Self-doping in the water-soluble conjugated polyelectrolytes PCPDTBT-2SO3K AND PCPDTBT-SO3K: a case of study

Conjugated polyelectrolytes (CPEs) are unique polymers as they contain a ?-conjugated backbone and pendant ionic functionalities, which make the polymer soluble in water and polar solvents1. CPEs combine the optical and charge transport properties of organic semiconductors with the possibility of modulating physical properties by electrostatic interactions. Furthermore, the solubility in water and in polar solvents makes CPEs appealing materials for a wide variety of applications going from sensing to organic electronics, etc1,2. Recently, the CPE poly[2,6-(4,4-bis-potassium butanylsulfonate-4H-cyclopenta-[2.1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiazole)] (PCPDTBT-2SO3K or CPE-2K, Figure 1) has become object of interest and study by the research community, as it has shown the ability to be self-doped during its purification by dialysis in water1. In particular, it was observed that it becomes doped in the presence of a proton source and is de-doped with the addition of a base1. The literature proposed self-doping mechanism is controversial and it is not completely clear how is the role played by the proton and the fuctional group of the anionic pendat1,3. Thus, starting from literature reported results, here we present a deep study of the di-alkylated CPE-2K in comparison with mono-alkylated CPE-K (Figure 1). Through combined spectroscopic (UV-vis-NIR, Raman) and electrochemical (cyclic voltammetry) analyses we aim to look into the doping mechanism trying to show how protons cause the organization of both polymers in aggregates, which then stabilize the polarons. A comparison with other CPE systems is also performed.