Electroabsorption spectroscopy of luminescent and nonluminescent π-conjugated polymers

Abstract

We have measured the quadratic electroabsorption ͑EA͒ spectrum of a variety of soluble luminescent and nonluminescent-conjugated polymer films in the spectral range of 1.5-4.5 eV. The luminescent polymers include MEH and DOO derivatives of poly͑phenylene-vinylene͒, poly͑phenylene ethylene͒, and polythiophene; the nonluminescent polymers include poly͑diethynyl silane͒ 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, Im͓ 3 (Ϫ;,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 (⌬p) using first derivative analysis. For the longest conjugation lengths in our films, we found ⌬p to be of order 10 4 (Å) 3 in luminescent polymers and 10 3 Å 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. ͓S0163-1829͑97͒03948-9͔

Photophysics of a poly(phenylenevinylene) with alternatingmeta-phenylene andpara-phenylene rings

“…This feature is not observed in the EA spectra of DOO-PPV and other PPV derivatives containing only para-phenylene rings. 19,31 However, we have observed similar features in the EA spectra of distyrylbenzene derivatives where the central para-phenylene ring has been replaced by a meta-phenylene ring. 32 The EA spectrum is a weighted average of a number of phonons that couple to the electronic transition.…”

“…Comparison of the derivative absorption and EA spectra in Fig. 3 results in a maximum polarizability ⌬pϭ623 Å 3 at 2.9 eV, more than an order of magnitude less than Liess et al 19 calculated for several luminescent conjugated polymers ͑including substituted PPV derivatives͒. The polarizability of the lowest excited state of 1,3-PPV is only 50% larger than that of trans,trans'-distyrylbenzene (420 Å 3 ).…”

“…This results in bleaching of the allowed absorption and the appearance of induced absorption bands in the EA spectrum. [13][14][15][16][17][18][19] Samples were mounted in a cold-finger liquid nitrogen cryostat equipped with optical and electrical access. A sinusoidal electric field modulated at 5 kHz was applied to the sample using the reference output of a lock-in amplifier driving a high voltage amplifier.…”

Linear and nonlinear optical properties of the conjugated polymers PPV and MEH-PPV

“…This zeroth derivative contribution to the EA spectrum will also result in an apparent second derivative component, as the transfer of oscillator strength results in a small perturbation of the first derivative component which can be approximated by a second derivative in a Taylor-like expansion. Recently Liess et al 11 showed that a significant contribution to the second derivative component of the EA response of conjugated polymers can also arise from the distribution of conjugation lengths that exists in most conjugated polymer systems. They showed that the EA spectra in the region of the low-lying excitations for a wide range of conjugated polymers could be interpreted solely in terms of neutral excitations.…”

“…Recently, Liess et al 11 modeled the EA spectra of a variety of conjugated polymers using a sum-over-states ͑SOS͒ approach incorporating three essential states and an asymmetric distribution of conjugation lengths. These three essential states are the ground state ͑the 1A g ), the lowest oddparity excited state ͑the 1B u ), and an even-parity state strongly coupled to the 1B u ͑the mA g ).…”