The effect of end group substitution on the electrical conductivity of mesoporous terthiophene

A number of terthiophene derivatives have been synthesized by standard chemical procedures in order to measure the electrical properties of their micro-crystalline powders by impedance spectroscopy, in this case the complex admittance versus frequency. The compounds studied were T3, BrT3, Br2T3, T3NO2, BrT3NO2, IT3NO2, and Br-terminated polythiophene. The substituents were all α-position or α,α′-position terminations. The sample powders were placed between a co-axial tube capacitor arrangement and connected to a standard impedance bridge with a frequency range of 20 Hz to 1 MHz. The conductivity showed a simple power law dependence on the frequency with the exponent giving a good indication of the change in conjugation length due to the substituted end group. The push–pull nature of BrT3NO2 and IT3NO2 were clearly seen by the enhancement of both conductivity and susceptibility. The polythiophene sample was significantly more conducting and had a much higher susceptibility. It was observed to undergo a color change, from red to silver, when subjected to shear stress. This was accompanied by an increased conductivity most likely due to an enhancement of π-bond stacking between molecules. The technique presented is a useful and quick way to analyze the potential of new organic semiconductors.