Characterization of model luminescent PPV analogues with donating or withdrawing groups

Several oligomers analogous to poly(p-phenylene vinylene) (PPV) were synthesised by the Wittig reaction, to be used as model compounds for the study of structure/properties relationship. They are based on benzene-1,4-bis(phenylene vinylene) (OPPV) substituted by electron-withdrawing (nitro) or electron-donating (methyl and dimethylamino) groups on the terminal rings of the molecule. Moreover, two other oligomers 2-hexadecyloxy-5-methoxybenzene-1,4-bis(phenylene vinylene) (1-16-OPPV) and 2-hexadecyloxy-5-methoxybenzene-1,4-bis(4-dimethylaminophenylene vinylene) (1-16-NC-OPPV) were prepared with alkoxy groups on the central ring of the molecule. After a complete structural characterization by FTIR, 1H and 13C NMR, particular emphasis was placed upon the investigation of their thermal stability by TGA and their optical properties in tetrahydrofuran (THF) solutions. Depending on the substituent and on its position in the molecule a different thermal behaviour was observed. Moreover, a red-shift from 10 to 50 nm was observed for the emission wavelength, with respect to unsubstituted OPPV. Several model compounds (with donating groups) exhibited high fluorescence quantum yields, from 51% to 68%. On the other hand, a very low yield was obtained for the oligomer with nitro groups, that was explained from semi-empirical calculations. These results showed promising properties for the incorporation of these oligomers in electroluminescent diodes.