A Molecular Yarn: Near-Field Optical Studies of Self-Assembled, Flexible, Fluorescent Fibers

Abstract

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 electronic properties of these fibers spin-coated into thin films on fused-quartz substrates are studied by fluorescence and topographic imaging with near-field scanning optical microscopy (NSOM) and also by atomic force microscopy (AFM). The scanned-probe images demonstrate that fibers with lengths in the hundred micrometer range, widths of hundreds of nanometers, and thicknesses of a few tens of nanometers are readily formed in aqueous mixtures of PVS and PIC. Unprecedented flexibility in these fibers is exemplified by the formation of numerous curved and looped structures in the spin-coated thin films. A sandwich-like composite structure of alternating anionic PVS and cationic PIC layers is proposed 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 the cooperative “cross-linking” of the PVS and PIC layers by electrostatic dye/polymer interactions, and by hydrophobic van der Waals interactions between the PIC molecules. The intermolecular interactions in the PIC layer result in the formation of a liquid-crystalline-like, well-ordered layer of the PIC which exhibits the spectral characteristics of J-aggregates. The proposed layered structure apparently possesses “reactive” surfaces which link individual fibers into a yarnlike assembly. This cross-linking effect is supported by the presence of continuous circular fibers and by the gel-forming ability of the solutions from which these fibers are grown.

A Molecular Yarn: Near-Field Optical Studies of Self-Assembled, Flexible, Fluorescent Fibers

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Abstract

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