(466d) Making Achiral Gold Nanoparticles Chiral: Gold Nanoparticles Capped with Aminocalixarene Enantiomers

Chiral metal nanostructures have been reported for chirally modified metal nanoparticles and crystalline surfaces, and have been suggested for a broad range of applications ranging from enantioselective separations and asymmetric catalysis to chiroptical materials. While the synthesis of chiral metal structures has been emphasized in prior research, the additional possibility of chiral electronic interactions between a metal core and an asymmetric ligand are less well understood. In order to investigate this, we postsynthetically modified initially achiral gold nanoparticles with chiral diamino calixarene ligands, which exhibit a circular dichroism (CD)-active surface plasmon resonance absorption (SPR) band. Electronic communication between adsorbed ligand and gold nanoparticle surface is evidenced in an almost 10-fold increase in the ligand molar ellipticity in the π-π* transition spectral range when bound to the gold surface relative to free solution. At ligand concentrations above that required for surface saturation, the ellipticity of the band in the SPR spectral range plateaus to a constant value, whereas the ellipticity of the band in the π-π* transition spectral range continues to increase in a manner that corresponds to free ligand in solution. This critical observation correlates ligand adsorption and the onset of the CD-active SPR band. On the basis of packing characteristics of the bulky calixarene ligand, which are controlled by achiral tert-butyl groups, and the postsynthetic nature of nanoparticle surface modification of 4.7 nm gold cores used in this study, which precludes synthesis of chiral arrangements of gold atoms, a mechanism responsible for the CD-active SPR bands is proposed. This mechanism is based upon the influence of the asymmetric center of the chiral adsorbate on the electronic states of the metal nanoparticle core, which is further supported by the observed interactions between the gold surface and adsorbed ligand.