Helically Flexible Boron-Dipyrromethenes for Solution-Processed Isotropic Films With Preserved Chiral Exciton Coupling

Abstract

Chiral organic thin films are emerging as key materials in advanced optoelectronic and chiroptical technologies, where the straightforward, solution-processed fabrication of chiroptically active neat films from small π-conjugated helical molecules is highly desirable for sustainable and scalable device integration. However, in such densely packed systems, the intrinsic chiroptical response is often compromised by macroscopic anisotropies (e.g., linear dichroism and birefringence) and aggregation-induced effects, which obscure genuine molecular signatures and induce or alter chiral exciton coupling. Here, we demonstrate that a readily accessible and conformationally flexible helically chiral boron-dipyrromethene dimer (helicoBODIPY) platform enables the reproducible fabrication of robust solution-processed neat thin films exhibiting a unique behavior: chiroptical isotropy while retaining the characteristic molecular chiroptical response arising from intramolecular chiral exciton coupling. Through systematic structural tuning, these systems exhibit enhanced electronic circular dichroism (ECD) and circularly polarized luminescence in solution. Remarkably, the sharp ECD couplets and absorption dissymmetry values measured in the corresponding films closely reproduce those observed in solution, indicating minimal contribution from anisotropic contributions and negligible perturbation of excitonic interactions in the condensed phase. These unique findings point to helically flexible helicoBODIPYs as an excellent molecular platform that bridges molecular design and thin-film performance, offering a potential tool for disentangling aggregation effects in chiral thin films and advancing the rational design of chiral organic materials with predictable chiroptical properties.