(538b) Spectroelectrochemistry of Conjugated Radical Polymers

Conjugated radical polymers are an interesting marriage of a conjugated backbone with radical pendant groups. This brings new opportunities for organic electronics, energy storage, and thermoelectrics. The challenge is to understand how the singly occupied molecular orbital (SOMO) of the radical interacts with the properties of the conjugated backbone, as this can strongly affect the resultant conductivity, mobility and redox activity. Here, a series of polythiophenes with pendant TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) groups of varying alkyl spacer length are compared in the context of electrochemical energy storage. The conjugated radical polymers are examined as cathodes for nonaqueous batteries. Energy storage could potentially occur at both the radical group and the conjugated backbone. However, the overall capacity of the conjugated radical polymers is lower than expected, as is the conductivity. The explanation for this reveals an important design criteria for conjugated radical polymers in that the redox potential (or energy level) of the radical group and the conjugated backbone should be finely tuned so as to control electron exchange between the two moieties. This is further evidenced by in situ spectroelectrochemistry, in which the oxidation state of the polythiophene backbone is observed to change with time as electrons are transferred from one moiety to the other.