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Realizing and Characterizing Bond-selective Chemistry in Single Functionalized Molecules

Nov 30, 2012

Chemical reactions involve the dissociation, rearrangement and formation of individual bonds. One of the key challenges in chemistry is to break and form bonds selectively in complex organic molecules that possess arrange of different functional groups. To do this at the single-molecule level not only provides an opportunity to create custom nanoscale devices, but offers opportunities for the in-depth study of how the molecular electronic structure changes in individual reactions.

Most of the studies on bond-selective chemistry to date addressed small molecules. However, the feasibility of initiating a bond-specific reaction within a complex molecule with functional groups is more relevant to molecular nanotechnology,in areas such as molecular electronics, organic solar cells and nanomachines. Furthermore, there are very limited experimental studies on changes in the molecular electronic structure associated with bond dissociation and formation, which provide crucial evidence for orbital hybridization in chemical transformations.

Prof. HUAN Qing from the Institute of Physics, Chinese Academy of Sciences (IOP), cooperating with Prof. JIANG Ying (ICQM, Peking University), Prof. Wilson Ho (University of California, Irvine), and Prof. Guillermo C. Bazan (University of California, Santa Barbara), realized target-selective bond dissociation and formation steps in a relatively large thiol-based π-conjugated molecule (1,4-bis[4’ -(acetylthio) styryl]benzene) adsorbed on a NiAl(110)surface. By injecting high confined and tunable hot electrons for resonant electronic excitation, they were able to abstract four different functional groups from the molecule step by step and monitor the evolution of the electronic structure in real space and in energy space.

Then, the researchers further manipulated and attached single gold atoms to the exposed sulfur atoms at the two ends of the molecule to form the Au-S bonds. They found that the molecular symmetry and Au-S bond geometry play a key role on the type of formed bonds: covalent or coordinate. The modifications of the molecular resonance on the Au–S bond formation, the directionality of the Au–S bond and the extent of the Au–S coupling were revealed and may clarify the long-standing discrepancy of the electron transport in thiol-based molecular junctions.

This work also opens up the possibility of measuring electron transport, controlled with atomic precision, through a single thiol-based molecule in an electrode–molecule contact geometry. The results published on Nature Chemistry [Nature Chem. DOI: 10.1038/NCHEM.1488 (2012)].

Figure1: Stepwise dissociation sequence of a straight DSB-2S-2Ac molecule. Left and middle-left columns: Schematics and topographies for different resultants at each dissociation step. Middle-right column: Corresponding dI/dV images of the resultants. Right column: dI/dV mappings taken at key locations (1, 2, 3, 4) on the molecules, as noted in the corresponding topographies (middle-left column). (Image by JIANG Ying et al ) 

Figure 2: Selective bond dissociation and formation in a single functionalized molecule. The electron beams emitted from the tip of a scanning tunneling microscope (STM) selectively cleave the sulfur-acetyl bond (lower-right), and weld the sulfur-gold bond (lower-left) in a 1,4-bis[4’-(acetylthio)styryl]benzene molecule. The background is the NiAl(110) substrate, on which the molecule is adsorbed.(Image by JIANG Ying et al )

Reference: http://www.nature.com/nchem/journal/vaop/ncurrent/full/nchem.1488.html

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