The spectral characterization of thiophene radical cation generated by pulse radiolysis

Pulse radiolysis matched with kinetic spectrometry has been employed to produce and characterize the radical cations of the series of unsubstituted oligothiophenes from one to six rings in dichloromethane dilute solutions. The concentration of radicals has been tuned to low values so as to slow down dimerization processes. The spectra obtained for the radical cations with 2 to 6 rings agree with previous literature reports. The novel isolation of the radical cation of thiophene itself and its spectral characterization, both experimental and theoretical, stresses its close relationship to the electronic structure of oligothiophenes. This fact strongly recommends that the charge carrier properties of oligothiophenes be interpreted on the basis of the molecular orbital theory rather than of the polaron model. The state responsible for the main UV absorption band is found to be described by a mixture of configurations where the HOMO→LUMO transition is present with different spin couplings.