Effective stimulated emission and excited-state absorption cross-section spectra of para-phenylene-ethynylene polymers

The effective stimulated emission cross-section spectra, sigma(em,eff)(lambda), the stimulated emission cross-section spectra, sigma(em)(lambda), and the excited-state absorption cross-section spectra, sigma(ex)(lambda) = sigma(em)(lambda) - sigma(em,eff)(lambda), of the conjugated polymers poly(2,5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-thienyl) (OPT) and poly(2.5-dioctadecyloxy-paraphenylene-ethynylene-co-2,5-pyridinyl) (OPP) in tetrahydrofuran are determined in the fluorescence region. sigma(em,eff)(lambda) determined by a fluorescence amplification technique is found to be small compared to sigma(em,eff)(lambda) indicating that sigma(ex)(lambda) is nearly as large as sigma(em)(lambda). The laser excitation to the wide inhomogeneously broadened S-1 -absorption band leads to the formation of spectrally structured inhomogeneously broadened lattice relaxed exciton conformations extending on average over two monomer units which are thought to be responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.