(Invited) Colloidal Semiconductor Nanocrystal Photocatalysts: Teaching an Old Dot New Tricks

Abstract

Colloidal semiconductor nanocrystal quantum dots (QDs) have shown great promise as photocatalysts for a variety of chemical transformations including oxidations, reductions, and most recently carbon-carbon bond forming reactions. QDs inherently possess several desirable properties for photocatalysis including oxidation and reduction potentials that are size tunable, and also that are largely independent of their surface chemistry. Due to their size, QDs can also interact with several substrates at once, enhancing charge-transfer rates. We recently found that CdSe QDs and simple aqueous Ni2+salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. Under appropriate conditions, the nanocrystal-based system has undiminished activity for at least 360 hours with about a million turnovers. Ultrafast optical spectroscopy studies of electron transfer from the nanoparticles to a Ni-catalyst reveals the optimal nanoparticle size and shape for photochemical proton reduction, which was confirmed with photocatalytic experiments. We will also discuss recent measurements of carbon-carbon bond formation driven photochemically using CdSe QDs. A single-sized CdSe QD can replace several different dye catalysts needed for five different photoredox reactions (beta-alkylation, beta-aminoalkylation, dehalogenation, amine arylation, and decarboxylative radical formation).Even without optimization of the QDs or the reaction conditions, efficiencies rivaling the best available metal dyes were obtained. The facile customization afforded by QDs for photoredox catalysis suggests they could have important applications in future syntheses of novel pharmaceuticals.