Directing an Elusive Photoredox [4+2] Cycloaddition Reaction in Water by Altering the Shape of Nano-confinement

Geometry of confinement and intermolecular packing of guest molecules inside supramolecular cavities can lead to product selectivity that is unattainable in bulk. Here we report for the first time an unprecedented radical-mediated intermolecular [4+2] photo-cycloaddition reaction of naphthyl acetylenes to form naphthyl-substituted phenanthrenes inside the confines of a cup-shaped water soluble nanocage. Using a combination of multidimensional NMR, FT-IR along with broadband femtosecond transient absorption spectroscopy, we characterize the reactive intermediates while providing mechanistic details of this unique host-guest charge transfer mediated photoreaction. Detailed electronic structure studies of the host-guest complex along with Deuterium-labelling experiments reveal that both the alkyne moieties are buried towards the smaller pore of the cup-shaped cavity, thereby driving a novel proximity-based [4+2] photo-cycloaddition product. Our results are in contrast to the typical oxidation via exposed solvent water or linear dimerization products observed in porous octahedral nanocage thereby highlighting the role of cavity shape and its confinement in driving product selectivity.