A Rational Framework to Estimate the Chiroptical Activity of [6]Helicene Derivatives

Helicenes are a class of molecules potentially suitable in several technological applications with intrinsic structural chirality, and they are known for their exceptional chiroptical properties, with CD signals being notably more intense than those of other small organic molecules. Accurately estimating the chiroptical properties of helicenes is relevant for the application of these molecules in many diverse fields, yet still challenging. In this paper, we combine experimental optical characterization and ab initio calculations to study how different substituents influence the chiroptical properties of [6]helicene. By systematically varying the size and chemical nature of the substituents, we find that both electron withdrawing and electron-donating substituents red-shift and dwindle the optical activity of the molecule. We hypothesize that the observed dumping in transition energy and intensity is connected to the strength of the perturbation induced by the substituent on the π-conjugation of the aromatic rings. The comparison between experiments and computations allows rationalization of the trends and suggestion of how the substituents influence properties. This work provides a framework for the fine-tuning of helicenes' chiroptical properties via chemical modification of the substituents, enabling the design of helicene-based systems with tailored optical properties.