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 (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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