Effects of torsional disorder on poly-para-phenylene

We study the effect of thermal disorder on the electronic structure of one-dimensional poly-para-phenylene (PPP). In a real chain the torsion angles between rings are bound to be distributed over a range of values, which depend on temperature, and thus the chain is intrinsically disordered. In this study we simulated this kind of thermally induced off-diagonal disorder through the simple Hückel method. We base our Hamiltonian on ab initio results for the effect of temperature on torsion angles, and the effect of torsion angles on the energy gap. We analyze the electronic structure of 200-monomer-long chains focusing on the density of states, and the associated localization character (measured by the inverse participation ratio). Our results contrast with the usually assumed Gaussian-shaped density of localized states for disordered systems.