X. Long

Chain geometry, solution aggregation and enhanced dichroism in the liquidcrystalline conjugated polymer poly(9,9‐dioctylfluorene)

We report on the physical characterization of a dioctyl-substituted polyfluorene, both in solution and in the solid state. We focus on studies of chain geometry both by molecular modeling and by gel permeation chromatography coupled with light scattering. We determine experimentally a Kuhn segment length, lk = 17.1 ± 2.1 nm and a characteristic ratio C∞ = 21.5 %plusmn; 4.3 indicative of a stiff polymer chain. The effects on absorption and emission spectra of intermolecular interactions that lead to gelation or precipitation from solution are reported. We discuss these results in the context of the strong current interest in the nature of aggregation phenomena and their role in controlling the emissive properties of conjugated polymers. We further show that a markedly enhanced dichroism can be achieved through suitable control of the polymer microstructure.

Optical limiting properties of a zinc porphyrin polymer and its dimer and monomer model compounds

We report on the strong decrease in laser light transmission with increasing excitation intensity observed in thin films of a zinc porphyrin polymer and its dimer and monomer model compounds. Nonlinear transmission measurements performed at 532 nm using 500 ps laser pulses show decreases of 40%, 60% and 65% at input fluences of 0.01 J/cm2 for the monomer, dimer and polymer thin films, respectively. Its application as an optical limiting device for the protection of the eyes and optical sensors from high-fluence laser pulses is assessed. Additional pump–probe measurements performed at 595 nm using 0.8 ps excitation allow us to discuss the strong response in terms of a six-level model of the electronic structure.

Influence of aggregation on the optical properties of a polyfluorene

We report on the photophysical properties of a dioctyl substituted polyfluorene, with a focus on the effects of intermolecular interactions on absorption and emission spectra. We discuss these results in context of the strong current interest in the nature of aggregation phenomena and their role in controlling the emissive properties of conjugated polymers.

EMISSION PROCESSES IN CONJUGATED POLYMER SOLUTIONS AND THIN FILMS

Abstract We report a study of the emissive properties of both solutions and thin films of a substituted polyfluorene. Our work, motivated by the observation of emission line narrowing phenomena in conjugated polymer thin films, shows that in solution the polymer acts largely as expected for an organic laser dye. From our results for films and from extrapolations as a function of increasing chromophore concentration in solution we rule out lasing as an explanation of the line narrowing. The possible roles played by superradiant and superfluorescent cooperative emission and by amplified spontaneous emission are discussed.

Spectral narrowing phenomena in the emission from a conjugated polymer

Abstract We report a study of the morphology dependent emission properties of a liquid crystalline polyfluorene in solid thin films. Spectral narrowing (SN) is observed in spin coated films, but after thermal treatment of the same films the SN is no longer observed. For films which are spin coated on a rubbed polyimide alignment layer, the situation is different. There is then no influence of thermal treatment on the observation of SN. However, specific combinations of the pump polarization direction, film alignment direction and the long axis of the rectangular excitation area now play an important role. The spectral location of SN peaks also varies with film morphology and for different regions of the same sample. The occurrence of SN is strongly dependent on film morphology and the excitation configuration.

Effective stimulated emission and excited-state absorption cross-section spectra of poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) and t,t′-didecycloxy-II-distyrylbenzene

Abstract Effective stimulated emission cross-section spectra, σ em, eff ( λ ), stimulated emission cross-section spectra, σ em ( λ ), and excited-state absorption cross-section spectra, σ ex ( λ ) = σ em, eff ( λ ), are determined in the fluorescence region for the conjugated polymer poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) in tetrahydrofuran, xylene, toluene, and for the model molecule t,t′-(didecycloxy)-II-distyrylbenzene. For theconjugated polymer σ em, eff ( λ ) is found to be small compared to σ em ( λ ) indicating that σ ex ( λ ) and σ em ( λ ) are of similar strength and spectral shape, while for the model compound σ ex ( λ ) is small compared to σ em ( λ ). In the polymer the laser excitation to the broad S 1 absorption band leads to the formation of discrete lattice relaxed exciton conformations extending on the average over four monomer units which are responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.

Excitation intensity-dependent fluorescence behaviour of some luminescent polymers

The fluorescence lifetime, fluorescence signal output and fluorescence spectral distribution of three highly diluted luminescent polymers (one para-phenylenevinylene and two para-phenylene-ethynylene polymers) in tetrahydrofuran are studied as a function of picosecond laser pulse excitation intensity. The results are compared with coumarin 2 in methanol and a distyrylbenzene molecule in tetrahydrofuran. The fluorescence lifetime and the spectral distribution are practically independent of excitation intensity, indicating the absence of excitation-dependent cooperative effects and the absence of emission from preferred exciton states. The fluorescence signal saturation of the polymer solutions at high excitation intensities indicates the photoexcitation of polymer segments (excitons or polarons) extending on average over two para-phenylene-ethynylene or four para-phenylenevinylene monomeric units.

Effective stimulated emission and excited-state absorption cross-section spectra of luminescent polymers

The effective stimulated emission cross-section spectra of a para-phenylene-vinylene polymer and of two paraphenylene- ethynylene polymers are determined by a fluorescence amplification technique. For these luminescent polymers laser action has been achieved in solution. The effective stimulated emission cross-section spectra are found to be small compared to the stimulated emission cross-section spectra calculated from fluorescence quantum distribution and fluorescence lifetime measurements. The laser excitation of the polymer solutions seems to form lattice relaxed exciton conformations extending over a few monomer units which are thought to be responsible for upper-to-lower-state stimulated emission and lower-to-upper-state (excited-state) absorption.

Gain migration dynamics in a conjugated polymer laser

the dynamics of this large gain coefficient, we performed measurements by the pump-probe technique. Figure 2 shows the change of the absorption at zero time delay between the pump and probe pulses. We observe bleaching ofthe absorption at the laser wavelength and in the spectral region below it. In the transparent region ofthe 1 10-nm-thicksample, we observe a significant increase of the transmission at the spectral position where stimulated emission occurs. This feature is a clear signature of optically induced gain. Taking into account the sample thickness, the measured absorption change in the transparent region translates into a gain coefficient of 10,000 cm-’, which is, to our knowledge, by far the largest value ever reported for a solid-state semiconducting polymer sample. We also determined the decay kinetics of the gain and the absorption bleaching (see Fig. 3) that persist for several picoseconds. A further investigation on the very fast gain structure around 605 nm, which appears for very high excitation intensities, is in progress. This work was supported by ONR through the MURI Center CAMP. *Donnelly Corporation, Tucson, Arizona