ESR AND OPTICAL SPECTROSCOPY EVIDENCE FOR A CHAIN-LENGTH DEPENDENCE OF THE CHARGED STATES OF THIOPHENE OLIGOMERS - EXTRAPOLATION TO POLYTHIOPHENE

Correlated electron spin resonance (ESR) and optical absorption spectroscopy measurements have been carried out on soluble thiophene oligomers (nT, n standing for the number of rings) of ring size n = 6 and n = 12. Chemical oxidation leads to the formation of singly and doubly charged molecules. On 6T, the paramagnetic radical-cation 6T.+ and diamagnetic dication 6T++ are successively generated, whereas the dication (bipolaron) 12T++ of the twice longer molecule 12T is directly produced. Both the radical cation and dication show two optical absorption bands, which we have used to estimate the bandgap of the corresponding species. This latter is shown to be a pertinent parameter for studying the chain-length dependence of the electronic properties of charged oligomers. From a comparison of our results with previously published optical absorption data on thiophene oligomers and polymers, we find that short (n less-than-or-equal-to 8) oligomers are better described in terms of molecular orbitals, whereas the one-electron model of conjugated polymers is more appropriate for longer chains (n = 12 and the polymer). The transition between short and long oligomers, which occurs at n = 10 +/- 1 thiophene rings, corresponds to a chain long enough to bear two independent polarons.