Theoretical investigation of the lowest singlet and triplet states in poly(paraphenylene vinylene)oligomers

Using the semiempirical intermediate neglect of differential overlap (INDO) Hamiltonian in combination with configuration interaction techniques, we calculate the optical and photoinduced absorption spectra of poly(paraphenylene vinylene) oligomers containing from two to five phenyl rings; the evolutions with chain length of the singlet-singlet and triplet-triplet excitation energies as well as the values extrapolated for the polymer are in good agreement with experiment. The geometry relaxation phenomena in the first one-photon allowed singlet excited state and in the lowest triplet state are modeled on the basis of either bond-order/bond-length relationships or the formation of (bi)polaron-type defects; the results are compared to those of direct geometry optimizations in the excited state. The different methods consistently lead to more pronounced bond-length modifications in the triplet state than in the singlet state.