Calculation of excitonic properties of conjugated polymers using the Bethe–Salpeter equation

The excitonic spectra of a number of conjugated polymers, polythiophene (PT), polyphenylenevinylene (PPV), ladder-type poly-para-phenylene (LPPP), trans-polyacetylene (PA), and polydiacetylene (PDA), are calculated by solving the Bethe–Salpeter equation for the two-body Green function of the electron–hole pair. The only input to the calculation is the bulk dielectric constant. We find intermediate binding energies of 0.4–0.6 eV for the lowest optical 1 1Bu exciton, obtaining good agreement with recent experiments for each polymer individually. The agreement for the singlet–triplet splittings and the 1 1Bu polarizabilities is satisfactory. This also holds for the 1 1Bu–2 1Ag splittings in PT, LPPP, and PPV. For PA and PDA we find the 2 1Ag exciton above the 1 1Bu exciton, in contrast with experiment; a possible explanation for this discrepancy is the absence of lattice relaxation effects and double excitations in our calculations.