Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

Abstract

We have measured the quadratic electroabsorption (EA) spectrum of a variety of soluble luminescent and nonluminescent π-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 are 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, $\mathrm{Im}\left[{\chi }^3\right(-\omega ;\omega ,0,0\left)\right],$ using a summation over states model dominated by the ground state, the ${1B}_u$ exciton, two strongly coupled $A_g$ states (${\mathrm{mA}}_g$ and ${\mathrm{kA}}_g$), 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 $\left(\Delta p\right)$ using first derivative analysis. For the longest conjugation lengths in our films, we found $\Delta p$ to be of order ${10}^4(\mathrm{\AA }{)}^3$ in luminescent polymers and ${10}^3{\mathrm{\AA }}^3$ 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.