A Chemical Balancing Act: A Quinoidal Motif on the Edge of Stability and its Applications to Organic Electronics and Materials

The field of organic electronics is focused around the fabrication of electronic devices utilizing organic molecules as their active constituents. Organic electronics has existed as a subfield of applied organic synthesis since the 1970s and during this time, it has produced widely-applied conductive polymers, such as PEDOT:PSS, and semiconducting materials—as was prominently exemplified by the widespread adoption of organic light-emitting diodes (OLEDs). Beyond these successes, organic electronics produces a wealth of synthetic knowledge in the realm of conjugated organic molecules, which propels the forefront of technology in many surrounding fields. With these successes, organic electronics is now maturing into a field driven to find applications of conjugated organic molecules inaccessible to their inorganic counterparts, most typified by organic electrochemical transistors (OECTs).Progress in the field of organic electronics, as well as many of the surrounding fields, is driven by novel organic synthesis of conjugated systems. One of the most important features in the active materials of electronic devices is their band gap, which in organic molecules can be approximated by their HOMO-LUMO gap. The HOMO- LUMO gap can be reduced through a variety of strategies, one of which is the direct conjugation of aromatic and quinoidal units, which raises the ground state energy and lowers the energy levels of excited states. However, quinoidal conjugated systems frequently suffer from heightened reactivity, inhibiting their incorporation into useful organic materials. By carefully designing the synthesis of substituted quinoidal conjugated moieties though, the synthetic organic chemist can circumvent these issues. This was demonstrated clearly by the synthesis of the para-azaquinodimethane (AQM) motif. This building-block exhibits a central ring analogous to the highly reactive hydrocarbon para-quinodimethane, with a more controllable reactivity than its structural cousin. The AQM system can then be understood to be one of the smallest and simplest ambiently-stable quinoidal systems that can be incorporated into otherwise aromatic conjugated systems to produce small band gap organic semiconductors. A simple synthesis beginning from glycine anhydride produces the quinoidal AQM ring in its last step, and thus does not require the prolonged handling of any reactive quinodimethane intermediate. The AQM ring was originally generated in such a way that it could only be substituted with alkoxy groups. This synthesis was quickly used to produce mixed aromatic-quinoidal polymers that exhibited high charge carrier mobilities as active materials in organic field-effect transistors (OFETs). However, the limited world of synthetically-accesible AQM molecules was greatly expanded with the discovery of a synthetic route to an AQM system substituted with two trifluoromethanesulfonate (triflate) groups. This activated AQM intermediate could be functionalized via nucleophilic displacement and cross-coupling reactions. Additionally, the stability of the AQM ring itself was found to be greatly affected by its substituents, with some substitution patterns causing the AQM ring to rearrange, dimerize, or even polymerize. Displacement of the triflate groups on this activated AQM intermediate with neutral nucleophiles was found to yield dicationic AQM small molecules. These ionic AQMs (iAQMs) were stable enough to be functionalized via cross-coupling reactions and even incorporated into small band-gap conjugated polyelectrolytes (CPEs) with absorptions reaching into the infrared. The iAQM CPEs so produced were found to show notable photothermal properties, and were effective in killing the model bacterium staphylococcus aureus. While the AQM ring remained intact in these iAQMs, other transformations of the AQM ditriflate intermediate were found to result in rearrangements of the AQM ring. When the AQM ditriflate intermediate was subjected to Stille cross-coupling conditions, it appears that both triflate groups successfully react, but that the compound so produced is reactive enough to dimerize, forming a highly-substituted [2.2]paracyclophane derivative. This AQM dimer shows many of the intriguing properties of cyclophanes, including through-space interactions, but has a smaller band gap than most of the cyclophane literature. Depending on the substitution pattern of the AQM ring, the AQM ditriflate intermediates were themselves found to be reactive. A subfamily of AQM ditriflates with phenylidine substituents were found to undergo light- and heat-induced polymerization in the solid state. So-called single-crystal photopolymerization (SCP) is the only method capable of producing polymer single-crystals, and holds promise as a method to produce ultra-high molecular weight polymers via a solvent- and catalyst-free reaction. However, most of the products produced via SCP suffer from insolubility, and are restricted in terms of functional group tolerance. The AQM ditriflates found to undergo SCP provided a counterpoint to these two common problems, by incorporating solubilizing side-groups, as well as reactive pyrazine ditriflate units, which could be functionalized via nucleophilic substitution reactions after solid-state polymerization. Finally, when a similar series of phenylidine AQM ditriflates were converted into a redox-active pentacene-derivative dubbed PDIz. The PDIz family of compounds were found to have notably low oxidation potentials—as exemplified by their use in charge- transfer complexes with strong electron-acceptors. A PDIz derivative was also synthesized that allowed for the incorporation of this motif into very small band gap polymers (0.71 eV).

Advisor: Sarpong, Richmond