Chiroptical properties of cyanine aggregates: hierarchical modelling from monomers to bundles

Some achiral cyanine dyes form well-ordered chiral assemblies exhibiting pronounced Circular Dichroism (CD) and Circularly Polarized Luminescence (CPL). Notably, achiral C8O3 cyanines self-assemble into tubular J-aggregates, which further organize into bundles displaying bisignate CD spectrum - hallmark of an exciton coupled system - and an unusual bisignated CPL. In contrast, the tubular aggregates display a monosignate CD spectrum. The mechanism underlying these intriguing features remains elusive. In the present work, a quantum-mechanical exciton model is proposed to elucidate the (chir)optical behaviour of C8O3 aggregates. A herringbone arrangement of C8O3 dyes within the tubular aggregates well reproduces the observed spectral signatures. The anomalous observation of a singular CD peak in tubular aggregates is ascribed to the intrinsic chirality of the monomeric units inside the aggregate, whereas the CD doublet characterizing the bundles is attributed to the exciton coupling between the constituent tubes. The bisignated CPL signal observed in bundles reveals significant anti-Kasha emission at room temperature and is quantitatively addressed accounting for a very tiny exciton splitting leading to a sizable thermal population of both exciton states. This study provides crucial insights on the complexity of C8O3 aggregation and on the origin of chiroptical response at various aggregation stages.