ja8b00742_si_002.cif (522.03 kB)
A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer
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posted on 2018-04-19, 00:00 authored by Yu Zhang, Tia S. Lee, Jeffrey L. Petersen, Carsten MilsmannTime-resolved emission spectroscopy
for the luminescent zirconium
complex Zr(MePDP)2 (MePDP = 2,6-bis(5-methyl-3-phenyl-1H-pyrrol-2-yl)pyridine) revealed a long-lived excited state
with a lifetime τ = 325 ± 10 μs. Computational studies
using time-dependent density functional theory were conducted to identify
the nature of the luminescent excited state as a mixed triplet intraligand/ligand-to-metal
charge-transfer state. Stern–Volmer experiments showed a strong
dependence of the quenching rate on the redox potential of the quencher
indicating photoinduced single-electron transfer (SET) as the quenching
pathway. Mechanistic investigations of the photocatalytic homocoupling
of benzyl bromide allowed the detection of organic radical intermediates
during turnover and provided further evidence for SET mediated by
Zr(MePDP)2. Isolation of the one-electron-reduced
form of the photosensitizer, [Zr(MePDP)2]−, enabled studies of its electronic structure by a
combination of experimental and computational techniques and confirmed
its role as a strong reductant. Additionally, the role of the benzimidazolium
hydride derivatives as two-electron sacrificial reductants during
photoredox catalysis was investigated. In combination, the results
presented in this report establish a detailed mechanistic picture
of a photoredox catalytic reaction promoted by an earth-abundant early
transition metal photosensitizer.
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Computational studiescombinationphotoinduced single-electron transferpyrrol -2-ylreductantbenzyl bromidelifetime τtransition metal photosensitizerZrZirconium Photosensitizertime-dependent densityLong-Lived Excited StatePDPPhotoinduced Single-Electron Transfer Time-resolved emission spectroscopyMechanistic Insightquenching pathwayquenching ratephotocatalytic homocouplingroleMechanistic investigationsone-electron-reduced formphotoredox catalysis