A MOLECULAR YARN - NEAR-FIELD OPTICAL STUDIES OF SELF-ASSEMBLED, FLEXIBLE, FLUORESCENT FIBERS

The formation of flexible molecular fibers via the solution-phase self -assembly of the pseudo-isocyanine dye (PIC) 1,1'-diethyl-2,2'-cyanine , and poly(vinyl sulfate) (PVS) is reported. The physical and electron ic properties of these fibers spin-coated into thin films on fused-qua rtz substrates are studied by fluorescence and topographic imaging wit h near-field scanning optical microscopy (NSOM) and also by atomic for ce microscopy (AFM). The scanned-probe images demonstrate that fibers with lengths in the hundred micrometer range, widths of hundreds of na nometers, and thicknesses of a few tens of nanometers are readily form ed in aqueous mixtures of PVS and PIC. Unprecedented flexibility in th ese fibers is exemplified by the formation of numerous curved and loop ed structures in the spin-coated thin films. A sandwich-like composite structure of alternating anionic PVS and cationic PIC layers is propo sed as a model for the assembly of the dye and polymer in these fibers . The alternating layers in this model are held tightly together via t he cooperative ''cross-linking'' of the PVS and PIC layers by electros tatic dye/polymer interactions, and by hydrophobic van der Waals inter actions between the PIC molecules. The intermolecular interactions in the PIC layer result in the formation of a liquid-crystalline-like, we ll-ordered layer of the PIC which exhibits the spectral characteristic s of J-aggregates. The proposed layered structure apparently possesses ''reactive'' surfaces which link individual fibers into a yarnlike as sembly. This cross-linking effect is supported by the presence of cont inuous circular fibers and by the gel-forming ability of the solutions from which these fibers are grown.