(73a) Polymer-Grafted Plasmonic Nanocomposites with Enhanced Chiroptical Properties

We are interested in studying nanocomposites with a higher chiroptical activity in the visible region, which can be employed in developing chiral metamaterials. We present a thin film system of chiral polymer-plasmonic nanocomposites, fabricated, and characterized to reveal enhanced chiroptical response. The polymers are synthesized using the Suzuki coupling technique with fluorene-based monomers having chiral side chains copolymerized with thiophene units to generate high optical activity in the visible region. Further, plasmonic gold nanostructures are incorporated into these polymers to achieve plasmonic enhancement of optical activity. The nanocomposites are synthesized in situ, in a solution of the polymer, to ensure the integration of the nanostructures within the helical polymer fibrils. Initially, we used monodispersed gold nanoparticles of 5nm diameter. Gold nanostructures have a strong affinity towards the thiophene units, and the in situ technique ensues reduced aggregation during the annealing of the thin film nanocomposites. These polymer-gold nanocomposite thin films show a 20-fold enhancement of optical rotatory dispersion compared to pristine polymer films. Further, we study the chiroptical properties of polymer nanocomposites with gold nanostars. These plasmonic multibranched nanocrystals with high-aspect-ratio spikes render a dominant localized surface plasmon resonance, thereby further enhancing chiroptical properties. The nanocomposites of chiral polymer with these anisotropic plasmonic nanostructures promote a red shift in absorbance (approximately 50 nm) and 80-fold enhancement of circular dichroism compared to pure annealed polymer film. Thus, the gigantically enhanced chiroptical response obtained here via tuning the plasmon coupling will pave the road for new metamaterials and warrant various applications in chiral polymer photonics.