Giant infrared absorption bands of electrons and holes in conjugated molecules

Infrared (IR) absorption bands often convey identifying information about molecules, but are usually weak, having molar absorption coefficients <200 M −1 cm −1 . Here we report observation of radical anions and cations of conjugated oligomers and polymers of fluorene and thiophene that possess intense mid-infrared absorption coefficients as large as 50,000 M −1 cm −1 , perhaps the largest known for molecular species. For anions of fluorene oligomers, F n , n =2–4, IR intensities increase almost linearly with n , but with a slope much larger than one, indicating that the absorptions are not extensive properties. Large intensities seem to arise from a mechanism known for charged solitons and polarons in conjugated polymers. In this mechanism, vibrations of ungerade symmetry drive substantial displacements of charge, creating the large dipole derivatives responsible for intense IR absorption. Both experiments and calculations find that pairing with counter-ions attenuates IR band intensities. The IR bands may be diagnostic for bound ion pairs and their escape to form free ions.