Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

We have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent pi-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 (1B(u)) and imply the presence of phonon sidebands. There ate also higher-energy bands due to transfer of oscillator strength to even-parity exciton states (A(g)), the strongest of which (mA(g)) is located at an energy about 1.3 times that of the 1B(u) exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent A(g) state (kA(g)) at an energy of about 1.6 times that of the 1B(u). We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im[chi(3)(-omega;omega,0,0)], using a summation over states model dominated by the ground state, the 1B(u) exciton, two strongly coupled A(g) states (mA(g) and kA(g)), and their most strongly coupled vibrations, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results,in a distribution of excited state energies, was also incorporated into the model. The decomposition of the EA spectra due to the conjugation length distribution was then used to calculate the 1B(u) exciton polarizability (Delta p) using first derivative analysis. For the longest conjugation lengths in our films, we found Delta p to be of order 10(4) (Angstrom)(3) in luminescent polymers and 10(3) Angstrom(3) in nonluminescent polymers, respectively, in good agreement with recent subnanosecond transient photoconductivity measurements. We also found that the Huang-Rhys parameter of the 1B(u) exciton varies between 0.25 and 0.9, being in general smaller for the luminescent polymers, The consequent exciton relaxation energies were calculated to be of order 100 meV. [S0163-1829(97)03948-9].