The theoretical investigation of the stability and the racemization of pristine, functionalized, and doped expanded helicenes

The Hückel aromatic and Möbius aromatic compounds, consisting of topologically different arrangements of adjacent benzene rings in the polycyclic aromatic hydrocarbon (PAH) family, have long attracted interest in both synthesis and theoretical chemistry. Among these, [n]-azene and [n]-circulene derivatives are more thoroughly explored due to their simple architectures and more accessible synthesis pathways. However, there are active ongoing studies on the synthesis of both [n]-helicene and expanded [n]-helicene structures, as well as Möbius circulenes, which are Möbius isomers of [n]-circulenes. In this regard, there is limited information available on both the experimental and theoretical characterization of the synthesized molecules. In this study, we employed Density Functional Theory (DFT) to investigate the thermodynamic stabilities, electronic structures, and optical and molecular properties of both synthesized and hypothetical expanded [n]-helicenes and Möbius [n+1]-circulenes. Particular emphasis was placed on the racemization barriers—an essential and tunable parameter in the design of functional chiral materials and their molecular properties. The racemization energies were computed for pristine, functionalized, and heteroatom-doped variants of expanded [n]-helicenes. Given their critical role in determining configurational stability, the racemization barriers are crucial for the material applications of functional chiral molecules and especially relevant to applications involving [n]-helicenes or helicene-like structures.