ELECTROABSORPTION SPECTROSCOPY OF LUMINESCENT AND NONLUMINESCENT PI-CONJUGATED POLYMERS

We have measured the quadratic electroabsorption (EA) spectrum of a va riety of soluble luminescent and nonluminescent pi-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers in clude MEH and DOO derivatives of poly(phenylene-vinylene), poly(phenyl ene ethylene), and polythiophene; the nonluminescent polymers include; poly(diethynyl silane) and monosubstituted polyacetylene. All EA spec tra 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 st ates (A(g)), the strongest of which (mA(g)) is located at an energy ab out 1.3 times that of the 1B(u) exciton in both luminescent and nonlum inescent polymers; in the luminescent polymers the EA spectra also sho w a second prominent A(g) state (kA(g)) at an energy of about 1.6 time s that of the 1B(u). We have successfully fitted the EA spectra by cal culating the imaginary part of the third order optical susceptibility, Im[chi(3)(-omega;omega,0,0)], using a summation over states model dom inated by the ground state, the 1B(u) exciton, two strongly coupled A( g) states (mA(g) and kA(g)), and their most strongly coupled vibration s, using Frank-Condon overlap integrals. A distribution of conjugation lengths, which results,in a distribution of excited state energies, w as also incorporated into the model. The decomposition of the EA spect ra due to the conjugation length distribution was then used to calcula te the 1B(u) exciton polarizability (Delta p) using first derivative a nalysis. For the longest conjugation lengths in our films, we found De lta p to be of order 10(4) (Angstrom)(3) in luminescent polymers and 1 0(3) Angstrom(3) in nonluminescent polymers, respectively, in good agr eement with recent subnanosecond transient photoconductivity measureme nts. 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 luminesc ent polymers, The consequent exciton relaxation energies were calculat ed to be of order 100 meV. [S0163-1829(97)03948-9].