The diagrammatic exciton basis valence-bond approach, developed previously by us for obtaining physical, pictorial descriptions of correlated eigenstates of linear polyenes, is extended to the case of polyphenylenes. In linear chain polyenelike systems with large effective bond alternation, low-energy even-parity states are linear combinations of one-excitations that are charge transfer within the exciton representation and two-excitations that are composed of two triplets, which are localized on two different unit cells, and the spin angular momenta of which are coupled to give an overall singlet. This description of even-parity states gets modified substantially in the polyphenylenes, where the unit cell contains molecular orbitals that are delocalized over the entire unit cell as well as localized molecular orbitals with nonzero electron densities on only four of the six carbon atoms of each benzene molecule. We show that, as a consequence, there occur multiple classes of triplet-triplet two-excitations in the polyphenylenes, involving both the delocalized and localized molecular orbitals. We further show that all triplet-triplet two-excitations mix with the one-excitation charge-transfer type configurations that involve only the delocalized molecular orbitals, giving rise to different classes of two-photon allowed states. Low-energy two-photon states are predominantly composed of one-excitation charge-transfer configurations and triplet-triplet configurations involving the delocalized molecular orbitals. At higher energy, there occur two-photon states consisting of the same one-excitations, but triplet-triplet configurations that involve both delocalized and localized molecular orbitals. The high-energy two-photon states in polyphenylenes therefore cannot be obtained within effective linear chain models that retain only the delocalized molecular orbitals, and will also be missed unless electron correlations are taken into account to high order. Low- and high-energy photoinduced transitions in poly(para-phenylenevinylenes) are assigned to transitions from the optical exciton to these two distinct classes of two-photon states. The current assignment of the high-energy photoinduced absorption is in disagreement with our previous assignment of this transition to the biexciton, which occurs at still higher energy.

• Received 20 August 1998

DOI:https://doi.org/10.1103/PhysRevB.59.4839