Title: Modulating the Conducting Orbitals of Single Molecules Through Chemical Design
Description: The last two decades have seen huge improvements in measuring the conductance of single molecules, especially with the establishment of the scanning tunneling microscope break-junction (STM-BJ) method. The availability of such a robust and reliable measurement technique allows for the study of more exotic molecules with built-in functionality. In this thesis, we employ creative chemical design to manipulate transport in a single molecule junction by tuning the conducting frontier orbitals. We investigate three classes of materials – thiophene dioxides, mixed-valence bis(triarylamines), and benzotriazinyl-based Blatter radicals. Within each system, we probe changes in conducting behavior or interfacial interactions that arise from modifying the molecular structure. First, we demonstrate that a family of thiophene pentamers, which typically conduct through their highest occupied molecular orbital (HOMO), can be induced to conduct through their lowest unoccupied molecular orbital (LUMO) instead.

This is akin to switching between from hole to electron transport. The switching was achieved using chemical modifications that drastically lower the LUMO level toward the Fermi energy of gold: oxidation at the sulfur position to form thiophene dioxides combined with installing electron-withdrawing groups at the 3- and 4-positions of the thiophene moiety. The ability to tune HOMO versus LUMO transport is potentially useful for assembling molecular circuits with n- and p-type components. Next, we found that oxidation of bis(triarylamine) molecular wires into their mixed-valence state shifts their conducting orbitals close to the Fermi energy of gold, making these wires highly conducting. We measured the length dependent transport of three bis(triarylamine) molecules. In their neutral state, the conductance of these compounds decreases with increasing length, which is observed for many different systems. However, when they are chemically oxidized, the mixed-valence molecular wires show an increase in conductance with increasing length.

Such wires that maintain good electrical transport over long distances are valuable for building efficient molecular devices. We then investigated the interaction of half-filled orbitals in organic radicals with gold substrates to explore the potential of these compounds for spintronic and magnetic applications. We found that a Blatter radical functionalized with gold-binding thiomethyl groups displays different charge transfer behavior depending on the environment. Under ultra-high vacuum, X-ray spectroscopy shows that the radical molecules in contact with the gold substrate gain a charge from gold and their singly unoccupied molecular orbitals get filled. Contrastingly, in solution-based single molecule measurements, the radical loses the electron from its singly occupied molecular orbital instead, and only the conductance of the oxidized species is detected. We further probed the nature of charge transfer between the Blatter radical and gold in ultra-high vacuum by comparing spectroscopic measurements from three different derivatives.

The derivative that was functionalized with two thiomethyl groups in order for it to be measured in the STM-BJ was the only molecule to undergo charge transfer in ultra-high vacuum. Two other Blatter derivatives that had only one and no thiomethyl groups did not show the same charge transfer; these retained their radical character even when in contact with the gold substrate. Therefore, the results indicate that only one of the thiomethyl groups mediates charge transfer between radical and substrate. The body of work presented herein shows that chemical modifications to old and new systems can be used to modulate transport in junctions via the intrinsic character of the molecules rather than external engineering factors. Thiophene dioxides are a relatively nascent class of materials that already show versatility as molecular conductors, while organic mixed-valence and radical systems have been heavily researched in other fields but less so in molecular electronics. This thesis therefore seeks to encourage further research that takes advantage of the unique electronic structure of these materials systems to discover new transport phenomena.