Thiocyanate monolayers and the synthesis and assembly of transition metal complexes
Ciszek, Jacob W.
Tour, James M.
Doctor of Philosophy
Herein is described the synthesis and assembly of compounds designed to advance the fields of surface science and molecular electronics. The first series of compounds are based on a phenanthrene core and are terminated with a thioacetate moiety which allows for assembly onto metallic surfaces. Eight polyaromatic compounds were synthesized to examine the role of internal rotation in electrical switching as observed by a scanning tunneling microscope (STM). It was thought that internal rotation, which disrupts the molecule's conjugation, was responsible for the switching behavior. One of these polyaromatic compounds (2-thioacetyl-phenanthrene) as well a biphenyl control (4-thioacetobiphenyl) were examined in a STM testbed. The polyaromatic compound (which cannot rotate) showed switching allowing internal rotation to be ruled out as a mechanism for switching as seen by STM. A second series of compounds, organic thiocyanates, were synthesized to examine their assembly onto metallic surfaces. It was found that this class of compounds assembles onto metallic surfaces under ambient conditions. The resultant monolayer contains the same thiolate-gold bond seen when assembling from the free thiol. The mechanism for this assembly involves surface-mediated conversion of the thiocyanate to a thiolate and M(CN)ads followed by expulsion of the cyanide from the surface as a metal salt ([M(CN) 2]- or [M(CN)4]2-). Comparable free thiol assembles are oxidatively unstable and are prone to disulfide formation. In the case of alpha,o-dithiols, polymerization is often seen. Thiocyanates are advantageous as they are stable and require neither exogenous material for the assembly nor oxygen free conditions. Finally, a series of transition metal complexes designed to assemble on gold surfaces was synthesized. This bis(2,5-di-pyridyl-3,4-dithiocyanto-pyrrolate) metal complex was prepared with zinc, copper, nickel, cobalt, and iron. The electronic structure and transitions of these compounds were studied by UV-vis and their magnetic properties studied by EPR and SQUID magnetometer. These molecules were then assembled onto a gold surface taking advantage of the above thiocyanate assembly protocol. The assembly process gives a loosely packed monolayer with no sign the compound had disproportionated or decomposed. This molecular assembly was used in single molecule transistors (SMTs) and allowed for examination of the Kondo effect.
Analytical chemistry; Inorganic chemistry; Organic chemistry