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

Abstract

“π-Expanded” (hetero)helicenes have 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 (ΔG^‡_e < 20 kcal mol⁻¹), imposing a significant challenge to their enantiomeric separation, and hence, the study of their chiroptical properties. Therefore, there is a rising interest in increasing the ΔG^‡_e of “π-expanded” (hetero)helicenes to enable their chiral separation and chiroptical study. Previously, we disclosed a new type of expanded poly-aza[9]helicenes that possess low ΔG^‡_e (17–18 kcal mol⁻¹) values at 25 °C, and thus, their P and M enantiomers are difficult to separate, suggesting the necessity of suitable structural modification. Herein, addressing this problem, we report 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 ΔG^‡_e values for these compounds are significantly higher (>35 kcal mol⁻¹), allowing P/M enantiomer separation via chiral-HPLC. They show a g_abs of ±3 × 10⁻³ and g_lum of ±5 × 10⁻³, and have stable chiroptical properties at elevated temperatures. Moreover, the basic imidazole units within these novel poly-aza[9]helicenes enable 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.