Diphenylacetylene‐Incorporating Octaphyrin: A Rigid Macrocycle with Readily Separable Conformational Isomers

An expanded carbaporphyrinoid analogue, octaphyrin(2,1,1,1,2,1,1,1), containing two rigid diphenylacetylene moieties is reported. In contrast to traditional pyrrolic macrocycles where flexible conformers coexist in dynamic equilibrium, this macrocycle exists as two separable, conformationally stable stereoisomers, denoted as 1A and 1B. The conformational effect of both conformers, as well as their protonated forms, were thoroughly studied using NMR spectroscopy, UV‐Vis, and single crystal X‐ray diffraction analyses. Importantly, heating conformer 1B leads to its irreversible conversion to 1A, whereas in its protonated form, 1A·2MSA undergoes irreversible transformation to 1B·2MSA at lower temperatures. These temperature‐dependent features establish a foundation for developing new accumulated heat sensors, as demonstrated by the use of the present octaphyrins as a customized thermochromic indicator in steam sterilization. The present study thus underscores how the conformational rigidity of these new polypyrrolic macrocycles imparts properties that are distinct from historically flexible expanded porphyrinoids.