We are engaged in the development of new luminescent conjugated organic polymers based on indole and related chromophores [1-3] Conjugated polymers are organic semiconductors, the semiconducting behaviour being associated with the p molecular orbitals delocalised along the polymer chain. Conjugated polymers which have large semiconductor bandgaps can exhibit high photoluminescence quantum yields and there is increasing interest in the excitonic properties of these materials. The occurrence of efficient luminescence in conjugated polymers has become of technological interest because electroluminescence can be produced from such materials in thin-film light-emitting diode structures. Electroluminescent polymer films have potential applications in the development of large-area light-emitting displays.

The aim of this research project is to investigate in detail the photophysical properties of these polymer systems with particular emphasis on the factors which influence their photoluminescence efficiency. Our previous time-resolved fluorescence measurements, using picosecond dye laser excitation, have shown that the polymers have complex multiexponential fluorescence response functions, characteristic of a multiplicity of emitting species, corresponding to the localisation of excitons over a distribution of conjugation lengths in the polymer system. The polymer films show intense fluorescence in the visible region, but their fluorescence lifetimes, and hence fluorescence efficiency, are substantially reduced compared with those in solution phase. This indicates that interchain interactions in the polymer film enhance non-radiative exciton decay.

The scope of these measurements has been severely restricted by the limited range of excitation wavelengths available from ps dye lasers. The extensive range of excitation wavelengths offered by the ultrafast Ti:Sapphire laser systems in COSMIC will enable the differential excitation of monomer and polymer species, and even different subsets of the distribution of polymer chain lengths. This gives us the opportunity to explore the intra- and inter-chain excitonic processes which govern luminescence efficiency and to gain the knowledge required to take a strategic approach to the development of new, more efficient electroluminescent polymers.

References

1. P.Jennings, A.C.Jones, A.R. Mount and A.D. Thomson, J. Chem. Soc. Faraday Trans.1997, 93, 3791
2. P. Jennings, A.C. Jones and A.R. Mount,J. Chem. Soc. Faraday Trans., 1998, 94, 3619.
3. P. Jennings, A.C. Jones and A.R. Mount, Phys. Chem. Chem. Phys., 2000, 2, 1241.