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


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

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 All remaining data are available in the main text or the Supplementary Information.


  1. 1.

    Nazeeruddin, M. K., Baranoff, E. & Grätzel, M. Dye-sensitized solar cells: a brief overview. Sol. Energy 85, 1172–1178 (2011).

    CAS  Google Scholar 

  2. 2.

    Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L. & Pettersson, H. Dye-sensitized solar cells. Chem. Rev. 110, 6595–6663 (2010).

    CAS  PubMed  Google Scholar 

  3. 3.

    Ashford, D. L. et al. Molecular chromophore–catalyst assemblies for solar fuel applications. Chem. Rev. 115, 13006–13049 (2015).

    CAS  PubMed  Google Scholar 

  4. 4.

    Xuan, J. & Xiao, W.-J. Visible-light photoredox catalysis. Angew. Chem. Int. Ed. 51, 6828–6838 (2012).

    CAS  Google Scholar 

  5. 5.

    Tucker, J. W. & Stephenson, C. R. J. Shining light on photoredox catalysis: theory and synthetic applications. J. Org. Chem. 77, 1617–1622 (2012).

    CAS  PubMed  Google Scholar 

  6. 6.

    Prier, C. K., Rankic, D. A. & MacMillan, D. W. C. Visible light photoredox catalysis with transition metal complexes: applications in organic synthesis. Chem. Rev. 113, 5322–5363 (2013).

    CAS  PubMed  PubMed Central  Google Scholar 

  7. 7.

    Monro, S. et al. Transition metal complexes and photodynamic therapy from a tumor-centered approach: challenges, opportunities, and highlights from the development of TLD1433. Chem. Rev. 119, 797–828 (2019).

    CAS  PubMed  Google Scholar 

  8. 8.

    Yersin, H., Rausch, A. F., Czerwieniec, R., Hofbeck, T. & Fischer, T. The triplet state of organo-transition metal compounds. Triplet harvesting and singlet harvesting for efficient OLEDs. Coord. Chem. Rev. 255, 2622–2652 (2011).

    CAS  Google Scholar 

  9. 9.

    Xu, H. et al. Recent progress in metal–organic complexes for optoelectronic applications. Chem. Soc. Rev. 43, 3259–3302 (2014).

    CAS  PubMed  Google Scholar 

  10. 10.

    Dias, F. B., Penfold, T. J., Berberan-Santos, M. N. & Monkman, A. P. Photophysics of thermally activated delayed fluorescence in organic molecules. Methods Appl. Fluoresc. 5, 012001 (2017).

    PubMed  Google Scholar 

  11. 11.

    Li, G., Zhu, Z.-Q., Chen, Q. & Li, J. Metal complex based delayed fluorescence materials. Org. Electron. 69, 135–152 (2019).

    CAS  Google Scholar 

  12. 12.

    Uoyama, H., Goushi, K., Shizu, K., Nomura, H. & Adachi, C. Highly efficient organic light-emitting diodes from delayed fluorescence. Nature 492, 234–238 (2012).

    CAS  PubMed  Google Scholar 

  13. 13.

    Penfold, T. J., Dias, F. B. & Monkman, A. P. The theory of thermally activated delayed fluorescence for organic light emitting diodes. Chem. Commun. 54, 3926–3935 (2018).

    CAS  Google Scholar 

  14. 14.

    Kirchhoff, J. R. et al. Temperature dependence of luminescence from Cu(NN)2 + systems in fluid solution. Evidence for the participation of two excited states. Inorg. Chem. 22, 2380–2384 (1983).

    CAS  Google Scholar 

  15. 15.

    Peltier, J. L. et al. Eliminating nonradiative decay in Cu(i) emitters: 99% quantum efficiency and microsecond lifetime. Science 363, 601–606 (2019).

    PubMed  Google Scholar 

  16. 16.

    Czerwieniec, R., Leitl, M. J., Homeier, H. H. H. & Yersin, H. Cu(i) complexes—thermally activated delayed fluorescence. Photophysical approach and material design. Coord. Chem. Rev. 325, 2–28 (2016).

    CAS  Google Scholar 

  17. 17.

    Wenger, O. S. Photoactive complexes with earth-abundant metals. J. Am. Chem. Soc. 140, 13522–13533 (2018).

    CAS  PubMed  Google Scholar 

  18. 18.

    Harlang, T. C. B. et al. Iron sensitizer converts light to electrons with 92% yield. Nat. Chem. 7, 883–889 (2015).

    CAS  PubMed  Google Scholar 

  19. 19.

    Chábera, P. et al. A low-spin Fe(iii) complex with 100 ps ligand-to-metal charge transfer photoluminescence. Nature 543, 695–699 (2017).

    PubMed  Google Scholar 

  20. 20.

    Kjær, K. S. et al. Luminescence and reactivity of a charge-transfer excited iron complex with nanosecond lifetime. Science 363, 249–253 (2019).

    PubMed  Google Scholar 

  21. 21.

    Lazorski, M. S. & Castellano, F. N. Advances in the light conversion properties of Cu(i)-based photosensitizers. Polyhedron 82, 57–70 (2014).

    CAS  Google Scholar 

  22. 22.

    Zhang, Y., Schulz, M., Wächtler, M., Karnahl, M. & Dietzek, B. Heteroleptic diimine–diphosphine Cu(i) complexes as an alternative towards noble-metal based photosensitizers: design strategies, photophysical properties and perspective applications. Coord. Chem. Rev. 356, 127–146 (2018).

    CAS  Google Scholar 

  23. 23.

    Arias-Rotondo, D. M. & McCusker, J. K. The photophysics of photoredox catalysis: a roadmap for catalyst design. Chem. Soc. Rev. 45, 5803–5820 (2016).

    CAS  PubMed  Google Scholar 

  24. 24.

    Pfennig, B. W., Thompson, M. E. & Bocarsly, A. B. A new class of room temperature luminescent organometallic complexes: luminescence and photophysical properties of permethylscandocene chloride in fluid solution. J. Am. Chem. Soc. 111, 8947–8948 (1989).

    CAS  Google Scholar 

  25. 25.

    Paulson, S., Sullivan, B. P. & Caspar, J. V. Luminescent ligand-to-metal charge-transfer excited states based on pentamethylcyclopentadienyl complexes of tantalum. J. Am. Chem. Soc. 114, 6905–6906 (1992).

    CAS  Google Scholar 

  26. 26.

    Heinselman, K. S. & Hopkins, M. D. Luminescence properties of d 0 metal–imido compounds. J. Am. Chem. Soc. 117, 12340–12341 (1995).

    CAS  Google Scholar 

  27. 27.

    Loukova, G. V., Huhn, W., Vasiliev, V. P. & Smirnov, V. A. Ligand-to-metal charge transfer excited states with unprecedented luminescence yield in fluid solution. J. Phys. Chem. A 111, 4117–4121 (2007).

    CAS  PubMed  Google Scholar 

  28. 28.

    Romain, C. et al. Redox and luminescent properties of robust and air-stable N-heterocyclic carbene group 4 metal complexes. Inorg. Chem. 53, 7371–7376 (2014).

    CAS  PubMed  Google Scholar 

  29. 29.

    Loukova, G. V. & Smirnov, V. A. Phosphorescent ligand-to-metal charge-transfer excited states in the group IVB metallocene triad. Chem. Phys. Lett. 329, 437–442 (2000).

    CAS  Google Scholar 

  30. 30.

    Gazi, S. et al. Selective photocatalytic C–C bond cleavage under ambient conditions with earth abundant vanadium complexes. Chem. Sci. 6, 7130–7142 (2015).

    CAS  PubMed  PubMed Central  Google Scholar 

  31. 31.

    Zhang, Y., Petersen, J. L. & Milsmann, C. A luminescent zirconium(iv) complex as a molecular photosensitizer for visible light photoredox catalysis. J. Am. Chem. Soc. 138, 13115–13118 (2016).

    CAS  PubMed  Google Scholar 

  32. 32.

    Zhang, Y., Lee, T. S., Petersen, J. L. & Milsmann, C. A zirconium photosensitizer with a long-lived excited state: mechanistic insight into photo-induced single electron transfer. J. Am. Chem. Soc. 140, 5934–5947 (2018).

    CAS  PubMed  Google Scholar 

  33. 33.

    Zhang, Y., Petersen, J. L. & Milsmann, C. Photochemical C–C bond formation in luminescent zirconium complexes with CNN pincer ligands. Organometallics 37, 4488–4499 (2018).

    CAS  Google Scholar 

  34. 34.

    Zhang, Y., Akhmedov, N. G., Petersen, J. L. & Milsmann, C. Photoluminescence of seven-coordinate zirconium and hafnium complexes with 2,2′-pyridylpyrrolide ligands. Chem. Eur. J. 25, 3042–3052 (2019).

    CAS  PubMed  Google Scholar 

  35. 35.

    Chan, K.-T. et al. Strongly luminescent tungsten emitters with emission quantum yields of up to 84%: TADF and high-efficiency molecular tungsten OLEDs. Angew. Chem. Int. Ed. 58, 14896–14900 (2019).

    CAS  Google Scholar 

  36. 36.

    Zhang, L. L.-M. et al. Core-dependent properties of copper nanoclusters: valence-pure nanoclusters as NIR TADF emitters and mixed-valence ones as semiconductors. Chem. Sci. 10, 10122–10128 (2019).

    PubMed  PubMed Central  Google Scholar 

  37. 37.

    Hakey, B. M., Darmon, J. M., Zhang, Y., Petersen, L. & Milsmann, C. Synthesis and electronic structure of neutral square-planar high-spin iron(ii) complexes supported by a dianionic pincer ligand. Inorg. Chem. 58, 1252–1266 (2019).

    CAS  PubMed  Google Scholar 

  38. 38.

    Vernitskaya, T. V. & Efimov, O. N. Polypyrrole: a conducting polymer; its synthesis, properties and applications. Russ. Chem. Rev. 66, 443–457 (1997).

    Google Scholar 

  39. 39.

    Herr, P., Glaser, F., Büldt, L. A., Larsen, C. B. & Wenger, O. S. Long-lived, strongly emissive, and highly reducing excited states in Mo(0) complexes with chelating isocyanides. J. Am. Chem. Soc. 141, 14394–14402 (2019).

    CAS  PubMed  Google Scholar 

  40. 40.

    Lees, A. J. The luminescence rigidochromic effect exhibited by organometallic complexes: rationale and applications. Comments Inorg. Chem. 17, 319–346 (1995).

    CAS  Google Scholar 

  41. 41.

    Brown, A. M., McCusker, C. E. & McCusker, J. K. Spectroelectrochemical identification of charge-transfer excited states in transition metal-based polypyridyl complexes. Dalton Trans. 43, 17635–17646 (2014).

    CAS  PubMed  Google Scholar 

  42. 42.

    Hammond, G. S. et al. Mechanisms of photochemical reactions in solution. XXII. Photochemical cistrans isomerization. J. Am. Chem. Soc. 86, 3197–3217 (1964).

    CAS  Google Scholar 

  43. 43.

    Larsen, C. B. & Wenger, O. S. Photoredox catalysis with metal complexes made from earth-abundant elements. Chem. Eur. J. 24, 2039–2058 (2018).

    CAS  PubMed  Google Scholar 

  44. 44.

    Hockin, B. M., Li, C., Robertson, N. & Zysman-Colman, E. Photoredox catalysts based on earth-abundant metal complexes. Catal. Sci. Technol. 9, 889–915 (2019).

    CAS  Google Scholar 

  45. 45.

    Kalyanasundaram, K. Photophysics, photochemistry and solar energy conversion with tris(bipyridyl)ruthenium(ii) and its analogues. Coord. Chem. Rev. 46, 159–244 (1982).

    CAS  Google Scholar 

  46. 46.

    Connelly, N. G. & Geiger, W. E. Chemical redox agents for organometallic chemistry. Chem. Rev. 96, 877–910 (1996).

    CAS  PubMed  Google Scholar 

Download references


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.

Author information




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.

Corresponding author

Correspondence to Carsten Milsmann.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading