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Published April 15, 2020 | Supplemental Material
Journal Article Open

Highly Stable Organic Bisradicals Protected by Mechanical Bonds


Two new highly charged [2]catenanes—namely, mHe[2]C·6PF₆ and mHo[2]C·6PF₆—were synthesized by exploiting radical host–guest templation between derivatives containing BIPY•+ radical cations and the meta analogue of cyclobis(paraquat-p-phenylene). In contrast to related [2]catenanes that have been isolated as air-stable monoradicals, both mHe[2]C·6PF₆ and mHo[2]C·6PF₆ exist as air-stable singlet bisradicals, as evidenced by both X-ray crystallography in the solid state and EPR spectroscopy in solution. Electrochemical studies indicate that the first two reduction peaks of these two [2]catenanes are shifted significantly to more positive potentials, a feature which is responsible for their extraordinary stability in air. The mixed-valence nature of the mono- and bisradical states endows them with unique NIR absorption properties, e.g., NIR absorption bands for the mono- and bisradical states observed at ∼1800 and ∼1450 nm, respectively. These [2]catenanes are potentially useful in applications that include NIR photothermal conversion, UV–vis–NIR multiple-state electrochromic materials, and multiple-state memory devices. Our findings highlight the principle of "mechanical-bond-induced stabilization" as an efficient strategy for designing persistent organic radicals.

Additional Information

© 2020 American Chemical Society. Received: February 19, 2020; Published: March 30, 2020. The authors thank Northwestern University (NU) for their support of this research. This work also made use of the IMSERC at NU, which has received support from the State of Illinois and International Institute for Nanotechnology (IIN). Z.L. acknowledges support from the National Natural Science Foundation of China (No. 21971211) and the Supercomputer Center of Westlake University. The research at NU was also supported by National Science Foundation (NSF) grant no. CHE-1900422 (M.R.W.) The computational investigations at California Institute of Technology were supported by NSF grant no. CBET-1805022. The authors declare no competing financial interest.

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August 19, 2023
October 19, 2023