Lawson Brent, Zahl Percy, Hybertsen Mark S, Kamenetska Maria
Department of Physics, Boston University, Boston, Massachusetts 02215, United States.
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States.
J Am Chem Soc. 2022 Apr 13;144(14):6504-6515. doi: 10.1021/jacs.2c01322. Epub 2022 Mar 30.
Single-molecule circuits with group 8 metallocenes are formed without additional linker groups in scanning tunneling microscope-based break junction (STMBJ) measurements at cryogenic and room-temperature conditions with gold (Au) electrodes. We investigate the nature of this direct gold-π binding motif and its effect on molecular conductance and persistence characteristics during junction evolution. The measurement technique under cryogenic conditions tracks molecular plateaus through the full cycle of extension and compression. Analysis reveals that junction persistence when the metal electrodes are pushed together correlates with whether electrodes are locally sharp or blunt, suggesting distinct scenarios for metallocene junction formation and evolution. The top and bottom surfaces of the "barrel"-shaped metallocenes present the electron-rich π system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on a sharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small, flat patch on a dull tip can bind more strongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of model electrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs during junction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecific interaction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from that identified for the more commonly studied, rod-shaped organic molecular wires.
在基于扫描隧道显微镜的低温和室温条件下的断结（STMBJ）测量中，使用金（Au）电极形成了含有第8族金属茂的单分子电路，无需额外的连接基团。我们研究了这种直接的金-π结合基序的性质及其在结演化过程中对分子电导和持久性特征的影响。低温条件下的测量技术通过拉伸和压缩的完整循环跟踪分子平台。分析表明，当金属电极推到一起时结的持久性与电极局部是尖锐还是钝有关，这表明了金属茂结形成和演化的不同情况。“桶”形金属茂的顶面和底面呈现出环戊二烯基环的富电子π体系，并以两种不同方式与金电极相互作用。尖锐尖端上配位不足的金原子与环中的特定碳原子形成供体-受体键。然而，钝尖端上的一个小而平的区域可以通过范德华相互作用与整个环更强地结合。基于密度泛函理论（DFT）的模型电极结构计算提供了这些情况的原子尺度图像，证明了这些键合基序在结演化过程中的作用，并表明电导相对独立于尖端原子尺度结构。环戊二烯基环与电极的非特异性相互作用使得能够形成扩展的电导平台，这是一种与更常研究的棒状有机分子导线所确定的机制不同的机制。
