Where is the counterion located?

Organic electronics have emerged overtime as the leading candidate for future biosensing technologies due to the improved biological interface, flexible energy storage and capacity to conduct via ionic and electronic mechanisms to enhance the device sensitivity. A typical example of an organic material is a polymer. While most polymers are insulating, certain types of polymers can be doped to be made conductive. These polymers have conjugated backbones that creates an environment of delocalised electrons. In their neutral state they are insulating but once they are doped, an electron hole is created which makes them conducting.

When a conducting polymer is doped via chemical or electrochemical processes, electronic conduction is achieved as electrons are free to move along the backbone. However, usually these electrons do not move freely along the backbone but are localised to a few units due to the irregularity and distortion of the backbone.

While conducting polymers are very promising for new biotechnologies such as brain-interfaces or skin patches, the complexity of them opens a lot of fundamental questions. When a CP is doped, it becomes either positively or negatively charged, meaning that it requires a counterion (salt) to neutralise the charge. Thus a key question is where does the counterion (or dopant) reside?

Flagg et al., 2023 attempted to address this question by studying thin films of regioregular poly(3-{[2(2-methoxyethoxy)ethoxy]methyl}thiophene-2,5-diyl) (P3MEEMT). The group used grazing incidence resonant Xray scattering to probe the S-Cl interactions from doping the polythiophene backbone with ClO₄. They observed that the doping occurs far from the induced polaron on the backbone: It resides in the mid-plane of the side-chains.1

References [1] Flagg, L.Q., et al., Resonant X-ray Diffraction Reveals the Location of Counterions in Doped Organic Mixed Ionic Conductors. Chemistry of Materials, 2023.