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Modeling the Effects of Torsional Disorder on the Spectra of Poly- and Oligo-(p-phenyleneethynylenes)

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journal contribution
posted on 28.09.2006, 00:00 by Lu Tian Liu, David Yaron, Mikhail I. Sluch, Mark A. Berg
The absorption spectra of phenyleneethynylene oligomers show an unusual change in shape with oligomer length. The unusual aspects of the spectra arise from rotation of the phenylene rings about the long axis of the oligomer. In the ground electronic state, the barrier to this rotation is low and the spectra in room temperature come from an ensemble of different structures. In the excited electronic state, the barrier to rotation is substantially higher, giving rise to strong nonlinear electron−phonon coupling. A multidimensional semiempirical model that includes these effects is developed for the photophysics of phenyleneethynylene oligomers. The ground-state energy is modeled with a molecular mechanics expression, and the excitation energy is modeled with an exciton model. Intermediate Neglect of Differential Overlap/Singles Configuration Interaction (INDO/SCI) calculations verify the exciton model and provide initial estimates of the model parameters. These parameters generate the qualitative features seen in experimental spectra. Inclusion of entropy effects from the multiple torsional coordinates is essential. Refinement of the parameters yields quantitative agreement with experiment. This agreement shows that coupling to torsional motion is a major factor in the spectroscopy and photophysics of these conjugated polymers.