When tunneling is the dominant mechanism of charge transport in a molecular junction, the conductivity of the junction is largely insensitive to chemical and structural perturbations which do not impact the overall length of the junction. This severely hampers the seemingly limitless potential of molecules to modulate charge transport at interfaces and their application in a host of device designs. This is a particular challenge for molecules baring insulating features like saturated hydrocarbons which decouple functional groups from the surface. Such decoupling groups increase the energy required to isolate charge on the molecule, pushing transport into the tunneling regime in many cases. Herein, we demonstrate that, through enhancement of nearest neighbor interactions, lateral delocalization of charge states in molecular islands can be used to shift transport out of the tunneling regime to the more efficient, and more chemically tunable, charge-hopping regime. In a previous study, it was found that through-bond tunneling was the dominant mechanism of charge transport through a hydrocarbon-tethered free-base porphyrin thiol. With coordination of zinc(II), the formation of large molecular islands in an alkanethiol matrix on a Au(111) surface was facilitated. Bias-induced switching and unphysical tunneling efficiencies observed by scanning tunneling microscopy of these molecular islands, as well as Coulomb blockade observed in low-temperature crossed-wire tunnel junction measurements, indicate charge hopping becomes the dominant mechanism of transport in the molecular islands, whereas transport in single molecules was consistent with through-bond tunneling. These results elucidate the basis for functional conductivity-structure and supramolecular relationships that may be employed in the design of molecular junctions in organic thin films.

A series of experiments employing scanning tunneling microscopy (STM) have been developed for the physical chemistry laboratory. These experiments are designed to engage students in cutting edge research techniques while introducing and reinforcing topics in physical chemistry, quantum mechanics, solid-state chemistry, and the electronic structure of molecules and materials. In the first of three experiments, students are introduced to the basics of STM operation while imaging and conducting spectroscopy on the highly oriented pyrolytic graphite (HOPG) surface. Images of the surface are used to determine the crystal structure of the material, and scanning tunneling spectroscopy is used to determine the electronic properties of the material and study the tunneling phenomenon. In the second experiment, the students image the Au(111) surface as well as a series of alkanethiol self-assembled monolayers (SAMs) of different chains lengths on the Au(111) surface. They examine the structural and electronic properties of the metal surface and the adlattice structure of the film. Finally, in the third experiment, the students examine the conductance of molecules adsorbed onto the Au(111) surface, including the alkanethiol SAMs and a thiol-tethered porphyrin molecule or a dimercaptostilbene embedded into the SAM matrix. By measuring the tunneling efficiency and spectroscopic characteristics of these molecules, the students can explore the relationship between chemical structure and charge transport efficiency. The experiments provide advanced chemistry students an opportunity to view and study materials at the atomic and molecular length scales and provide an opportunity to apply their understanding of quantum mechanical concepts to real systems.

The proposal that molecules can perform electronic functions in devices such as diodes, rectifiers, wires and capacitors, or serve as functional materials for electronic or magnetic memory, has stimulated intense research across physics, chemistry, and engineering for over 35 years. Because biology uses porphyrins and metalloporphyrins as catalysts, small molecule transporters, electrical conduits, and energy transducers in photosynthesis, porphyrins are an obvious class of molecules to investigate for molecular electronic functions. Of the numerous kinds of molecules under investigation for molecular electronics applications, porphyrins and their related macrocycles are of particular interest because they are robust and their electronic properties can be tuned by chelation of a metal ion and substitution on the macrocycle. The other porphyrinoids have equally variable and adjustable photophysical properties, thus photonic applications are potentiated. At least in the near term, realistic architectures for molecular electronics will require self-organization or nanoprinting on surfaces. This review concentrates on self-organized porphyrinoids as components of working electronic devices on electronically active substrates with particular emphasis on the effect of surface, molecular design, molecular orientation and matrix on the detailed electronic properties of single molecules.

Porphyrins are actively studied for use in molecular and organic electronic components of devices because of their diverse tunable optical and electronic properties. In this study, mixed self-assembled monolayers (SAMs) of dodecanethiol and a tripyridyl porphyrin attached to a thiol tether via a perfluorinated phenyl ring (TPy3PF4-SC5SH) were prepared on Au(111) substrates. The synthetic strategy allows for rapid formation of derivatives with different tethers. The surface structural and electronic properties of mixed monolayer SAMs of the porphyrin inserted into the dodecanethiol matrix were investigated using scanning tunneling microscopy (STM), atomic force microscopy (AFM), Fourier transform infrared reflection-absorption spectroscopy (FT-IRAS), and X-ray photoelectron spectroscopy (XPS). Density functional theory (DFT) calculations were also employed to evaluate the analytical vibrational frequencies of the TPy3PF4-SC5SH molecule as well as its electronic structure. For the mixed monolayers, the morphology of the porphyrin molecules was probed by STM where it was found that the molecules assembled into domains of similar to 2 and 6 nm. AFM shows that the molecules protrude above the n-dodecanethiol layer by similar to 0.9 nm, while by STM, apparent heights of only similar to 0.5 nm were observed, suggesting limited tunneling efficiency. Stochastic switching of the porphyrin molecules was also observed during STM measurements in the mixed monolayer and is likely associated with conformational changes within the monolayer since these molecules tended to insert near defects within the SAM.