Effects of the Environment on Charge Transport in Molecular Wires

Supramolecular engineering offers opportunities for creating polymer-based materials with tailored conductive properties. However, this requires an understanding of intermolecular interaction effects on intramolecular charge transport. We present a study of hole transport along molecular wires consisting of fluorene-p-biphenyl or Zn-porphyrin monomer units, in dilute solutions. The intramolecular hole mobility was studied by pulse radiolysis-time-resolved microwave conductivity. Experiments were supplemented by charge transport simulations employing a quantum-mechanical description of the hole and a classical description of the polymer and solvent dynamics. The model was parametrized using ab initio and molecular dynamics calculations. It was found that the solvent-induced energy disorder along a polymer chain in common solvents (benzene, cyclohexane, acetonitrile, water) is 1 eV, significantly greater than the values of 0.05-0.2 eV commonly cited in the literature. Environment-induced disorder of this magnitude has profound consequences for intramolecular charge transport. The hole initial state upon injection onto a molecular wire also influences the mobility. Experiments and simulations demonstrate that supramolecular modification of polymers (coordination, rotaxination) can significantly enhance or suppress charge transport. Incorporating a molecular level description of the immediate supramolecular and solvent environment into charge transport models improves their predictive potential, providing a valuable tool for material design. © 2012 American Chemical Society