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Thermodynamics and Mechanism of a Photocatalyzed Stereoselective [2 + 2] Cycloaddition on a CdSe Quantum Dot
journal contribution
posted on 2020-08-28, 14:34 authored by Leighton
O. Jones, Martín A. Mosquera, Yishu Jiang, Emily A. Weiss, George C. Schatz, Mark A. RatnerColloidal
quantum dots (QDs) have shown promise over the last few
decades for a range of applications including single photon emission, in vivo imaging, and photocatalysis. Recent experiments
demonstrated that QDs impart stereoselectivity to triplet excited-state
[2 + 2] cycloaddition reactions of alkenes photocatalyzed by the QD
through self-assembly of the reagent molecules on the QD surface,
but these experiments did not reveal the precise geometries of surface-bound
molecules or their interactions with surface atoms. Here, a theoretical
mechanistic approach is used to study such interactions for [2 + 2]
cycloadditions of 4-vinylbenzoic acid derivatives on CdSe QDs. Spin-polarized
periodic density functional theory (DFT) and nonperiodic DFT calculations
are deployed to determine the origin of the selectivity for the syn diastereomer of the resultant tetrasubstituted cyclobutane
product via atomistic modeling of the CdSe surface and substrates,
determination of the thermodynamic energies of reactions for each
step, the intermolecular interactions between the substrates, and
the triplet state reaction paths. The calculations indicate that reaction
selectivity arises from preferred binding of pairs through intermolecular
interactions of substrate molecules on the QD surface in a syn-precursor structure followed by dimerization after triplet
excitation. These mechanisms are generalizable to other metal-enriched
QD surfaces that have a similar surface structure as that of CdSe,
such as InSe or CdTe. Design principles for anti diastereomer
derivatives are also discussed.