Fig. 2. Oxygen-evolution transition traced by FTIR.

a, IR time traces at selected wavenumbers, demonstrating the delayed onset of O–O bond formation (1,381 cm⁻¹) and reversible changes assignable to transient sidechain deprotonation (1,571 cm⁻¹ and 1,707 cm⁻¹). The corresponding wavenumbers in the spectra in b–d are marked with coloured asterisks. b–d, DAS corresponding to the proton release phase ($t_{{\mathrm{H}}^{+}}$ = 340 µs, blue line) and the oxygen-evolution phase ($t_{{\mathrm{O}}_2}$ = 2.5 ms, green) as well as the steady-state difference spectrum of the S₃ → S₀ + O₂ transition (dashed black line). Red areas b,d mark inverted 340 µs DAS and 2.5 ms DAS, indicating reversible behaviour; purple shaded areas in c highlight the similarity of the 2.5 ms DAS and the steady-state spectrum, in line with the assignment to non-transient changes in Mn oxidation state. Scale bars, 50 µOD.

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