The effect of chain length on the dielectric and optical properties of oligothiophenes

The oligothiophene compounds T₃-T₇ inclusive have been synthesized by standard chemical procedures in order to measure the complex admittance versus frequency of their micro-crystalline powders as compressed into a co-axial tube capacitor. The results are compared to that obtained from commercially available polythiophene powder. Optical measurements of the UV-visible absorption and luminescence (fluorescence) spectra were also obtained for these compounds suspended in solutions of dimethylsulfoxide and dichloromethane as well as for compressed solid powders. The impedance spectra show a typical Jonscher model variation with a large jump in conductivity between T₄ and T₅ followed by more gradual increases. A semi-log plot of conductivity or susceptibility versus chain length at fixed frequency shows that polythiophene fits in at T₉ or T₁₀. The photoluminescence peaks expressed in energy units (eV) show a linear increase with inverse chain length and in this case polythiophene fits in at T₆. The uniform increase of the absorption maximum or a photoluminescence peak with the inverse chain length of successive oligothiophenes can also be used to calculate the effective "chain length" or conjugation of the end substitution of various radical groups to these same oligothiophenes. This has been done using results from previous work.