Charged states of thiophene oligomers

Summary form only given. Optical chemical doping and photoexcitation studies of thiophene oligomers were carried out. We observed a number of broad charge-induced infrared active vibrational modes and two associated electronic absorption peaks suggesting bipolaron formation durring chemical doping and photoexcitation. We found that the two subgap absorption peaks persist even in thiophene tetramers which involve only 4 thiophene rings. Since the upper bipolaron absorption peak is not symmetry allowed except for the case of extended polymer with disorder, one should expect to see only one absorption peak. In the case of polarons, one anticipates two absorptions with different absorption widths, which is not the present case. We discuss an alternative way of understanding the results in terms of molecular dynamics.