Electroabsorption spectroscopy of luminescent and nonluminescent {pi}-conjugated polymers
- Department of Physics, University of Utah, Salt Lake City, Utah 84112 (United States)
- Department of Electronic Engineering, Faculty of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565 (Japan)
- Department of Chemistry, Iowa State University, Ames, Iowa 50011 (United States)
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{endash}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{sub 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{sub g}), the strongest of which (mA{sub g}) is located at an energy about 1.3 times that of the 1B{sub u} exciton in both luminescent and nonluminescent polymers; in the luminescent polymers the EA spectra also show a second prominent A{sub g} state (kA{sub g}) at an energy of about 1.6 times that of the 1B{sub u}. We have successfully fitted the EA spectra by calculating the imaginary part of the third order optical susceptibility, Im[{chi}{sup 3}({minus}{omega};{omega},0,0)], using a summation over states model dominated by the ground state, the 1B{sub u} exciton, two strongly coupled A{sub g} states (mA{sub g} and kA{sub 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{sub 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{sup 4}({Angstrom}){sup 3} in luminescent polymers and 10{sup 3}{Angstrom}{sup 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{sub 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. {copyright} {ital 1997} {ital The American Physical Society}
- DOE Contract Number:
- FG03-93ER45490
- OSTI ID:
- 664577
- Journal Information:
- Physical Review, B: Condensed Matter, Vol. 56, Issue 24; Other Information: PBD: Dec 1997
- Country of Publication:
- United States
- Language:
- English
Similar Records
Polymer fluorescence: Even-parity states, alternation, and heteroatoms
Electronic excitations of poly(methylphenylsilane) films