The elusive dynamics of aqueous permanganate photochemistry

The iconic permanganate ion (i.e. ) is a powerful oxidizing agent, owing to the high formal oxidation state (+7) of the manganese center. Its oxidative properties are utilized in a wide range of fields, such as organic synthesis and degradation of contaminants in water.

Despite over a century of investigation, mechanistic details of aqueous permanganate photo-decomposition are remain unclear. We follow photoinduced dynamics of aqueous permanganate with femtosecond transient absorption spectroscopy. Photo-excitation in the visible absorption band which colors KMnO₄ solutions unleashes a sub-picosecond cascade of non-radiative transitions, leading to a well-defined species which reverts exponentially to S₀ with a lifetime of 16 ps. Tuning pump wavelengths to the UV shows growing evidence for formation of metastable photoproducts which outlive our ~1 ns window of detection. The same products are formed with low quantum yield upon re-exciting the 16 ps intermediate in the visible. Guided by High-level electronic structure calculations, these experiments identify the 16 ps intermediate as the lowest component of the ³T₁ triplet state and suggest a plausible sequence of curve crossings which can rapidly populate it. In conjunction with wavelength dependent photodecomposition quantum yields from the literature, our results support its proceeding through a long-lived ground state intermediate. They also prove this intermediate is formed directly from the nascent excited state within ~100 fs upon UV absorption. The possibility that this is the long sought-after peroxo intermediate, a fraction of which leads to the stable (MnO₂^- + O₂) fragments is discussed. Finally, periodic modulations detected in the pump-probe data are mostly assigned to ground state vibrational coherences excited by impulsive Raman. Their wavelength dependent absolute phases clearly outline the borders between adjacent electronic transitions in the linear spectrum of permanganate.