Chirality plays an important role in the fields of chemistry, pharmacology, biology, medicine, nanobiotechnology and catalysis. Understanding of the origin of cluster chirality is critical for the development of pharmaceuticals, catalysts, chemicals and food additives. In resent years, gold nanoparticles protected by chiral organic molecules have attracted significant attention especially in the development of enantioselective nanocatalysts and in biochemical analysis as electron microscopy labels. In the literature, a lot of data about chiroptical activity of metal nanoclusters can be found. However, despite these extensive studies, the origin of the chirality of gold clusters remains unclear, especially due to lack of structural information. Therefore, theoretical approaches can be very useful to help better understand this important question. Tsukuda and co-workers experimentally investigated the optical activity of the BINAP- and DIOP-stabilized undecagold clusters such as Au11X4Cl2+ (X = BINAP and DIOP) using circular dichroism (CD) spectroscopy. Their results showed that the BINAP-protected undecagold cluster exhibited very intense optical circular dichroism, whereas the DIOP-stabilized system showed very weak CD signals. The goal of our study is to understand why this difference in chiroptical activity of Au11(BINAP)4Cl2+ and Au11(DIOP)4Cl2+ is observed. In this work, DFT and TDDFT methods were used to investigate optical activity and elucidate the origin of the chirality these two clusters. To reduce the size of the calculated complexes, the ligands BINAP and DIOP were substituted with simpler models: BINAP was simulated by 1,4-bisdiphosphino-1,3-butadiene (H2P-HC=CH-HC=CH-PH2), whereas the DIOP ligand was modeled by 1,4-bisdiphosphinobutane (H2P-(CH2)4-PH2). Three hypotheses about the origin of the chiral activity in the undecagold protected clusters were checked: 1) the CD activity originates from core deformation due to ligation; 2) Cl atoms positions can affect the CD intensity and 3) the nature of the chiral ligands can play a crucial role in the optical activity of the achiral core.

Theoretical study of the chiroptical activity of BINAP– and DIOP– stabilized undecagold clusters

253rd National Meeting (2017)

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