Highly Stable Organic Bisradicals Protected by Mechanical Bonds

Abstract

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.

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