Frenkel and charge transfer states of conjugated polymers and molecules

Abstract

Neutral excitations of conjugated polymers resemble molecular π-π ∗ spectra or Frenkel excitons in organic molecular crystals, while charge transfer (CT) between strands is related to ion pairs. The relative energies of Frenkel and CT states depend on Coulomb, polarization, and dispersion forces, as well as molecular and lattice relaxation. Polar substituents and redistribution of delocalized charge are molecular contributions that stabilize CT states. Their roles are illustrated by low-energy CT in PTCDA films and by stilbene dimers. Covalent bonding along the polymer backbone requires generalization of molecular exciton theory and generates extended states with different excitation thresholds indicative of Coulomb interactions. Expanded atomic sites are introduced through a second π-electron approximation for D 2h networks whose low-lying state are in an odd manifold. Expanded sites allow systematic comparisons of perylene and pyrene spectra with those of polyenes, indicate an even-parity exciton to be the lowest singlet of polyperinaphthalene, and represent a step towards molecular sites used in ion-radical salts with narrow bands. The energetics of Frenkel and CT states in organic molecular solids are closely related to excitons and polaron pairs in conjugated polymers, whose appropriate frontier orbitals and correlated excitations are currently sought.