Unraveling the Handedness of Chiral Assemblies: The Hidden Role of Interchromophoric Interactions in Bisignate CD

In chiral assemblies, the transition dipole moments of the individual molecules are arranged obliquely, leading to the appearance of two absorption bands and a bisignation in the circular dichroism (CD) spectra. However, the influence of the handedness of the self-assembled systems and their interaction energy on the sign of the CD bisignation and circularly polarized luminescence remains enigmatic. These aspects are explored using α- and β-dicyanostilbenes appended with R- and S-isomers of chiral groups, which spontaneously self-assemble in a THF-H2O mixture. The β-dicyanostilbene aggregates, with predominantly H-type character, show positive interchromophoric interaction energy, confirming the exciton chirality rule; i.e., a bisignate CD signal with a positive Cotton effect for right-handed chiral arrangement and vice versa. Conversely, α-dicyanostilbene aggregates with predominantly J-type character exhibit negative interaction energy, displaying a bisignate CD signal with a negative Cotton effect for a right-handed chiral arrangement, which reverses for the left-handed system.