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 s emiconductors is applied to poly(para-phenylene vinylene). The energies of the low-lying excited states are calculated using the density matrix renorm alization group method. Calculations of both the exciton size and the charg e gap show that there are both B-1(u)- and (1)A(g)(+) excitonic levels belo w the band threshold. The energy of the 1 B-1(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 B-1(u)-, exciton is 0.9 eV for a. 13 phenylene unit chain and 0.6 eV for a n 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 tri plet-triplet gap being around 1.6 eV. A comparison between theory and two-p hoton absorption and electroabsorption it; made, leading to a consistent pi cture of the essential states responsible for most of the third-order nonli near optical properties. An interpretation of the experimental nonlinear op tical 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 B-1(u)- exciton and the band gap, respectively. [S0163-1829(99)12115-5].