Chiroptical responses of D-/L-penicillamine-capped gold clusters under perturbations of temperature change and phase transfer

We report chiroptical properties of optically active D-/L-penicillamine-capped gold clusters as a function of temperature or dispersion phase. The clusters examined are size-selected via gel electrophoresis. Circular dichroism (CD) signals of the gold clusters (0.57 nm in the core diameter), whose sizes have been determined by a small-angle X-ray scattering (SAXS) analysis, are practically temperature-dependent (20 and 40 degrees C), whereas ordinary absorption spectra are almost independent of the temperature. Furthermore, phase transfer of the gold clusters (0.57 nm) from water to toluene is successfully achieved by hydrophobization using ion association between tetraoctylammonium cations and carboxylate anions in surface penicillamine molecules. The phase-transferred clusters covered with D- or L-enantiomer still display intense optical activity with the opposite sign (mirror-image relationship) in the metal-based electronic transition region. Upon phase transfer, we find a significant change in the profiles of the anisotropy factor. On the basis of these temperature- and phase-dependent chiroptical behaviors of the gold clusters, we discuss the origins of their optical activity in the context of a dissymmetric field model.