EvanCurtin+Log

=EvanCurtin's Log=

Article Summary
Impact of Molecular Symmetry on Single-Molecule Conductance Emma J. Dell et al. [|DOI: 10.1021/ja4055367]

-Through both theory and experiment, the broad conductance distribution of bithiophene compared to a methyl sulfide substituded biphenyl was shown to be a consequence of decreased symmetry in the former case.
 * Abstract**

Body
-Characterizing single molecules used as junctions between metals is increasing in importance as interest in nanoscale electronics is increasing. -Conductance of such junctions is dependent on backbone conformation of the molecule, and for lower symmetry molecules, this is more dependent on binding to the metal on either side compared to high symmetry molecules. -The conductance of bithiophene and a biphenyl analog was measured experimentally, the bithiophene was synthesized and the biphenyl analog was purchased from Sigma Aldrich. -The conductance was measured using a scanning tunneling microscope break junction technique. -All individual conductance values for each molecule were compiled into logarithmically scaled histograms, the biphenyl shows a more narrow distribution than the bithiophene, which must result from the differences in the molecules themselves. -The rotation of biphenyl is unhindered by attachment to the gold at either end, while the rotation of bithiophene about its central bond requires either the gold to move or the molecule to strain. -This means that the conformers available to the bithiophene are more dependent on binding geometry to the gold. -A consequence of this is that the conductance of biphenyl shows little dependence on junction distance compared to the bithiophene. -To test this rotational barrier theory experimentally, two derivatives of bithiophene were synthesized; one restricted in a twist conformation and one in a flat conformation. -The conductance distribution of both derivatives is more narrow than bithiophene, and adding the two together is similar to the bithiophene distribution. -Torsional potential energy curves for the molecules tested were generated using an OPLS-SB-T force field. -This showed that the torsional rotations allowed for the biphenyl junction was unchanged by attachment to the gold at either end, while the bithiophene was more restricted in its rotations -This means that there is greater variability in a large sample of bithiophene junctions, giving rise to a broader conductance range. -This same effect was seen using room temperature molecular dynamics simulations.

Conclusion
-In summary, the decreased symmetry of bithiophene results in a broader range of conductance for gold-bithiophene-gold junctions, and this highlights the importance of symmetry of the molecular backbone for creating useful molecular electronics.

Resources for Paper
Initially started as a paper on electron transfer, then narrowed down the topic more specifically towards properties of metal-organic-metal junctions.

1) Marcus, R. A. “Electron transfer theory and its inception” Phys. Chem. Chem. Phys, 2012.[|DOI: 10.1039/c2cp90116a]

2) Libby, W.F. “Theory of Electron Exchange Reactions in Aqueous Solution” J. Phys Chem, 1952. [|DOI: 10.1021/j150499a010]

3) Marcus, R.A. “On the Theory of Oxidation‐Reduction Reactions Involving Electron Transfer. I” J. Chem. Physics. (24) 966-978. 1956. [|DOI: 10.1063/1.1742723]

4) Marcus, R.A. “On the Theory of Electrochemical and Chemical Electron Transfer Processes” Can. J. Chem. 1959. [|DOI: 10.1139/v59-022]

5) Masamitsu Tachibana, Kazunari Yoshizawa, Atsushi Ogawa, Hiroshi Fujimoto, and Roald Hoffmann. "Sulfur−Gold Orbital Interactions which Determine the Structure of Alkanethiolate/Au(111) Self-Assembled Monolayer Systems" J. Phys. Chem. B, 2002. [|DOI: 10.1021/jp020993i]

6) Solomon, G. C.; Herrmann, C.; Hansen, T.; Mujica, V.; Ratner, M. A. “Exploring local currents in molecular junctions” Nature Chemistry 2010. [|DOI:10.1038/nchem.546]

7) Zhifeng, Fang Chen, Peter A. Bennett, and Nongjian. "Single Molecule Junctions Formed via Au−Thiol Contact:  Stability and Breakdown Mechanism" J. Am. Chem. Soc. 2007. [|DOI: 10.1021/ja074456t]

8) Bingqian Xu, Xiaoyin Xiao, and Nongjian J. Tao. "Measurements of Single-Molecule Electromechanical Properties" J. Am. Chem. Soc., 2003. [|DOI: 10.1021/ja038949j]

9) Xiulan Li, Jin He, Joshua Hihath, Bingqian Xu, Stuart M. Lindsay, and Nongjian Tao. "Conductance of Single Alkanedithiols:  Conduction Mechanism and Effect of Molecule−Electrode Contacts" J. Am. Chem. Soc., 2006. [|DOI: 10.1021/ja057316x]

10) Ronald Hoffman. “Interaction of orbitals through space and through bonds.” Acc. Chem. Res. 1971. [|DOI: 10.1021/ar50037a001]

11) Dell, E. J. et al. “Impact of Molecular Symmetry on Single-Molecule Conductance.”J. Am. Chem. Soc. 2013. [|DOI: 10.1021/ja4055367]

12) Josh Vura-Weis, Mark A. Ratner and Michael R. Wasielewski. “Geometry and Electronic Coupling in Perylenediimide Stacks: Mapping Structure−Charge Transport Relationships” J. Am. Chem. Soc. 2010. [|DOI: 10.1021/ja907761e]

13) Thorsten Schepers, Josef Michl, “Optimized ladder C and ladder H models for sigma conjugation: chain segmentation in polysilanes.” J. Phys. Org. Chem. 2002. [|DOI: 10.1002/poc.527]

14) Li, Chen et al. “Charge Transport in Single Au|Alkanedithiol|Au Junctions: Coordination Geometries and Conformational Degrees of Freedom.” J. Am. Chem. Soc. 2008. [|DOI: 10.1021/ja0762386]

15) Bilić A, Reimers JR, Hush NS. “The structure, energetics, and nature of the chemical bonding of phenylthiol adsorbed on the Au(111) surface: implications for density-functional calculations of molecular-electronic conduction.” J. Chem. Phys. 2005. [|DOI:10.1063/1.1850455]

16)

17) Wenyong Wang, Takhee Lee, and Mark A. Reed. "Elastic and Inelastic Electron Tunneling in Alkane Self-Assembled Monolayers" J. Phys. Chem. B, 2004. [|DOI: 10.1021/jp048904k]

18) Roger L. York, Phuong T. Nguyen, and Krzysztof Slowinski. "Long-Range Electron Transfer through Monolayers and Bilayers of Alkanethiols in Electrochemically ControlledHg-Hg Tunneling Junctions" J. Am. Chem. Soc., 2003. [|DOI: 10.1021/ja0211353]

19) Clark, A. E. & Davidson, E. R. “Population analyses that utilize projection operators.” J. Quantum Chem. 2003. [|DOI: 10.1002/qua.10576]

20) Jordan R. Quinn, Frank W. Foss, Jr., Latha Venkataraman, Mark S. Hybertsen, and Ronald Breslow. “Single-Molecule Junction Conductance through Diaminoacenes” J. Am. Chem. Soc., 2007. [|DOI: 10.1021/ja0715804]

21) Franklin Anariba and Richard L. McCreery. "Electronic Conductance Behavior of Carbon-Based Molecular Junctions with Conjugated Structures" J. Phys. Chem. B, 2002. [|DOI: 10.1021/jp026285e]

22) Shan X, Patel U, Wang S, Iglesias R, Tao N.” Imaging local electrochemical current via surface plasmon resonance” // Science // . 2010 [|DOI: 10.1126/science.1186476]