Large Change of Electrical Conductivity and Absorption Spectrum of Poly(3-alkylthiophene) at the Solid-Liquid Phase Transition

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Abstract
Electrical conductivity of poly(3-alkylthiophene) such as poly(3-dodecylthiophene) and poly(3-docosylthiophene) decreases in a step-wise manner at the melting point. The absorption spectra also changes rapidly at the phase transition. These phenomena are discussed in terms of the increased energy band gap in the liquid state due to the decrease of the co-planarity of thiophene rings accompanied by remarkable conformation changes. Large hysteresis was also observed in the temperature dependence of conductivity and absorption spectra. Thermochromism in the premelting region can be explained by effective shortening of the conjugation length due to the introduction of torsion of the thiophene rings at several points on a conjugated chain.