Photophysical studies on nanostructured PPV-systems
Introduction
One of the most interesting and promising trends in materials science is the ordering of matter on the nanometer scale in order to achieve unprecedented and tuneable combinations of material properties. In living organisms, such structures are obtained through self-assembly processes of organic matter, often in a liquid crystalline phase. The composites studied here are formed from lyotropic liquid crystalline molecules that form an inverse hexagonal phase when mixed with an aqueous solution of an optically active substance or its precursor; in our case, a precursor for poly(para-phenylene vinylene) (PPV) [1]. In this arrangement, the liquid crystals are photopolymerised to form a matrix of nanotubes (diameter 1.5 nm approx. [2]) that clad the PPV after thermal conversion of the precursor. Thus, a solid state film of isolated PPV chains (nano-PPV) is obtained, which shows optical and electronic properties of the single polymer chain unaffected by interaction of the other chains. A strongly increased photoluminescence (PL) quantum yield compared to pristine PPV has been achieved by means of this technique, accompanied by a blue shift of the PL and absorption spectra [3], [4]. The excitation energy dependence of the PL spectrum of the nanocomposite provides further evidence for chain isolation.
In this paper, we focus on the consequences of the isolation of polymer chains and the resulting one-dimensionality of the exciton migration on the energy levels and generation and recombination kinetics of the (spin-1/2) polaron and (spin-1) triplet excitations.
Section snippets
Experimental
The lyotropic liquid crystalline monomer is mixed with a 1 wt.% aqueous (tetra-hydro-thiophene) PPV precursor solution and a radical photoinitiator solution at ambient temperature at a weight ratio of 90:8:2. Taking into account the high viscosity of the material, films are doctorbladed at 80°C, a temperature at which the liquid crystalline material exists in its isotropic phase. During cooling to room temperature, the inverse hexagonal phase is formed, which is subsequently stabilised by
Results and discussions
Fig. 1 shows the PIA spectra for PPV and nano-PPV for excitation at 2.7 eV and 3.5 eV. The different peak heights are the consequence of different excitation efficiencies, consistent with the efficiencies for PL as reported in [3], [7]. For the 2.7-eV excitation, both samples show two peaks, the bulk material at 1.47 eV and 1.58 eV, the nano-PPV at 1.68 eV and 1.79 eV. For the nano-system, both PIA features show a blue-shift of 210 meV, which is similar to what has been observed for PL [3], [4].
Conclusion
Nanostructuring by means of self-assembling lyotropic liquid crystalline templates offers an interesting method to gain control over the alignment and order of conjugated polymer materials. The blue shift observed previously for PL and absorption of nano-PPV compared to pristine PPV is confirmed for the intergap triplet and polaron states. The incorporation of the polymer chains into the matrix inhibits interchain excitation transfer processes resulting in a one-dimensional migration scheme.
Acknowledgements
E.J.W.L. acknowledges the Austrian Fonds zur Förderung der wissenschaftlichen Forschung P 12806-PHY. D.L.G. thanks the Raychem for funding and support. R.C.S. thanks the Chevron for a fellowship. Ames Lab is operated by Iowa State University for the US Department of Energy under Contract No. W-7405-Eng-82. The work in Graz was partially supported by the Sonderforschungsbereich Elektroaktive Stoffe. We thank E. Zojer and R. Resel for fruitful discussions.
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