Light triggered selective bond activation is at the heart of many light-induced natural processes such as photosynthetic water splitting, and vision. Synthetic designs mimicking such bond selective photochemistry often suffer from multiple competing non-radiative pathways in excited states. Here we present an impactful strategy using artificial mimics of enzyme active sites which can preorganize and polarize target chemical bonds inside water-soluble cationic nanocavities. Specifically, we demonstrate ultrafast photoactivation of weakly polarized C-H bonds and their subsequent controlled reaction with molecular O2 in water via host-guest charge transfer complexes and solvent-mediated interactions. In our scheme, electron-rich aromatic hydrocarbons self-organize themselves inside an electron deficient water-soluble Pd6L412+ nanocage to generate a visible light absorbing charge transfer (CT) state in water. Photoexcitation (using visible light) of the host-guest CT state drives an ultrafast and selective cleavage of C-H bonds within 350 picoseconds through cage assisted proton coupled electron transfer reaction. The photo-generated confined neutral carbon-centered radicals, having the lifetime of nanoseconds, allow for a facile yet selective reaction with molecular O2 and photocatalytic turnover of oxidized products in water. We provide evidence for the presence of strong directional electric fields which can drive selective bond activation reactions inside charged nanostructures. The enzyme-like behaviour of these photoactivable supramolecular host-guest complexes opens up new avenues to design chemical reactions using water-soluble “artificial photoenzymes”.

Ultrafast C-H bond activation by an artificial photoenzyme via PCET

256th National Meeting (2018)

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