Enhanced Chiral Exciton Coupling in Neat Molecular Films

Chiral molecule-based organic thin films are of increasing interest in optoelectronics and light technologies, where the development of isotropic neat films of chiral molecules is important for practical applications. Understanding the chiroptical responses of dense molecular aggregates often becomes challenging due to the reflection or scattering of light arising from significant reflectivity changes at the excitonic transition. Furthermore, the combination of linear birefringence (LB) and linear dichroism (LD) from micro- to mesoscopic ordering is a potential source of artifacts. Here, we report the circular dichroism (CD) of optically isotropic neat films of a new BODIPY-BINOL conjugate (O-BODIPY), which reveals a negligible LD-LB contribution as measured with both conventional methods and Mueller polarimetry. A 5-fold increase in the anisotropy factor in the neat film relative to the solution is explained by intermolecular exciton coupling. Time-dependent density functional theory calculations of possible intermolecular geometries induced in the film indicate the formation of short-ordered structures in the isotropic film with the help of combined chiral units. These results provide insight into chiral light-matter interactions, which are currently at the core of many fundamental discussions and promising chiroptical applications.