Increased Electrical Conductivity in Poly(3,4-ethylenedioxythiophene) upon Cross-Linking

We show here that the electrical conductivity can be increased by up to 36% through introduction of crosslinks between the PEDOT chains. Cross-linked PEDOT films were synthesized by an oxidative polymerization of 3,4-ethylenedioxythiophene in the presence of four different conjugated cross-linkers. Experimental Materials. All reagents were dried in vacuum for 24 h before use. 1,4diethynyl-2,5-bis-(octyloxy)benzene was synthesized as described before. 8 3,4-Ethylenedioxythiophene (EDOT), ethanol, n-propanol, 3-iodothiophene, 3-thiophenecarboxaldehyde, potassium tert-butoxide in tetrahydofuran (1M), 1,4-bis(octyloxy)-2,5-xylenebis(triphenylphosphonium bromide), iron(III) ptoluenesulfonate hexahydrate , imidazole, 3,3' bisthiophene, and 3,2':5',3"terthiophene were of highest available purity and were purchased from Sigma Aldrich. Toluene and diisopropylamine were distilled over CaH2 under argon. Instrumentation. 1 H-NMR spectra were obtained on a Varian 600 MHz spectrometer.Resistance measurements were taken using the 4 point method on a Keithley 2410 1100V Source Meter. Thickness measurements were made on a Dektak IID Profilometer. UV-Vis absorption spectra were collected on a Perkin Elmer Lambda 800. Synthesis of Cross-Linker X3. Glassware and stir bar were dried at 120-130°C for 24 h. 1,4-diethynyl-2,5-bis-(octyloxy)benzene (0.5 g, 0.131 mmol), Pd(PPh3)4 (0.038 g, 0.0464 mmol), CuI (0.009 g, 0.0464 mmol) and toluene (4.5 mL) were mixed under inert conditions. 3-Iodothiophene (0.270 g, 1.28 mmol) and (iPr)2NH (2.5 mL) were added and the reaction mixture was heated to 65°C under argon. After 5 h the mixture was cooled to RT, precipitated in methanol, and the precipitate was filtered off, washed with methanol and dried under vacuum to yield 55 mg of green-brown crystals. The product was characterized by 1 H-NMR: δ = 7.50 (q), 7.29 (q), 7.19 (q), 6.98 (s), 4.00 (t), 1.82 (m), 1.51 (m), 1.36 (m), 1.28 (m), 0.86 (t). Synthesis of Cross-Linker X4. Glassware and stir bar were dried at 120-130°C for 24 h. 1,4-Bis(octyloxy)-2,5-xylenebis(triphenylphosphonium bromide) (0.261 g, 0.25 mmol) was added to ethanol (20 mL) and allowed to dissolve. Potassium tert-butoxide in THF (1M, 0.75 mL) was added slowly over 5 min. 3-Iodothiophene (0.062 g, 0.55 mmol) was added dropwise over 15 min and the mixture was left to react at room temperature. 5 mL of water was added after 5 h and the mixture was stirred for 30 min. The resulting precipitate was filtered off and was recrystalized from ethyl acetate twice to yield 20 mg of a yellow-green solid. The product was characterized by 1 HNMR: δ = 7.49 (m), 7.39 (q), 7.35 (q), 7.30 (m), 7.21 (m), 7.16 (m), 7.10 (s), 4.05 (t), 1.87 (m), 1.54 (m), 1.40 (m), 1.29 (m), 0.88 (m). Typical polymerization procedure for the synthesis of cross-linked PEDOT. EDOT was purified by vacuum distillation. Iron(III) p-toluenesulfonate hexahydrate (0.61 g, 0.896 mmol) and n-propanol (0.250 g) were added to one vial. Imidazole (0.070 g, 1.02 mmol), EDOT, cross-linker, and n-propanol (0.250 g) were added to a separate vial. The combined concentration of EDOT and the cross-linker was kept constant at 2.05 mmol. Both vials were heated to dissolve the reactants and then allowed to cool to room temperature. After cooling, the contents were combined and the mixture