Dynamic equilibrium of self‐assembled cyclic trimer and tetramer of 1‐methyl‐5‐imidazolylcobalt(III)porphyrin

Abstract Dynamic equilibrium of self‐assembled multi‐porphyrin systems is of interest in obtaining switchable photoresponsive material, but rarely reported. 1‐methyl‐5‐imidazolylcobalt(III)porphyrin (1Co) synthesized here assembled automatically into cyclic trimer and tetramer by intermolecular imidazolyl‐cobalt(III) coordination. The trimer‐tetramer equilibrium was dependent on concentration and solvent, as examined by NMR spectrometry. In CDCl3, the tetramer formation was favored at high concentrations, as the ratio of the trimer to the tetramer was 1:2 at 14.8 mM 1Co, and shifted to 1:8 at 74 mM. Further, when the sample was concentrated from a CHCl3 solution to dryness, the ratio increased to 1:24 on dissolution. In CD3OD, on the other hand, only the trimer was observed in the wide concentration range. Accordingly, both the trimeric and the tetrameric structures could be prepared selectively by the choice of concentration and solvent.