Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Article
  • Published:

Discovery and characterization of an acridine radical photoreductant

Nature volume 580, pages 76–80 (2020)Cite this article

Subjects

Abstract

Photoinduced electron transfer (PET) is a phenomenon whereby the absorption of light by a chemical species provides an energetic driving force for an electron-transfer reaction1,2,3,4. This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics and photosensitive materials. In recent years, research in the area of photoredox catalysis has enabled the use of PET for the catalytic generation of both neutral and charged organic free-radical species. These technologies have enabled previously inaccessible chemical transformations and have been widely used in both academic and industrial settings. Such reactions are often catalysed by visible-light-absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium or copper5,6. Although various closed-shell organic molecules have been shown to behave as competent electron-transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as excited-state donors or acceptors. This is unsurprising because the lifetimes of doublet excited states of neutral organic radicals are typically several orders of magnitude shorter than the singlet lifetimes of known transition-metal photoredox catalysts7,8,9,10,11. Here we document the discovery, characterization and reactivity of a neutral acridine radical with a maximum excited-state oxidation potential of −3.36 volts versus a saturated calomel electrode, which is similarly reducing to elemental lithium, making this radical one of the most potent chemical reductants reported12. Spectroscopic, computational and chemical studies indicate that the formation of a twisted intramolecular charge-transfer species enables the population of higher-energy doublet excited states, leading to the observed potent photoreducing behaviour. We demonstrate that this catalytically generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and can be used in other organic transformations that require dissolving metal reductants.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Mechanistic studies of Mes-Acr radical.
Fig. 2
Fig. 3: Scope of reductive detosylation catalysed by Mes-Acr•.

Similar content being viewed by others

Data availability

The data supporting the findings of this study are available within the paper and its Supplementary Information.

References

  1. Mattay, J. Photoinduced electron transfer in organic synthesis. Synthesis 1989, 233–252 (1989).

    Article  Google Scholar 

  2. Bauer, A., Westkämper, F., Grimme, S. & Bach, T. Catalytic enantioselective reactions driven by photoinduced electron transfer. Nature 436, 1139–1140 (2005).

    Article  ADS  CAS  Google Scholar 

  3. Fox, M. A. Photoinduced electron transfer. Photochem. Photobiol. 52, 617–627 (1990).

    Article  CAS  Google Scholar 

  4. Fukuzumi, S. New development of photoinduced electron-transfer catalytic systems. Pure Appl. Chem. 79, 981–991 (2007).

    Article  CAS  Google Scholar 

  5. Romero, N. A. & Nicewicz, D. A. Organic photoredox catalysis. Chem. Rev. 116, 10075–10166 (2016).

    Article  CAS  Google Scholar 

  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).

    Article  CAS  Google Scholar 

  7. Johnston, L. J. Photochemistry of radicals and biradicals. Chem. Rev. 93, 251–266 (1993).

    Article  CAS  Google Scholar 

  8. Arnold, B. R., Scaiano, J. C. & McGimpsey, W. G. Electron-transfer quenching of excited diphenylmethyl radicals. J. Am. Chem. Soc. 114, 9978–9982 (1992).

    Article  CAS  Google Scholar 

  9. Scaiano, J. C., Tanner, M. & Weir, D. Exploratory study of the intermolecular reactivity of excited diphenylmethyl radicals. J. Am. Chem. Soc. 107, 4396–4403 (1985).

    Article  CAS  Google Scholar 

  10. Samanta, A. et al. Quenching of excited doublet states of organic radicals by stable radicals. J. Phys. Chem. 93, 3651–3656 (1989).

    Article  CAS  Google Scholar 

  11. Weir, D. & Scaiano, J. C. Substituent effects on the lifetime and fluorescence of excited diphenylmethyl radicals in solution. Chem. Phys. Lett. 128, 156–159 (1986).

    Article  ADS  CAS  Google Scholar 

  12. Scordilis-Kelley, C. Alkali metal reduction potentials measured in chloroaluminate ambient-temperature molten salts. J. Electrochem. Soc. 139, 694–699 (1992).

    Article  CAS  Google Scholar 

  13. Margrey, K. A. & Nicewicz, D. A. A general approach to catalytic alkene anti-Markovnikov hydrofunctionalization reactions via acridinium photoredox catalysis. Acc. Chem. Res. 49, 1997–2006 (2016).

    Article  CAS  Google Scholar 

  14. Romero, N. A. & Nicewicz, D. A. Mechanistic insight into the photoredox catalysis of Anti-Markovnikov alkene hydrofunctionalization reactions. J. Am. Chem. Soc. 136, 17024–17035 (2014).

    Article  CAS  Google Scholar 

  15. Ghosh, I., Ghosh, T., Bardagi, J. I. & König, B. Reduction of aryl halides by consecutive visible light-induced electron transfer processes. Science 346, 725–728 (2014).

    Article  ADS  CAS  Google Scholar 

  16. Connell, T. U. et al. The tandem photoredox catalysis mechanism of [Ir(ppy)2(dtb-bpy)]+ enabling access to energy demanding organic substrates. J. Am. Chem. Soc. 141, 17646–17658 (2019).

    Article  CAS  Google Scholar 

  17. Lu, C., Fujitsuka, M., Sugimoto, A. & Majima, T. Dual character of excited radical anions in aromatic diimide bis(radical anion)s: donor or acceptor? J. Phys. Chem. C 121, 4558–4563 (2017).

    Article  CAS  Google Scholar 

  18. Christensen, J. A. et al. Phenothiazine radical cation excited states as super-oxidants for energy-demanding reactions. J. Am. Chem. Soc. 140, 5290–5299 (2018).

    Article  CAS  Google Scholar 

  19. Gumy, J.-C. & Vauthey, E. Investigation of the excited-state dynamics of radical ions in the condensed phase using the picosecond transient grating technique. J. Phys. Chem. A 101, 8575–8580 (1997).

    Article  CAS  Google Scholar 

  20. Romero, N. A., Margrey, K. A., Tay, N. E. & Nicewicz, D. A. Site-selective arene C-H amination via photoredox catalysis. Science 349, 1326–1330 (2015).

    Article  ADS  CAS  Google Scholar 

  21. Brancato, G. et al. Dual fluorescence through Kasha’s rule breaking: an unconventional photomechanism for intracellular probe design. J. Phys. Chem. B 119, 6144–6154 (2015).

    Article  CAS  Google Scholar 

  22. Demchenko, A. P., Tomin, V. I. & Chou, P.-T. Breaking the Kasha rule for more efficient photochemistry. Chem. Rev. 117, 13353–13381 (2017).

    Article  CAS  Google Scholar 

  23. Peng, Z. et al. Expression of anti-Kasha’s emission from amino benzothiadiazole and its utilization for fluorescent chemosensors and organic light emitting materials. J. Mater. Chem. C 6, 7864–7873 (2018).

    Article  CAS  Google Scholar 

  24. Scuppa, S., Orian, L., Donoli, A., Santi, S. & Meneghetti, M. Anti-Kasha’s rule fluorescence emission in (2-ferrocenyl)indene generated by a twisted intramolecular charge-transfer (TICT) process. J. Phys. Chem. A 115, 8344–8349 (2011).

    Article  CAS  Google Scholar 

  25. Shida, T. Electronic Absorption Spectra of Radical Ions (Elsevier, 1988).

  26. Bhandari, S. & Dunietz, B. D. Quantitative accuracy in calculating charge transfer state energies in solvated molecular complexes using a screened range separated hybrid functional within a polarized continuum model. J. Chem. Theory Comput. 15, 4305–4311 (2019).

    Article  CAS  Google Scholar 

  27. Song, Y. et al. Vibronic structure of photosynthetic pigments probed by polarized two-dimensional electronic spectroscopy and ab initio calculations. Chem. Sci. 10, 8143–8153 (2019).

    Article  CAS  Google Scholar 

  28. Bhandari, S., Cheung, M. S., Geva, E., Kronik, L. & Dunietz, B. D. Fundamental gaps of condensed-phase organic semiconductors from single-molecule calculations using polarization-consistent optimally tuned screened range-separated hybrid functionals. J. Chem. Theory Comput. 14, 6287–6294 (2018).

    Article  CAS  Google Scholar 

  29. Maiti, B. et al. Enhancing charge mobilities in organic semiconductors by selective fluorination: a design approach based on a quantum mechanical perspective. Chem. Sci. 8, 6947–6953 (2017).

    Article  CAS  Google Scholar 

  30. Kerzig, C. & Goez, M. Generating hydrated electrons through photoredox catalysis with 9-anthrolate. Phys. Chem. Chem. Phys. 17, 13829–13836 (2015).

    Article  CAS  Google Scholar 

  31. Kerzig, C., Guo, X. & Wenger, O. S. Unexpected hydrated electron source for preparative visible-light driven photoredox catalysis. J. Am. Chem. Soc. 141, 2122–2127 (2019).

    Article  CAS  Google Scholar 

  32. Poelma, S. O. et al. Chemoselective radical dehalogenation and C–C bond formation on aryl halide substrates using organic photoredox catalysts. J. Org. Chem. 81, 7155–7160 (2016).

    Article  CAS  Google Scholar 

  33. Discekici, E. H. et al. A highly reducing metal-free photoredox catalyst: design and application in radical dehalogenations. Chem. Commun. 51, 11705–11708 (2015).

    Article  CAS  Google Scholar 

  34. Narayanam, J. M. R., Tucker, J. W. & Stephenson, C. R. J. Electron-transfer photoredox catalysis: development of a tin-free reductive dehalogenation reaction. J. Am. Chem. Soc. 131, 8756–8757 (2009).

    Article  CAS  Google Scholar 

  35. Yin, H. et al. The hexachlorocerate(III) anion: a potent, benchtop stable, and readily available ultraviolet A photosensitizer for aryl chlorides. J. Am. Chem. Soc. 138, 16266–16273 (2016).

    Article  CAS  Google Scholar 

  36. Javorskis, T. & Orentas, E. Chemoselective deprotection of sulfonamides under acidic conditions: scope, sulfonyl group migration, and synthetic applications. J. Org. Chem. 82, 13423–13439 (2017).

    Article  CAS  Google Scholar 

  37. Shohji, N., Kawaji, T. & Okamoto, S. Ti(O-i-Pr)4/Me3SiCl/Mg-mediated reductive cleavage of sulfonamides and sulfonates to amines and alcohols. Org. Lett. 13, 2626–2629 (2011).

    Article  CAS  Google Scholar 

  38. Alonso, E., Ramón, D. J. & Yus, M. Reductive deprotection of allyl, benzyl and sulfonyl substituted alcohols, amines and amides using a naphthalene-catalysed lithiation. Tetrahedron 53, 14355–14368 (1997).

    Article  CAS  Google Scholar 

Download references

Acknowledgements

This work was supported in part by the National Institutes of Health (NIGMS) Award number R01 GM120186 (D.A.N.). L.W. was supported by the National Natural Science Foundation of China (21801011) and the International Postdoctoral Exchange Fellowship Program (20180033). O.F.W. and A.M. were supported by the National Science Foundation under grant CHE-1763207. Photophysical measurements were performed in the AMPED EFRC Instrumentation Facility established by the Alliance for Molecular PhotoElectrode Design for Solar Fuels (AMPED), an Energy Frontier Research Center (EFRC) funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001011. B.D.D. acknowledges support for this project by the Department of Energy, Office of Basic Energy Sciences through the Chemical Sciences Geosciences and Biosciences Division, under grant number DE- SC0016501.

Author information

Author notes

  1. These authors contributed equally: Ian A. MacKenzie, Leifeng Wang, Nicholas P. R. Onuska

Authors and Affiliations

  1. Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA

    Ian A. MacKenzie, Leifeng Wang, Nicholas P. R. Onuska, Olivia F. Williams, Andrew M. Moran & David A. Nicewicz

  2. Department of Physics, Kent State University, Kent, OH, USA

    Khadiza Begam

  3. Department of Chemistry and Biochemistry, Kent State University, Kent, OH, USA

    Barry D. Dunietz

Authors
  1. Ian A. MacKenzie
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Leifeng Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. Nicholas P. R. Onuska
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. Olivia F. Williams
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. Khadiza Begam
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. Andrew M. Moran
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. Barry D. Dunietz
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. David A. Nicewicz
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

I.A.M. and D.A.N. were responsible for the initial conception of the project. I.A.M., L.W. and N.P.R.O. devised and executed all experimental work. N.P.R.O., D.A.N., K.B., B.D.D., O.F.W. and I.A.M. assisted in the preparation and editing of the final manuscript. O.F.W. assisted in the collection and O.F.W. and A.M.M. performed analysis of transient absorption data. B.D.D. designed the computational approach, K.B. executed the calculations and K.B., B.D.D., N.P.R.O. and D.A.N. were responsible for the analysis of computations.

Corresponding author

Correspondence to David A. Nicewicz.

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

This file contains Supplementary 1-16 which includes tables and figures.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

MacKenzie, I.A., Wang, L., Onuska, N.P.R. et al. Discovery and characterization of an acridine radical photoreductant. Nature 580, 76–80 (2020). https://doi.org/10.1038/s41586-020-2131-1

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-020-2131-1

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing