Molecular arrangements in polymorphous monolayer structures of carbocyanine dye J-aggregates

Subangstrom accuracy AFM height measurements and high-resolution visualization of crystallographic habits enabled us to give an insight into how stacked dye molecules are arranged in J-aggregate monolayers of 3,3′-di(γ-sulfopropyl)-4,4′,5,5′-dibenzo-9-ethylthiacarbocyanine betaine pyridinium. Ladder and staircase molecular arrangements were self-consistently proposed for the explanation of the stripe-like and leaf-like J-aggregate polymorphism correspondingly. It is inferred that a J-aggregate building unit constitutes a symmetric monolayer of ∼1.35 nm in a height that consists of all-trans monomers with anti-parallel up-down orientation and a slip angle of ∼15°. As it follows from AFM images of folded/overlapped J-aggregates, these species manifest themselves as mechanically very flexible structures.