Solid-State Synthesis of a Conducting Polythiophene via an Unprecedented Heterocyclic Coupling Reaction

Prolonged storage (∼2 years) or gentle heating (50−80 °C) of crystalline 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) affords a highly conducting, bromine-doped poly(3,4-ethylenedioxythiophene) (PEDOT), as confirmed by solid-state NMR, FTIR, CV, and vis−NIR spectroscopies. The novel
solid-state polymerization (SSP) does not occur for 2,5-dichloro-3,4-ethylenedioxythiophene (DCEDOT),
and requires a much higher temperature (>130 °C) for 2,5-diiodo-3,4-ethylenedioxythiophene (DIEDOT).
X-ray structural analysis of the above dihalothiophenes reveals short Hal···Hal distances between adjacent
molecules in DBEDOT and DIEDOT, but not in DCEDOT. The polymerization may also occur in the melt
but is significantly slower and leads to poorly conductive material. Detailed studies of the reaction were
performed using ESR, DSC, microscopy, and gravimetric analyses. SSP starts on crystal defect sites; it is
exothermic by 14 kcal/mol and requires activation energy of ∼26 kcal/mol (for DBEDOT). The temperature
dependence of the conductivity of SSP-PEDOT (σrt = 20−80 S/cm) reveals a slight thermal activation. It
can be further increased by a factor of 2 by doping with iodine. Using this approach, thin films of PEDOT
with conductivity as high as 20 S/cm were fabricated on insulating flexible plastic surfaces.