Chiral 1,1′-binaphthyl-2,2′-dithiol-stabilized gold clusters:: Size separation and optical activity in the UV-vis

Gold particles covered with 1,1'-binaphthyl-2,2'-dithiol (BINAS) were prepared. Using size exclusion chromatography, it was possible for the first time to separate the sample into fractions with different sizes and colors. Transmission electron microscopy shows that the particles are very small, in the order of 1 nm or slightly above. The absorption spectra of the separated samples show rich structure. The particles show size-dependent optical activity in metal-based electronic transitions. The shape of both the absorption and circular dichroism spectra of one of the smallest fractions exhibits similarities with the spectra reported for Au-11 covered by 2,2-bis(diphenylphosphino)-1,1'-biphenyl although the spectra are shifted to shorter wavelengths in the case of the dithiol. The anisotropy factors, Delta epsilon/epsilon of these particles are as large as 4 x 10(-3), which is larger than the values reported for gold particles stabilized by phosphines and water-soluble thiols. Ibis indicates that BINAS is particularly well-suited to impart chirality on to gold particles.