Increasing the Stability of Electroluminescent Phenylenevinylene Polymers by Encapsulation in Nanoporous Inorganic Materials

Conjugated polyphenylenevinylene oligomer (PPV) encapsulated inside basic zeolites X and Y has been prepared starting from the p-phenylenedimethylene-1,1'-bis(tetrahydrothiophen-1-ium) monomer introduced through ion exchange followed by heating the intermediate poly[1-(S-tetrahydrothiophenium)-p-phenylenethylene] oligomer at 220 °C under vacuum for 24 h. PPV loading ranged between 1.8 and 5 wt %, the sulfur content of the solid being lower than 0.03 wt %, corresponding to 1 S atom every 400 carbons. The organic oligomer has been characterized by diffuse reflectance UV−vis, IR, and MAS 13C NMR, all the data being in agreement with that reported in the literature for pure PPV and different from those of the tetrahydrothiophenium containing intermediate polymer encapsulated inside zeolites. The solids exhibit the green emission characteristic of PPV. Although the length of the oligomer could not be determined by solid−liquid extraction or by treatment of the zeolite with HF, samples with lower precursor loading showed a 20-nm red-shifted emission with respect to that reported for well-prepared PPV. Thus, it is assumed that the length of the encapsulated oligomer has to be enough to make its properties indistinguishable from those of the nonencapsulated PPV polymer. The zeolite-encapsulated PPV was fairly stable to laser flash photolysis (laser power ∼ 10 mJ pulse-1). The transient spectrum has been mainly attributed to PPV polarons. A cell of a thin film of encapsulated PPV prepared by deep coating shows a current intensity vs voltage response characteristic of a diode and a weak electroluminescence operating at a dc voltage of 2.5 V.