Synthetic Metals, 62 (1994) 245-252 245 ESR and optical spectroscopy evidence for a chain-length dependence of the charged states of thiophene oligomers. Extrapolation to polythiophene Gilles Horowitz and Abderrahim Yassar Laboratoire des Matdriaux Mol~culaires, CNRS, 2 rue Henry-Dunant, F-94320 Thiais (France) H.J. von Bardeleben Groupe de Physique des Solides, Universitd Paris 7, 2 place Jussieu, F-75251 Paris Cedex 05 (France) (Received June 23, 1993; in revised form September 28, 1993; accepted October 14, 1993) Abstract 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 ~< 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+ l thiophene rings, corresponds to a chain long enough to bear two independent polarons. Introduction The electrical charge carrier generation in ~--con- jugated oligomers and polymers is still the subject of active research, from both a theoretical and experi- mental point of view. According to the most widely accepted model for systems with a non-degenerate ground state (i.e., practically all the conjugated polymers except the special case of trans-polyacetylene) charges in a conjugated polymer chain form self-localized po- larons, which under some circumstances couple to form more stable bipolarons. Each of these defects manifests itself by quantum states located deep in the semicon- ductor gap. Accordingly, the charging of a conjugated chain is associated with a dramatic change of its optical absorption spectrum. Furthermore, the polaron defect having a spin S=½ also gives rise to an electron spin resonance (ESR) spectrum, whereas the bipolaron has a diamagnetic ground state. Polarons and bipolarons are theoretically predicted by modelling the conjugated polymers as one-dimen- sional coupled electron-phonon systems, as described by the Su-Schrieffer-Heeger (SSH) Hamiltonian [1]. This Hamiltonian can be analytically resolved in its continuum limit, the validity of which is restricted to long chains where charged defects extend over several lattice spacings. This limit has been used by Fesser et al. [2] (FBC model) to calculate the optical absorption of polarons and bipolarons in cL~-polyacetylene. Their results can be extended to the other non-degenerated ground-state conjugated polymers and have been widely used to rationalize the experimental absorption spectra of doped conjugated polymers. The formation of a charged defect in a conjugated chain is accompanied by a change of the bond alternation over several carbon atoms. Figure 1 shows the molecular diagrams of a positively charged polaron and bipolaron on a poly- thiophene (PT) chain, together with the associated energy-level schemes. The transition N1 of the neutral state corresponds to the energy gap Eg. It can be seen that, as a consequence of state occupancy, the polaron presents four optical transitions, which, because the levels are symmetric around the gap centre, reduce to three (labelled P1, P2, P3 and P4 in Fig. 1, with P3 = P4), whereas only two transitions are expected for the bi- polaron (BP1 and BP2). The bipolaron is predicted to 0379-6779/94/$07.00 © 1994 Elsevier Sequoia. All rights reserved