Redox- and Protonation-Driven Baird and Clar Aromaticity in Asymmetric Multicyclic Octaphyrins

Abstract

Aromaticity tuning in octaphyrins provides a powerful platform for controlling redox behavior, spin states, and stimulus-induced structural reorganization in fully π-conjugated macrocycles. In contrast, asymmetric multicyclic systems incorporating fused, bridged architectures and heteroaromatic components remain largely unexplored. Herein, we describe the synthesis of a series of fully π-conjugated asymmetric multicyclic N-fused 36π octaphyrins incorporating bithiophene or benzithiophene units. These multicomponent systems undergo redox- and protonation-induced aromaticity switching with commensurate changes in their electronic- and spin-states. The neutral species are globally nonaromatic; protonation selectively drives one asymmetric octaphyrin into a rare triplet ground state stabilized by Baird aromaticity, whereas the other is stabilized by Clar sextet aromaticity, while two-electron oxidation induces global aromaticity in both systems. Comprehensive electronic and conformational analyses reveal that these multifaceted changes in aromaticity and spin character originate from cooperative and competing π-electronic interactions between the constituent sub-macrocycles. These findings highlight fundamental design principles for modulating electronic and spin properties in fully conjugated multicyclic macrocycles through the controlled interplay of multiple π-electronic circuits.