The elusive dynamics of aqueous permanganate photochemistry

O. Haggag, P. Malakar, P. Pokhilko, J. F. Stanton, A. I. Krylov, and S. Ruhman
Phys. Chem. Chem. Phys.  22, 10043 – 10055 (2020)

Despite decades of investigation, mechanistic details of aqueous permanganate photo- decomposition remain unclear. Here we follow photoinduced dynamics of aqueous permanganate with femtosecond spectroscopy. Photoexcitation of KMnO₄(aq) in the visible unleashes a sub-picosecond cascade of non-radiative transitions, leading to a distinct species which relaxes to S₀ with a lifetime of 16 ps. Tuning excitation to the UV shows increasing formation of a metastable intermediate, which outlives our ~1 ns window of detection. Guided by electronic structure calculations, and the observations form three pulse excitation experiments, we assign the 16 ps species as the lowest Jahn-Teller component of the ³T₁ triplet state and suggest a plausible sequence of radiationless transitions, which rapidly populate it. In conjunction with photodecomposition quantum yields obtained from the literature, these results demonstrate that aqueous permanganate photo-decomposition proceeds through a long-lived intermediate which is formed in parallel to the triplet in less than one ps upon UV absorption. The possibility that this is the postulated highly oxidative peroxo species, a fraction of which leads to the stable (MnO₂^- + O₂) fragments, is discussed. Finally, periodic modulations detected in the pump-probe signal are assigned to ground-state vibrational coherences excited by impulsive Raman. Their wavelength- dependent absolute phases outline the borders between adjacent electronic transitions in the linear spectrum of permanganate.

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