π-Electronic structure of conjugated polymers
The novel electronic properties of conjugated polymers are associated with π-electrons whose early modeling by Huckel has been extended by Pariser, Parr and Pople (PPP) to include electron-electron (e-e) interactions and by Su, Schrieffer and Heeger (SSH) to include linear electron-phonon (e-p) coupling. Charged polymeric excited states are localized by e-p coupling, lead to excitations below the optical gap, and are coupled to backbone vibrations. Neutral polymeric excited states resemble those in conjugated molecules and clearly demonstrate e-e interactions. We introduce here inductive, or site energy, contributions to the optical gap of polythiophene (PT) and polydiacetylenes (PDAs) whose conjugated network differs from polyacetylene (PA) in having chemically inequivalent carbons. In addition, site energies model PA-dopant interactions and enhanced localization of charged PT and PDA excitations due to polarizable side groups. Such extensions of π-electron theory are attractive for modeling wider ranging phenomena of increasingly diverse conjugated polymers. Nonlinear susceptibilities are another interesting feature of π-electrons in conjugated polymers. Diagrammatic valence-bond (DVB) methods are used to bypass sums over excited π-electron states and give the exact dynamic nonlinear susceptibilities of interacting π-electrons in finite PPP or Hubbard fragments.