Plasmonic Chiroptical Response of Silver Nanoparticles Interacting with Chiral Supramolecular Assemblies

Silver nanoparticles were prepared in aqueous solutions of chiral supramolecular structures made of chiral molecular building blocks. While these chiral molecules display negligible circular dichroism (CD) as isolated molecules, their stacking produced a significant CD response at room temperature, which could be eliminated by heating to 80 degrees C due to disordering of the stacks. The chiral stack-plasmon coupling has induced CD at the surface plasmon resonance absorption band of the silver nanoparticles. Switching between two plasmonic CD induction mechanisms was observed: (1) Small Ag nanoparticles coated with large molecular stacks, where the induced plasmonic CD decayed together with the UV molecular CD bands on heating the solution, indicating some type of electromagnetic or dipole coupling mechanism. (2) Larger Ag nanoparticles coated with about a monolayer of molecules exhibited induced plasmonic CD that was temperature-independent. In this case it is estimated that the low chiroptical response of a molecular monolayer is incapable of inducing such a large chiroptical effect, and a model calculation shows that the plasmonic CD response may be the result of a slight chiral shape distortion of the silver nanoparticles.