Preparation and electrical conductivities of oligothiophene radical-cation salts

In order to elucidate the relationship between the conjugation length of oligothiophenes and their electrical properties, chemical oxidation of α-ethyl-disubstituted oligothiophene, e.g., 5,5“-diethyl-2,2′:5′,2′ʹ-terthiophene, 5,5″ʹ-diethyl-2,2′:5′,2′ʹ:5′ʹ, 2′"-quaterthiophene, and 5,5tʹ"ʹ-diethyl-2, 2′:5′,2′ʹ:5′ʹ,2′":5′",2′"ʹ-quinquethiophene, with iodine has been investigated. The resulting solids were identified as partially oxidized radical-cation salts containing polyiodide ions as counter anions. The electrical conductivity of the iodine-oxidized oligothiophenes was found to increase with increasing conjugation length; the thiophene pentamer exhibited a room-temperature conductivity of 4 × 10−4 S cm−1