On the self-doping of poly[n-(3′-thienyl)alkanesulfonic acid] and poly[(3-carboxyalkyl)thiophene]

Poly[n-(3′-thienyl)alkanesulfonic acid] is already self-doped without applying any electric potential. Such doping is different from that introduced by electrochemical doping of this polymer acid or its salt, and is termed 'self-acid-doping' in this work. Its structure is proposed to be composed of four basic units: an undissociated unit, a ring with an electron/hole pair and -(CH₂)_nSO₃^- side chain, a ring with a hole and addition of proton from a side chain on the α-carbon, and a bipolaron structure, as determined by studying the simulated system: polythiophene doped with sulfuric acid. Poly[(3-carboxyalkyl)thiophene] cannot be self-acid-doped or self-doped, as evidenced by the facts that 1. (i) poly(3-octylthiophene) (P3OT) cannot be electrochemically doped with alkylcarboxylate or arylcarboxylate as counter-anion, and 2. (ii) P3OT cannot be chemically doped with alkylcarboxylic or arylcarboxylic acids.