Spectroscopic studies of polaronic and bipolaronic species in n-doped poly(paraphenylenevinylene)

In this paper, we present a detailed study of the vibrational properties of neutral and n-doped polyparaphenylene-vinylene (PPV). Resonance Raman scattering and infrared-absorption results are analyzed and interpreted by the use of dynamical calculations based on a valence force-field model. Experimental data include those obtained from related oligomers, i.e., trans-stilbene and distyrylbenzene with the assumption of similar geometry parameters. Force constants are optimized according to assignments of Raman and ir vibrational bands of the series of compounds. In the case of sodium-doped PPV, new Raman bands are observed showing different resonance behaviors. These new features are investigated as a function of the doping level, leading to the conclusion that polarons and bipolarons of short lengths coexist in thermodynamical equilibrium. Doping-induced bands are interpreted in terms of changes in carbon-carbon force constants. Such modifications in the electronic distribution agree well with the quinoid character expected upon doping on the polymeric chains.