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A pinboard by
Leah Frenette

Ph. D. Candidate at the University of Rochester

PINBOARD SUMMARY

Exciting quantum dots to catalyze reactions efficiently and sustainably

Quantum dots are semiconducting nanocrystals whose properties change with size. They absorb a lot of light for their small size and can release that energy as photons and fluoresce, or can transfer the energy as an electron to neighboring molecules. We are exploiting these properties to use quantum dots to photocatalyze C-C bond forming reactions that are important for pharmaceutical applications. Quantum dots absorb more light, are more robust, are less expensive than traditional photocatalysts. Additionally, they can transfer energy at a wider range of redox potentials, meaning one quantum dot will work for a number of different reactions that would otherwise need a specially tuned catalyst. By changing the semiconducting material the quantum dots are made of we can access a wide range of potentials that have previously not been accessed for photocatalysis. This could lead to the discovery of new reactions or pathways not previously accessed, making organic reactions more efficient or allowing us to reach new synthetic targets.

8 ITEMS PINNED

Electronic Processes within Quantum Dot-Molecule Complexes.

Abstract: The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Pub.: 09 Aug '16, Pinned: 30 Jun '17

Photocatalytic Conversion of Nitrobenzene to Aniline through Sequential Proton-Coupled One-Electron Transfers from a Cadmium Sulfide Quantum Dot

Abstract: This paper describes the use of cadmium sulfide quantum dots (CdS QDs) as visible-light photocatalysts for the reduction of nitrobenzene to aniline through six sequential photoinduced, proton-coupled electron transfers. At pH 3.6–4.3, the internal quantum yield of photons-to-reducing electrons is 37.1% over 54 h of illumination, with no apparent decrease in catalyst activity. Monitoring of the QD exciton by transient absorption reveals that, for each step in the catalytic cycle, the sacrificial reductant, 3-mercaptopropionic acid, scavenges the excitonic hole in ∼5 ps to form QD•–; electron transfer to nitrobenzene or the intermediates nitrosobenzene and phenylhydroxylamine then occurs on the nanosecond time scale. The rate constants for the single-electron transfer reactions are correlated with the driving forces for the corresponding proton-coupled electron transfers. This result suggests, but does not prove, that electron transfer, not proton transfer, is rate-limiting for these reactions. Nuclear magnetic resonance analysis of the QD–molecule systems shows that the photoproduct aniline, left unprotonated, serves as a poison for the QD catalyst by adsorbing to its surface. Performing the reaction at an acidic pH not only encourages aniline to desorb but also increases the probability of protonated intermediates; the latter effect probably ensures that recruitment of protons is not rate-limiting.

Pub.: 19 Jan '16, Pinned: 29 Jun '17