The theoretical analysis of
2-Cu was conducted by the authors for three different states, including a close-shell singlet Cu(Ⅲ) (CS), an open-shell singlet antiferromagnetically coupled Cu(Ⅱ) corrole radical (OS) and a triplet ferromagnetically coupled Cu(Ⅱ) corrole radical (T). A lower T state was discovered for
2-Cu than the CS and OS states with a calculated singlet-triplet energy gap of 2.28 kcal/mol, providing theoretical support for the triplet ground state (
Fig. 1d). The strongest support for the triplet ground state came from temperature- and field-dependent superconducting quantum interference device (SQUID) magnetometry. The χT value of
2-Cu in 2 K was 0.77 cm
3 K/mol, and it reached approximately 1 cm
3 K/mol at 300 K. The singlet-triplet energy gap was estimated to be 1.66 kcal/mol by fitting the χT–T plot (
Fig. 1e). The field-dependent magnetization plot of
2-Cu at 2 K was fitted to a Brillouin function with
S = 0.89, which was close to the value (
S = 1) corresponding to the triplet ground state (
Fig. 1f). The magnetic hysteresis of
2-Cu was observed at 2 K. Moreover,
2-Cu exhibits remarkable stability in air despite its radical character. The calculated density plots of spin and SOMO both demonstrate that the density is concentrated mostly in the inner corrole ring, which is nicely protected by fused benzenes with low reactivity.