Doping or Aggregation: the case of Conjugated Polyelectrolytes PCPDTBT-2SO3K and PCPDTBT-SO3K

Conjugated polyelectrolytes (CPEs) are defined by a backbone that contains a ?-conjugated electronic structure with pendant ionic functionalities, which make the polymer soluble in water and in 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 both as optical reporters in bio-sensors and bio-imaging applications and as charge regulating layers in organic solar cells2, organic light-emitting diodes and organic thin-films transistors. 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) has become object of interest because is soluble in water and become doped in presence of a proton source3. In fact, it has been reported that the dopant is H+ and that the addition of sodium hydroxide to CPE-2K solutions reverses the process and de-dopes the polymer. The proposed "doping" mechanism consists in an initial double protonation of the polymer backbone, followed by comproportionation with a non-protonated chain, ultimately leading both to the formation of polarons (radical cations), which are stabilized by the pendant sulphonate groups4. Starting from these considerations, here we present a deep study of the di-alkylated CPE-2K in comparison with mono-alkylated CPE-K (Scheme 1). Through combined spectroscopic (UV-vis-NIR) 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.