(151e) Polymers and Polymer-Plasmonic Nanocomposites with Enhanced Chiral Optical Activity

We are studying conformationally modified thin films of polymers comprising fluorene monomers with chiral side chains copolymerized with thiophene units to generate high chiral optical activity. The synthesized polymers have varying number of thiophene units, which affects the rotational strength of the polymers owing to the changes in the helicity of their conformations as well as their electronic structure. Upon annealing, the polymer films attain a greater degree of helicity, enhancing their chiral optical activity. Further, we are doping the polymer films with gold nanoparticles of 3-5 nm diameter, synthesized directly in a solution of the polymer. Nanoparticle synthesis in the presence of the thiophene-containing polymer, and without other ligands, promotes incorporation of the gold nanoparticles in the polymer helical fibers. This method of doping with gold nanoparticles reduces aggregation of the nanoparticles during annealing. These polymer-gold nanocomposite thin films show substantially enhanced optical rotatory dispersion, i.e., 20 times higher than pure polymer films. We also study polymer nanocomposites with nonspherical gold nanostructures. Composites incorporating gold nanostars have an enhanced ellipticity intensity (approximately 80 times) with a red shift in the peak position of the circular dichroism spectrum. The combination of methods employed here allows tuning the backbone conjugation, chiral conformation, and localized plasmonic enhancement to achieve enhanced optical chirality and a red shift in the position of maximum circular dichroism.