Dynamic Architectures on Expanded Heterohelicenes

Expanded heterohelicenes are an emerging class of heteroatom-containing flexible helicenes, empowered by dual modification with doped heteroatom(s) as well as expanded helicity. Owing to the intriguing fusion of unique structural and electronic features, these molecules captured recent research focus, yielding some fascinating fundamental understanding and new insights in the helicene chemistry, as well as valued electronic and optoelectronic applications. However, limited synthetic strategies to access these molecules act as a major bottleneck for a rapid development of this field by uncovering hidden potentials of purposefully designed task-specific expanded heterohelicenes. In this regard, expanded heterohelicenes consisting of backbone-decorated dynamic architectures are promising toward exploring stimuli-switchable physicochemical properties and functions; however, they are yet to be developed. In this work, we reported a convenient and modular synthetic protocol for such a novel class of expanded heterohelicenes containing stimuli-responsive core and peripheral modules in order to unearth the hitherto unexplored dynamic properties of these molecules. The rhodium(III)-catalyzed method employed here, named as “rollover π-expansion (ROPE)”, involves a concurrent linear and angular ring fusions on readily accessible imidazolium-containing pyridines as templates by annulation with alkyne staples, generating a 6-6-5-6-6-6-5-6-6-ring-based expanded polyaza[9]helicene skeleton. The intermediacy of a unique dirhoda-hetero[9]-helicene species generated by quadruple C–H activation-rollover metalation was arrested. Single crystal X-ray crystallographic studies of the dirhoda-hetero[9]-helicene intermediate as well as the expanded polyaza[9]helicenes revealed the origin of the characteristic flexible helical rim and its translation into the organic products. The dynamic behavior of these stimuli-responsive molecules was demonstrated with redox and light triggers, which enabled the molecules to function as reversible hydride-transport and prospective photomechanical molecular spring-like systems.