The role of water in the electrodeposition and doping of polythiophene and two of its derivatives

Abstract

Abstract The galvanostatic electropolymerization of thiophene ( 1 ), 3‐methylthiophene ( 2 ) and 2,2′‐bithiophene ( 3 ) in acetonitrile, containing 0,002 to 20 mol/L water, was studied in detail. The current efficiency γ for the polymer film formation decreased rapidly for 1 , but much slower for 2 and 3 with increasing water concentration c . For c = 0,1 mol/L, γ was found to be 10, 82 and 95% for 1 – 3 . It was possible to describe the experimental γ vs. c plots quantitatively by a model where the electrogenerated radical cations (rc) of the thiophene (derivative) are consumed by two competing second‐order follow up reactions, the nucleophilic attack of the rc by water (i) and the rate‐determining dimerization of two rc's (ii). Their rates are given by: (i) v 1 = k 1 c rc c and (ii) v 2 = k 2 c 2 rc . The side reaction (i) leads to soluble products, while reaction (ii) contributes to the polymer film growth. The ratio k 1 / k 2 was found to be 3 · 10 −7 , 2 · 10 −8 and 4 · 10 −9 for 1, 2 and 3 , respectively, and this shows that the electropolymerization of 2,2′‐bithiophene is by a factor of about 100 less sensitive toward the present water than thiophene. The steady state concentration c rc is in the order of 10 −9 mol/L in all three cases. The relative insensitivity of doping/undoping cycles with respect to the presence of water is discussed in terms of a lessened reactivity of the polarons and a low solubility of water in the heteroaromatic polymer.