Controlling magnetism of Au(TBBT) nanoclusters at single electron level and implication for nonmetal to metal transition.

The transition from the discrete, excitonic state to the continuous, metallic state in thiolate-protected gold nanoclusters is of fundamental interest and has attracted significant efforts in recent research. Compared with optical and electronic transition behavior, the transition in magnetism from the atomic gold paramagnetism (Au 6s) to the band behavior is less studied. In this work, the magnetic properties of 1.7 nm [Au(TBBT)] nanoclusters (where TBBT = 4--butylbenzenethiolate) with 81 nominal "valence electrons" are investigated by electron paramagnetic resonance (EPR) spectroscopy. Quantitative EPR analysis shows that each cluster possesses one unpaired electron (spin), indicating that the electrons fill into orbitals instead of a band, for that one electron in the band would give a much smaller magnetic moment. Therefore, [Au(TBBT)] possesses a nonmetallic electronic structure. Furthermore, we demonstrate that the unpaired spin can be removed by oxidizing [Au(TBBT)] to [Au(TBBT)] and the nanocluster transforms from paramagnetism to diamagnetism accordingly. The UV-vis absorption spectra remain the same in the process of single-electron loss or addition. Nuclear magnetic resonance (NMR) is applied to probe the charge and magnetic states of Au(TBBT), and the chemical shifts of 52 surface TBBT ligands are found to be affected by the spin in the gold core. The NMR spectrum of Au(TBBT) shows a 13-fold splitting with 4-fold degeneracy of 52 TBBT ligands, which are correlated to the quasi- symmetry of the ligand shell. Overall, this work provides important insights into the electronic structure of Au(TBBT) by combining EPR, optical and NMR studies, which will pave the way for further understanding of the transition behavior in metal nanoclusters.