Coulomb interactions and linear, nonlinear, and triplet absorption in poly(para-phenylenevinylene)

Within a model Hamiltonian with variable on-site and long-range Coulomb interactions between the pi electrons for poly(para-phenylenevinylene), we conduct a thorough search in the parameter space to determine the magnitudes of the effective Coulomb interaction parameters necessary to fit all four absorption bands that are seen in the experimental absorption spectra of this material. We find best agreement between the calculated and experimental absorption spectra with Coulomb interactions that are slightly smaller than the standard Pariser-Parr-Pople parameters. For these values of the Coulomb parameters, the primary photoexcitation in poly(para-phenylenevinylene) is to an exciton with binding energy close to 0.9+/-0.2 eV. This result, obtained from fitting the linear absorption, is in agreement with nonlinear absorption studies, viz. electroabsorption, two-photon absorption, and picosecond photoinduced absorption, within our model. We have also calculated the energies of the lowest triplet state, and the final state to which triplet absorption occurs. The excited triplet state is an exciton. We show that the latter result, taken together with the known experimental triplet absorption energy, indicates that estimates of 0.2 eV or less for the binding energy are incorrect. We briefly discuss the possibility that the binding energy has an intermediate magnitude.