Inducing configurational stability in inherently flexible expanded heterohelicenes and unlocking stimuli-responsive chiroptical switching

Abstract

“π-Expanded” (hetero)helicenes attracted special attention by virtue of their increased helical diameter, enlarged cavity, structural flexibility, and dynamics. However, their structural flexibility results in low enantiomerization barriers (ΔGe‡ <20 kcal·mol−1), imposing a significant challenge to their enantiomeric separation, and hence the study of chiroptical properties. Therefore, there is a rising interest in increasing ΔGe‡ of “π-expanded” (hetero)helicenes, enabling their chiral separation and chiroptical study. Previously, we disclosed a new type of expanded poly-aza[9]helicenes, which possess low ΔGe‡ (17-18 kcal·mol−1) at 25 oC, and thus the P and M enantiomers are difficult to separate, suggesting the requirement of suitable structural modification. Herein, addressing this problem, we reported the synthesis of configurationally-stable analogues of the expanded poly-aza[9]helicenes by introducing bulky tert-Bu groups at the terminal overlapping rings to restrict ring-flapping. The ΔGe‡ values for these compounds are significantly higher (>35 kcal·mol−1), allowing the P/M enantiomer separation by chiral-HPLC. They show gabs = 3 × 10−3 and glum = 5 × 10−3, and stable chiroptical properties at elevated temperatures. Moreover, the basic imidazole units within these novel poly-aza[9]helicenes enabled pH-responsive chiroptical switching. Overall, the stable chiroptical property and stimuli-responsive chiroptical function of this class of expanded poly-aza[9]helicenes could make them promising candidates for potential chirality-based applications.