Spectroscopic and travelling-wave lasing characterisation of Gilch-type and Horner-type MEH-PPV

Two different types of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) (i) a strictly linear, well-defined material of moderate molar mass prepared through polycondensation (Homer-type) and (ii) a commercially available MEH-PPV-sample (Gilch-type, high-molecular and defect-rich) resulting from a dehydrohalogenation process, are carefully analysed by GPC and C-13/H-1-NMR spectroscopy. These studies show that for the Gilch-type polymer there is a marked deficiency of regular vinylene groups (approximate to30%), and therefore, a lack of long-range polyconjugation. The samples are characterized by thin-film optical constants measurements, absorption and fluorescence spectroscopic studies, and thin-film wave-guided travelling-wave laser action investigations. The influence of film-forming solvents (chlorobenzene (CB), tetrahydrofuran (THF)) and of film heating on the absorption, emission and laser behaviour is investigated. The fluorescence quantum yields of neat films (approximate to25% for Homer-type MEH-PPV and approximate to36% for Gilch-type MEH-PPV) and toluene solutions (approximate to38% for Homer-type MEH-PPV and approximate to32% for Gilch-type MEH-PPV) are similar. The exciton delocalisation (chromophore size) is found to be about four repeat units for both types of MEH-PPV. Remarkably good laser performance with low pump laser threshold (<20 muJ/cm(2)) was achieved for both polymers independent of the synthesis pathway, the deviations in the molecular fine structure and the film-forming solvents. However, the preparation of high quality optical films is much easier for the Homer-type MEH-PPV than for the Gilch-type MEH-PPV making the Homer-type MEH-PPV easier to use for thin-film lasers and photonic devices. (C) 2003 Elsevier B.V. All rights reserved.