Origin of intense chiroptical effects in undecagold subnanometer particles.

Time-dependent density functional theory (TDDFT) calculations are employed to examine the optical absorption and circular dichroism (CD) spectra of undecagold Au(11)L(4)X(2)(+) (X = Cl, Br) complexes and their Au(2)X(2)L precursors, where L is either 2,2'-bis(diphenylphosphino)-1,1'-binaphtyl (BINAP) or 1,4-diphosphino-1,3-butadiene (dpb). These systems exhibit intense and mirror-image Cotton effects in their CD spectra. Experimental peak positions are well reproduced in the calculations. The low energy peaks of Au(11)L(4)X(2)(+) arise primarily from transitions between delocalized metal superatom orbitals. Bidentate phosphine ligands have both a structural and electronic impact on the system. The lowest energy structure of Au(11)L(4)X(2)(+) has a chiral C(2) geometry, whereas monodentate phosphine ligands lead to a C(1) structure. In addition, the chiral core structure of Au(11)L(4)X(2)(+) is not sufficient to explain the strong Cotton effects, and the intensity of the CD spectrum is increased by the presence of the bidentate phosphine ligands.