This account reviews the current development of extraordinarily long oligothiophenes involving the 48-mer and 72-mer, which correspond to mono-disperse polymers. The systematic studies of the homologous series have provided valuable information on the optical and conductive properties of α-conjugated thiophene systems: an effective conjugation length extends to around 20 thiophene units in the neutral state and around 30 thiophene units in the oxidation state; this length plays a critical role in determining the electronic structures and conductivities. In addition, the active charge carrier species are both π-dimers and chain dimers, and charge transport occurs through the transfiguration of both species. This account also describes how useful oligothiophenes are as molecular wires allowing photoinduced electron- and/or energy-transfer. The molecular-wire behaviors of sophisticated oligothiophenes that are functionalized with a donor unit and an acceptor unit at the terminal α-positions are demonstrated.
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