p — normally a small correction of a few hundredths of an eV in a polar solvent — grows several-fold in a low-permittivity solvent like dichloromethane, or at short range.
- Mode buttons — switch which quenching pathway (oxidative vs. reductive) is running.
- E0,0 slider — the photocatalyst's singlet/triplet excited-state energy, set by its absorption/emission spectrum.
- Photocatalyst / quencher potential sliders — the relevant ground-state redox couples.
- Solvent + distance — set the Coulombic work of separating the ion pair; drag the quencher circle in the scheme below to change r directly.
Each cycle a photon excites PC to PC*; if ΔG_PET < 0 an electron actually hops to or from the quencher, forming a radical ion pair that back-transfers to close the catalytic cycle and the turnover counter advances. If ΔG_PET ≥ 0, PC* simply relaxes back to the ground state and no chemistry happens — exactly the failure mode that makes catalyst/quencher potential matching the central design problem in real photoredox synthesis.