Nature of a High- Conductivity State of Poly(diphenylene phthalide)

摘要

For thin films prepared from non-conjugated polymer, including poly(diphenylene phthalide)(PDP), a rather enigmatic transition to a metal-like high-conductivity state is observed. The existing modelsof the HCS in wide-band polymers were examined. The main feature of these models is a significant (oftenhighly improbable) rearrangement of the electronic structure of part of the macromolecules under the actionof external factors (an electric field, light, injected electrons, etc.). The model of the HCS in PDP proposedin the current work is based on the results of studying the electron-acceptor properties of fragments of PDPmacromolecules and the concepts of molecular electronics. Electron-acceptor unoccupied orbitals of thefragments of macromolecules and the corresponding energy levels are assumed to act as transport nodes forthe resonance coherent tunneling of electrons. Since PDP has no intrinsic charge carriers, the key role in theformation of the HCS is played by the injection of electrons from the electrodes and, hence, the processesoccurring in the interface. The resonance injection of electrons from the Fermi level of the electrode imme-diately onto the transport levels of the polymer is achieved due to the applied electric field, chemical connec-tion of macromolecules to the electrodes, and ensuing modification of the interface dipole. The connectionof PDP macromolecules to the electrodes occurs via defective hydroxyl and carboxyl groups. Carboxylateions, which interact with the electrode metal, can also be formed because of the dissociative capture ofinjected electrons by phthalide cycles adjacent to the electrode. Since a conducting channel in a 1-μm-thickfilm should be composed of at least ten PDP macromolecules, it is necessary to assume that an efficient elec-tron transfer occurs between macromolecules. The proposed concept was additionally substantiated byinvoking the published data on the conductivity of DNA molecules-and the field emission of electrons frommetals coated with a PDP film.