Chirality generated by flows in pseudocyanine dye J-aggregates: revisiting 40 years old reports.

Spontaneous symmetry breaking in J-aggregates of cyanine dyes has a long history in chemical literature. In 1976, Honda and Hada claimed that they had achieved chiral induction (CD) by stirring J-aggregates of pseudocyanine. However, this report is controversial, as the combinations of linear dichroism and birefringence can lead to artifactual circular dichroic signals that are unrelated to molecular chirality. A Mueller matrix spectroscopy study, with an approach for the application of a gradient of the shear rate (solution layer between a rotating and a fixed disk) that differs from the simple vortex stirring used in the original report, shows that true CD can be induced in the sample. The phenomenon is discussed, taking into account the flow dynamics that allows the alignment of the aggregate particles and the gradient of shear rates that determines their folding/torsion, which leads to a chiral excitonic transition.