Metalation Control of Open‐Shell Character in meso‐meso Linked Porphyrin meso ‐Oxy Radical Dimers

Control of open‐shell character of meso‐meso linked porphyrin meso ‐oxy radical dimers has been demonstrated by core metalation. Namely, Ni II ‐porphyrin dimer 6Ni exhibits a clear 1 H NMR spectrum and a distorted but rather coplanar quinonoidal structure consisting of two ruffled porphyrin rings, in accordance with the previous report. Freebase dimer 6H 2 shows a similar quinonoidal structure in the solid state but displays slightly broader and temperature‐dependent 1 H NMR spectra, indicating a partial diradical character in solution that increases at high temperature. In sharp contrast, bis‐imidazole‐coordinated Zn II ‐porphyrin dimer 6ZnIm 2 exhibits a perpendicular structure consisting of two planar Zn II ‐porphyrins and has been characterized as a distinct open‐shell diradical on the basis of its non‐observable 1 H NMR signals, a clear ESR signal, and a characteristic absorption spectrum reaching about 1700 nm. Despite the distinct diradical character, 6ZnIm 2 is an extremely stable molecule.