(129h) Counter-Ion Condensation on Doped ?-Conjugated Polymer Chains

Currently, there is great interest in doped π-conjugated polymer systems that exhibit both ionic and electronic conduction. Such polymeric mixed conductors have a host of applications in sensors, electrochromic devices, printable batteries, and electrochemical transistors. These systems exhibit electrostatic correlations between mobile ions and the mobile electronic charges in the π-conjugated backbone that dramatically impact their solid-state conductivities. These correlations depend on the polymer doping level and film microstructure, but their impact on electronic and ionic transport is not well established. In previous work, we have observed that the electronic conductance of electrochemically-doped polythiophene films exhibits a pronounced peak that depends on the choice of electrolyte infiltrating the film. We hypothesize that the collapse of conductivity at high doping levels is due to strong hole-anion interaction along the polymer chains.

Here, we apply Manning’s “counterion condensation theory” to characterize the molecular-level interactions in dilute liquid solutions between charge carriers on polythiophene derivatives and charge-compensating counterions. This theory offers a framework for understanding the degree of counterion-charge carrier interaction in terms of the Bjerrum length, i.e., the characteristic distance at which electrostatic energy is comparable in magnitude to the thermal energy. While this theory has been successfully used to interpret experimental data for non-conjugated polyelectrolytes, it has yet to be applied to systems with delocalized charge carriers in π-conjugated polymers. We have characterized variably doped poly(3-oligoethoxythiophenes) in exogenous electrolyte solutions using refractometry, NMR spectroscopy, and isothermal titration calorimetry. These efforts furnish fundamental insights into the charge carrier-counterion correlations that limit electronic conduction in doped polymer systems, which may lead to new molecular designs for improved mixed conductor performance.