Optical excitations in electroluminescent polymers: The poly(para-phenylenevinylene) family

The components of photoexcited states with large spatial extent in the optical absorption spectra of the electroluminescent conjugated polymers are investigated by using the intermediate-exciton theory. We calculate the ratio of the oscillator strengths due to long-range excitons to the sum of all of the oscillator strengths of the absorption, as a function of the monomer number. The oscillator strengths of the long-range excitons in poly(para-phenylene) are smaller than those for poly(para-phenylenevinylene), and those for poly(para-phenylenedivinylene) are larger than those for poly(para-phenylenevinylene). Such relative variations are explained by the differences of the numbers of vinylene units. The oscillator strengths of long-range excitons in poly(di-para-phenylenevinylene) are much larger than those for the above three polymers, due to the increase of the number of phenyl rings. We also find that the energy positions of the almost localized excitons are is nearly the same in the four polymers.