Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states

Abstract

Advances in chemical control of the photophysical properties of transition-metal complexes are revolutionizing a wide range of technologies, particularly photocatalysis and light-emitting diodes, but they rely heavily on molecules containing precious metals such as ruthenium and iridium. Although the application of earth-abundant ‘early’ transition metals in photosensitizers is clearly advantageous, a detailed understanding of excited states with ligand-to-metal charge transfer (LMCT) character is paramount to account for their distinct electron configurations. Here we report an air- and moisture-stable, visible light-absorbing Zr(iv) photosensitizer, Zr(MesPDPPh)2, where [MesPDPPh]2− is the doubly deprotonated form of [2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine]. This molecule has an exceptionally long-lived triplet LMCT excited state (τ = 350 μs), featuring highly efficient photoluminescence emission (Ф = 0.45) due to thermally activated delayed fluorescence emanating from the higher-lying singlet configuration with significant LMCT contributions. Zr(MesPDPPh)2 engages in numerous photoredox catalytic processes and triplet energy transfer. Our investigation provides a blueprint for future photosensitizer development featuring early transition metals and excited states with significant LMCT contributions.

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Fig. 1: Synthesis and structural characterization of Zr(MesPDPPh)2.
Fig. 2: Electrochemical and optical properties of Zr(MesPDPPh)2.
Fig. 3: Temperature-dependent emission characteristics of Zr(MesPDPPh)2 supporting thermally activated delayed fluorescence.
Fig. 4: Femtosecond TA spectroscopic data revealing the excited-state dynamics of Zr(MesPDPPh)2.
Fig. 5: Summary of excited-state dynamics and redox potentials of Zr(MesPDPPh)2.
Fig. 6: Representative examples for photoredox catalytic transformations promoted by Zr(MesPDPPh)2 under visible-light irradiation featuring three distinct mechanisms of substrate activation.

Data availability

Crystallographic data for the structure reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition no. CCDC 1922700 (Zr(MesPDPPh)2). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/. All remaining data are available in the main text or the Supplementary Information.

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Acknowledgements

C.M., Y.Z. and D.C.L. acknowledge West Virginia University and the National Science Foundation (CHE-1752738) for financial support. This work used X-ray crystallography (CHE-1336071) and NMR (CHE-1228336) equipment funded by the National Science Foundation. The WVU High Performance Computing facilities are funded by the National Science Foundation EPSCoR Research Infrastructure Improvement Cooperative Agreement no. 1003907, the state of West Virginia (WVEPSCoR via the Higher Education Policy Commission), the WVU Research Corporation and faculty investments. The temperature-dependent static and time-resolved photoluminescence experiments performed at NC State (F.N.C. and J.M.F.) were supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award no. DE-SC0011979. G.D.S and T.L. acknowledge the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, of the US Department of Energy through grant no. DE-SC0015429.

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Contributions

Y.Z. synthesized and characterized the compound, performed electrochemical measurements, collected steady-state absorption and emission spectra, conducted the majority of photoredox catalytic reactions, and obtained and analysed all computational data. T.S.L. collected and analysed the TA spectroscopic data. J.M.F. conducted the temperature-dependent emission studies and analysed the corresponding data. D.C.L. performed redox titrations and additional photoredox catalytic reactions. J.L.P. determined the crystal structure. G.D.S., F.N.C. and C.M. directed the project and wrote the manuscript.

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Correspondence to Carsten Milsmann.

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Supplementary information

Supplementary Information

Supplementary methods, characterization and computational results. Figs. 1–39, Tables 1–4 and refs. 1–29.

XYZ coordinates

Cartesian coordinates for optimized structures from DFT calculations.

Crystallographic data

Crystallographic data for Zr(MesPDPPh)2; CCDC reference 1922700.

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Zhang, Y., Lee, T.S., Favale, J.M. et al. Delayed fluorescence from a zirconium(iv) photosensitizer with ligand-to-metal charge-transfer excited states. Nat. Chem. 12, 345–352 (2020). https://doi.org/10.1038/s41557-020-0430-7

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