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A Zirconium Photosensitizer with a Long-Lived Excited State: Mechanistic Insight into Photoinduced Single-Electron Transfer

Zhang, Yu, Lee, Tia S., Petersen, Jeffrey L., Milsmann, Carsten
Journal of the American Chemical Society 2018 v.140 no.18 pp. 5934-5947
density functional theory, hydrides, luminescence, organobromine compounds, photocatalysis, photosensitizing agents, pyridines, redox potential, redox reactions, reducing agents, spectroscopy, zirconium
Time-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.