oxy)-1,4-phenylenevinylene] (MEHPPV) of varying conjugation length was prepared by the selective thermal elimination of one of the substituents in a suitable precursor polymer. The precursor, a dialkoxy poly(1,4-xylylene) derivative with varying amounts of acetoxy and methoxy groups, was prepared by a competitive nucleophilic substitution of the Wessling polyelectrolyte, using methanol and sodium acetate in acetic acid as nucleophiles. Selective thermal elimination (in solution) of the acetate groups alone yielded MEHPPV of varying conjugation lengths. The selective nature of the acetate elimination was confirmed by 1 H NMR spectroscopy. As expected, both the absorption and emission maxima of the eliminated samples shifted to the red with increasing conjugation length. While there was very little difference between the absorption spectra of thin film and solutions of MEHPPV-x, there was a significant bathochromic shift in the emission spectra of the thin films when compared to their dilute solution spectra. Additionally, separate emission from the various oligomers, which are occasionally visible in solution, is absent in thin films. Energy transfer from short to longer conjugated segments, within a single polymer chain in solution, was inferred by comparison of the fluorescence spectra of the partially conjugated polymers with those expected from a system where simultaneous independent emission occurs from a similar collection of noninteracting oligophenylenevinylene (OPV) molecules. The latter was calculated assuming a statistically random substitution/ elimination process of the precursor in conjunction with the fluorescence spectral data of OPV's reported by previous workers. The extent of energy transfer increases as the average conjugation length increases. Furthermore, unlike in the model oligomers, in the case of polymers the fluorescence quantum yield in solution rapidly decreases with increase in the average conjugation length.
Organic nanoparticles consisting of single conjugated polymer chains were investigated as a function of degree of conjugation by means of single-molecule spectroscopy. The degree of conjugation was synthetically controlled. For highly conjugated chains, singlet excitons are efficiently funneled over nanometer distances to a small number of sites. In contrast, chains with less conjugation and a high number of saturated bonds do not exhibit energy funneling due to a highly disordered conformation.
Segmented poly[2-methoxy-5-(2‘-ethylhexyl)oxy-1,4-phenylenevinylene] (MEHPPV-x, where x is the mole percent of conjugated segments) represent unique polymeric systems in which a large number of chromophores of different molecular conjugation lengths (different excitation/emission energies) are strung together in a single polymer chain, which thereby forces them to occupy a relatively small volume that is determined by the hydrodynamic size of the macromolecule. Changing the hydrodynamic volume by varying either solvent composition or temperature, therefore, provides a straightforward approach to modulate the interaction between the chromophores. Fluorescence spectroscopic studies of very dilute solutions (in dichloromethane/1,2-dichloroethane) of segmented MEHPPVs with increasing amounts of a nonsolvent (methanol/cyclohexane/ethanol) reveal an interesting inverted S-shaped variation in the emission yield with increasing nonsolvent composition, which is accompanied by a red shift of the emission maxima. Both these changes are indications of a conformational collapse of individual polymer chains, resulting in enhanced energy transfer from highly emissive short conjugation length segments to weakly emissive longer ones and/or to the formation of weakly emissive interchromophore excitons. The solvent composition at the onset of the steep decline in the S-shaped curve, termed the chain-collapse point, was found to vary with the extent of conjugation in a manner which suggests that the polymers bearing longer chromophores undergo collapse earlier than those bearing shorter ones. In samples with very low levels of conjugation, such as in MEHPPV-10, a rather unexpected observation of an initial increase in the emission yield prior to the sudden decline was seen. This is consistent with the unusual conjugation length dependence of the fluorescence quantum yield of oligoPPVs (OPV) reported by earlier workers. Fluorescence spectral variation as a function of temperature also reveals several interesting features of these systems, the most interesting of which is the observation that either an increase or a decrease of emission yields with increase in temperature could be observed, depending on the initial conformation of the polymer chain. When the chain is in a highly collapsed (or a highly solvated) state, an increase in temperature causes the expected decrease in emission yield, while in an intermediate partially collapsed conformation, a significant increase in the emission yield with temperature is observed. The latter observation is ascribed to polymer coil expansion that results in depletion of aggregated species and/or a reduction in energy transfer to weakly emitting chromophoric segments.
Near-infrared-emitting polymer light-emitting diodes ͑PLEDs͒ have been fabricated using blends of conjugated polymers and lanthanide tetraphenylporphyrin complexes. Host polymers include MEH-PPV and a bis-alkoxy-substituted poly͑p-phenylene͒ ͑PPP-OR11͒, and the lanthanide complexes include Yb͑TPP͒acac and Er͑TPP͒acac ͑where TPPϭ5,10,15,20-tetraphenylporphyrin and acacϭacetylacetonate͒. Electroluminescence ͑EL͒ is observed at 977 nm from devices fabricated using MEH-PPV or PPP-OR11 blended with Yb͑TPP͒acac, and EL is observed at 1560 nm from a device fabricated using a blend of MEH-PPV and Er͑TPP͒acac. Visible EL from the host polymers is strongly suppressed in all of the devices, however, in the device fabricated using the PPP-OR11 polymer blue emission from the host is completely quenched. Very efficient quenching of the EL from the host in the PPP-OR11 device is believed to occur due to efficient Förster energy transfer, which is facilitated by the excellent spectral overlap between the PPP-OR11 fluorescence and the Soret absorption band of the TPP ligand.
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