Electronic and Ionic Conduction in Radical-Containing Small Molecules and Polymers

Optoelectronically active small molecules and polymers have made critical inroads in many solid-state electronic devices (e.g., organic field-effect transistors), and excitingly, organic molecules capable of providing mixed electronic and ionic conduction offer unique opportunities in many application areas including bioelectronic devices. Here, we describe electronic and ionic conduction in both low molecular weight and polymeric materials containing stable open-shell sites. While the chemical nature of the two classes of materials differs, the key premise is that controlling the spatial distance between the radical groups is critical for electronic transport and that the redox behavior of open-shell species allows for facile ion transport if it is controlled appropriately.<br/><br/>First, we demonstrate how control of the local structure in radical-containing small molecules and polymers impacts charge transport in these materials. In one example, we implement radical polymers, which are macromolecular materials that have flexible polymeric backbones and pendant groups that bear stable open-shell moieties. In contrast to almost all other optoelectronically-active polymers, radical polymers lack backbone conjugation and are completely amorphous in the solid state. Despite this shift in macromolecular design archetype, we demonstrate that the solid-state electrical conductivity of a designer radical polymer exceeds 20 S m^{&lt;span style="font-size:10.8333px"&gt;-1&lt;/span&gt;}, and this places this nonconjugated polymer conductor in the same regime as many grades of common commercially-available, chemically-doped conjugated conducting polymers. Thus, this work presents an alternate design paradigm for next-generation organic electronic materials. In a second example, small molecules that bear stable radical groups and form macroscopic single crystals are evaluated. Specifically, we quantified the electrical conductivity in two organic radical single crystals and demonstrated that a subtle change in atomic connectivity drastically alters the macroscopic electronic properties of the materials. One radical-based crystal had an electrical conductivity of ~3 S m^{&lt;span style="font-size:10.8333px"&gt;-1&lt;/span&gt;}, which is the highest value for nonconjugated radical conductors over a 1 μm-scale reported to date. However, the other radical crystal has a 1,000-fold lower conductivity despite their extremely similar molecular structures. The temperature-dependent conductivity follows the variable range hopping mechanism for both radical crystals, and the difference in effective charge mobility is the reason for the conductivity difference. These results present a clear picture of the design rules and charge transport mechanism for radical-based materials.<br/><br/>Next, we describe how a blend of a common conjugated polymer, poly(3-hexylthiophene) (P3HT), and a radical polymer (i.e., a macromolecule with a nonconjugated backbone and with stable open-shell sites on its pendant groups), poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) PTEO, allows for the creation of high-performance organic electrochemical transistors (OECTs) through a unique mixed conduction mechanism. Second, we will discuss how this type of archetype can be translated to biomedical device applications through a practical demonstration of a stretchable and flexible polymer-based biosensor. This bioelectronic device is inkjet-printed atop a commercial soft contact lens in a high-throughput manner, and our electroretinogram (ERG) sensor shows performance that is superior to current clinical gold standards in human subjects. In this way, we couple the materials science mixed organic electronic conductors to translational performance to demonstrate clear patient impact.