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 Au11L4X2+ (X = Cl, Br) complexes and their Au2X2L 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 Au11L4X2+ 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 Au11L4X2+ has a chiral C-2 geometry, whereas monodentate phosphine ligands lead to a C-1 structure. In addition, the chiral core structure of Au11L4X2+ 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.