Siamese Twin Homocarbaporphyrin Dimer: Nonplanar Architecture, Localized Conjugation and Bis-BIII Complex.

Architecturally complex π-systems that defy planarity are increasingly sought for their unconventional electronic behaviour and functional adaptability. Among macrocyclic scaffolds, those capable of sustaining nonplanar yet stable conformations prompt to explore novel regimes of conjugation and stimuli-responsive behaviour. In this study, we present a structurally rigid, double-looped homocarbaporphyrin dimer that adopts a figure-eight conformation, generated by fusing two ortho-benzannulated monomeric units through their C2-bridges. This macrocycle features a Siamese twin molecular architecture wherein the flanking p-phenylene moieties introduce a twist that precludes macrocyclic π-delocalization, rendering the system globally nonaromatic. Notably, the overall geometry of homocarbaporphyrin dimer remains intact upon protonation and B^III coordination, demonstrating exceptional shape persistence under external perturbations. These findings are strongly supported by both spectroscopic and theoretical studies and are unequivocally validated by single-crystal X-ray diffraction analyses. This work advances the design of shape - persistent, nonaromatic macrocyclic systems, offering a blueprint for constructing unconventional π-architectures with tailored electronic properties.