Porphyrin-Based Air-Stable Helical Radicals

Stable helical radicals are promising multi-functional molecules in light of intriguing magnetic and chiroptical properties. Attempts were made to extend diphenylmethyl-fused Ni^II porphyrin radical to helical system as the first air-stable organic neutral helical radicals. Intramolecular Pd-catalyzed twofold C-H arylation of methyl- or methoxy-introduced meso-diphenylmethyl Ni^II porphyrins gave a mixture of the target and rearranged radicals. Oxidative fusion reaction of meso-(bis(1-naphthyl)methyl) Ni^II porphyrins provided doubly fused Ni^II porphyrin radicals. One of the helical radicals was separated into enantiomers that showed mirror-image circular dichroism (CD) spectra up to 1300 nm. The helical dinaphthylmethyl-fused Ni^II porphyrin radical displayed solid-state magnetic property mostly arising from monomeric radicals, different from the parent diphenylmethyl-fused Ni^II porphyrin radical that showed antiferromagnetic coupling due to π-stacked pairing.