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Jürgen Köhler Group  •  Research

Multichromophoric Systems

conjugated Polymers

Investigations of conjugated polymers cover basic as well as applied research. In particular, organic light-emitting diodes (OLEDs) and (flexible) polymer displays that are based on these materials underwent an astonishing development and are integrated into commercially available products meanwhile. Potential applications of conjugated polymers are determined both by their electronic properties and the possibility for easy processing. The former depend crucially on the mutual relationships of dimensionality, disorder, electron-electron and electron-phonon interaction. Another important parameter is their chemical structure: the emission wavelength can be tuned across the entire visible part of the spectrum simply by substituting the side-groups of the chain. The excellent solubility enables simple processing procedures and high quality thin films are easily prepared, which is an important factor for manufacturing OLEDs and displays. Additionally this allows for inkjet printing of simple integrated circuits ("plastic electronics") with much shorter product cycles than possible with anorganic semiconductors.

In cooperation with Lemmer (Karlsruhe) and Scherf (Wuppertal) we aim to elucidate the fundamental electronic properties of conjugated polymers by optical spectroscopy. However, the information that can be obtained in this way is limited due to the tremendous heterogeneity within an ensemble of such macromolecules resulting in strong inhomogeneous broadening of their spectral lines. In order to suppress ensemble averaging and inhomogeneous broadening we exploit single-molecule techniques that allow us to resolve structure-functional relationships in great detail.

The polymer we are studying is a methyl-substituted ladder-type poly(para-phenylene) (MeLPPP). Owing to the ladder structure MeLPPP is supposed to possess a rigid rod-like backbone yielding a rather well defined conformation compared to other conjugated polymers like poly(phenylene vinylene) (PPV), polythiophene, etc. The assumption of low intra-chain disorder in MeLPPP is experimentally supported by the observation of a small inhomogeneous broadening: the ensemble linewidths are by a factor of 3 - 4 smaller than those observed e. g. in PPV. Furthermore, MeLPPP shows a very small Stokes-shift of less than 150 cm-1 indicative of a fixed rigid backbone. Moreover this "double-stranded" ladder-polymer is characterized by a high thermal and chemical stability since a cleavage of two adjacent bonds in the same building block is very unlikely to occur. Finally, the low defect concentrations give reasons for the extraordinarily high photoluminescence quantum-yields of 85% in solution and 25% in films as only few channels for non-radiative deactivation of the electronically excited states exist.

Fig.1 Fig.2

References

  1. R. Hildner, U. Lemmer, U. Scherf, J. Köhler
    Picosecond Excitation Energy Relaxation Processes in a Ladder-Type p- Conjugated Polymer
    Chem.Phys.Lett. 429, (2006), 103
  2. R. Hildner, U. Lemmer, U. Scherf, M. Van Heel, J. Köhler
    Revealing the Electron-Phonon-Coupling in a Conjugated Polymer by Single-Molecule Spectroscopy
    Adv. Mat. 19, (2007), 1978 pdf
  3. R. Hildner, U. Lemmer, U. Scherf and J. Köhler
    Continous-wave two-photon spectroscopy on a ladder-type conjugated polymer
    Chem. Phys. Lett. 448 (2007) 213 pdf

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