Crystallography and Molecular Arrangement of Polymorphic Monolayer J-Aggregates of a Cyanine Dye: Multiangle Polarized Light Fluorescence Optical Microscopy Study.

The molecular orientation in monolayer J-aggregates of 3,3-di(γ-sulfopropyl)-5,5-dichlorotiamonomethinecyanine dye has been precisely estimated using improved linear polarization measurements in the fluorescence microscope in which a multiangle set of polarization data is obtained using sample rotation. The estimated molecular orientation supplemented with the previously established crystallographic constraints based on the analysis of the well-developed two-dimensional J-aggregate shapes unambiguously indicate the staircase type of molecular arrangement for striplike J-aggregates with the staircases oriented along strips. The molecular transition dipoles are inclined at an angle of ∼25° to the strip direction, whereas the characteristic strip vertex angle ∼45° is formed by the [100] and [1-10] directions of the monoclinic unit cell. Measurements of the geometry of partially unwound tubes and their polarization properties support the model of tube formation by close-packed helical winding of flexible monolayer strips. In the tubes, the long molecular axes are oriented at a small angle in the range of 5-15° to the normal to the tube axis providing low bending energy. At a nanoscale, high-resolution atomic force microscopy imaging of J-aggregate monolayers reveals a complex quasi-one-dimensional organization.

Polymorphism of Two-Dimensional Cyanine Dye J-Aggregates and Its Genesis: Fluorescence Microscopy and Atomic Force Microscopy Study.

Polymorphic J-aggregates of monomethine cyanine dye 3,3'-di(γ-sulfopropyl)-5,5'-dichlorotiamonomethinecyanine (TC) have been studied by fluorescence optical microscopy (FOM) and by atomic force microscopy (AFM). The in situ FOM observations in a solution drop distinguish two J-aggregate morphology classes: flexible strips and rigid rods. The AFM imaging of dried samples reveals a strong J-aggregate structural rearrangement under adsorption on a mica surface with the strips self-folding and the rods squashing into rectangular bilayers and much deeper destruction. In the present work, the following structural conclusions have been drawn on the basis of careful consideration of strip crystal habits and various structural features of squashed/destructed rods: (1) the tubular morphology of TC rods is directly proved by FOM measurements in the solution bulk; (2) the staircase model of molecular arrangement in strips is proposed explaining the characteristic ∼44° skew angle in strip vertices; (3) a model of tube formation by a close-packed helical winding of flexible monolayer strips is proposed and justified which explains the observed J-aggregate polymorphism and strip-to-rod polymorphic transformations in a wide spatiotemporal scale; (4) at a nanoscale, an unexpectedly complex quasi-one-dimensional organization in J-aggregate two-dimensional monolayers is observed by high-resolution AFM imaging of constituent nanostrips separated by a characteristic distance in the range of 6-10 nm. The obtained results indicate that the underlying monolayer structure is the same for all J-aggregate polymorphs.