Effect of organic solvents on J aggregation of pseudoisocyanine dye at mica/water interfaces: morphological transition from three-dimension to two-dimension.

Morphological and spectroscopic properties of pseudoisocyanine (PIC) J aggregates produced at mica/solution interfaces have been characterized by absorption/fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. Addition of organic solvents (1-propanol (PrOH) or 1,4-dioxane (Dox)) into aqueous solutions of the PIC dye induced a transition of the morphology of the interfacial J aggregates. The characteristic feature of this transition is the thickness (or height) change of the aggregate domain layers from three-dimensions to two-dimensions: The domain area of the J aggregates was dependent on the amount of the organic cosolvent, while the domain thickness was dependent on the type of the cosolvent. In pure aqueous solution, the J aggregates at the mica/water interface had a three-dimensional structure with the height of approximately 3 nm (multilayer structure). In mixed solvents of PrOH/water or Dox/water (5 or 10 vol%), the interfacial aggregates became a bilayer or monolayer structure, respectively, assuming that PIC molecules are adsorbed on their molecular plane perpendicular to the mica surface. Meanwhile, optical properties (band width and peak position) of the J band were invariant upon addition of the organic cosolvents, suggesting that molecular packing in the J aggregates is essentially unchanged. These results revealed that spectroscopic properties of the interfacial PIC J aggregates were determined only by the lateral (two-dimensional) interaction within the adsorbed monolayer of PIC molecules on mica, and interlayer interaction in the multilayered J aggregate was consequently small.