Zeng Chenjie, Weitz Andrew, Withers Gayathri, Higaki Tatsuya, Zhao Shuo, Chen Yuxiang, Gil Roberto R, Hendrich Michael, Jin Rongchao
Department of Chemistry , Carnegie Mellon University , 4400 Fifth Ave , Pittsburgh , PA , USA . Email:
Chem Sci. 2019 Sep 4;10(42):9684-9691. doi: 10.1039/c9sc02736j. eCollection 2019 Nov 14.
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.
硫醇盐保护的金纳米团簇从离散的激子态到连续的金属态的转变具有重要的基础研究意义，并且在最近的研究中吸引了大量的关注。与光学和电子跃迁行为相比，从原子金顺磁性（Au 6s）到能带行为的磁性转变研究较少。在这项工作中，通过电子顺磁共振（EPR）光谱研究了具有81个名义“价电子”的1.7 nm [Au(TBBT)]纳米团簇（其中TBBT = 4-丁基苯硫醇盐）的磁性。定量EPR分析表明，每个团簇拥有一个未成对电子（自旋），这表明电子填充到轨道而不是能带中，因为能带中的一个电子会产生小得多的磁矩。因此，[Au(TBBT)]具有非金属电子结构。此外，我们证明通过将[Au(TBBT)]氧化为[Au(TBBT)]可以去除未成对自旋，并且纳米团簇相应地从顺磁性转变为抗磁性。在单电子损失或添加过程中，紫外可见吸收光谱保持不变。应用核磁共振（NMR）来探测Au(TBBT)的电荷和磁态，发现52个表面TBBT配体的化学位移受金核中的自旋影响。Au(TBBT)的NMR光谱显示52个TBBT配体有13重分裂和4重简并，这与配体壳层的准对称性相关。总体而言，这项工作通过结合EPR、光学和NMR研究为Au(TBBT)的电子结构提供了重要见解，这将为进一步理解金属纳米团簇的转变行为铺平道路。
