TY - JOUR
T1 - Photocatalytic Conversion of Nitrobenzene to Aniline through Sequential Proton-Coupled One-Electron Transfers from a Cadmium Sulfide Quantum Dot
AU - Jensen, Stephen C.
AU - Homan, Stephanie Bettis
AU - Weiss, Emily A.
N1 - Funding Information:
This work was supported as part of the Argonne-Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DESC0001059. Initial catalyst development was conducted with support by the Army Research Office via the Presidential Early Career Award for Scientists and Engineers (PECASE) to E.A.W. GC−MS and NMR data were acquired at Northwestern University’s Integrated Molecular Structure Education and Research Center (IMSERC). S.C.J. thanks the Northwestern International Institute for Nanotechnology (I.I.N.) for a postdoctoral fellowship. The authors thank Dr. Igor Dubovyk and Prof. Regan Thomson for helpful discussions.
Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/2/17
Y1 - 2016/2/17
N2 - 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.
AB - 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.
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U2 - 10.1021/jacs.5b11353
DO - 10.1021/jacs.5b11353
M3 - Article
C2 - 26784531
AN - SCOPUS:84958230974
VL - 138
SP - 1591
EP - 1600
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 5
ER -