Electroabsorption spectroscopy of luminescent and nonluminescent ∏-conjugated polymers

We have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent p-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenylene ethylene), and polythiophene; the nonluminescent polymers include poly(diethynyl silane) and monosubstituted polyacetylene. All EA spectra show a Stark shift of the low-lying odd-parity exciton (1Bu) and imply the presence of phonon sidebands. There are also higher-energy bands due to transfer of oscillator strength to even-parity exciton states (Ag), the strongest of which (mAg) is located at an energy about 1.3 times that of the 1Bu exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent Ag state (kAg) at an energy of about 1.6 times that of the 1Bu. We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im[x3(ω;ω,0,0)], using a summation over states model dominated by the ground state, the 1Bu exciton, two strongly coupled Ag states (mAg and kAg), and their most strongly coupled vibrations, using Frank-Condon overlap integrals.