Theoretical investigation of the low-lying electronic structure of poly(p-phenylene vinylene)

The two-state molecular orbital model of the one-dimensional phenyl-based semiconductors is applied to poly(para-phenylene vinylene). The energies of the low-lying excited states are calculated using the density matrix renormalization group method. Calculations of both the exciton size and the charge gap show that there are both ${}^1{B}_u^{-}$ and ${}^1{A}_g^{+}$ excitonic levels below the band threshold. The energy of the $1^1{B}_u^{-}$ exciton extrapolates to 2.60 eV in the limit of infinite polymers, while the energy of the $2^1{A}_g^{+}$ exciton extrapolates to 2.94 eV. The calculated binding energy of the $1^1{B}_u^{-}$ exciton is 0.9 eV for a 13 phenylene unit chain and 0.6 eV for an infinite polymer. This is expected to decrease due to solvation effects. The lowest triplet state is calculated to be at around 1.6 eV, with the triplet-triplet gap being around 1.6 eV. A comparison between theory and two-photon absorption and electroabsorption is made, leading to a consistent picture of the essential states responsible for most of the third-order nonlinear optical properties. An interpretation of the experimental nonlinear optical spectroscopies suggests an energy difference of around 0.4 eV between the vertical energy and around 0.8 eV between the relaxed energy of the $1^1{B}_u^{-}$ exciton and the band gap, respectively.