Thin films of conjugated polymers, like poly(p-phenylene vinylene) (PPV) for example, have been widely integrated into LEDs, photo-voltaic devices, and chemical sensors due to their emissive and conductive properties. One disadvantage to these inexpensive materials, however, is that these conjugated systems are susceptible to degradation in ambient conditions. When exposed to oxygen, the double bonds in the polymer backbone are cleaved resulting in the polymer to lose its conductive properties. The primary focus of this research is to encapsulate the polymer within an inorganic framework that will serve to protect the polymer, thereby improving the optical properties, such as quantum yield, and extend the device lifetime.
Zeolites are aluminosilicate crystalline frameworks that are commonly used for their ion-exchange properties and are widely studied in photochemical host-guest interactions. The negatively charged zeolite framework allows for the incorporation of positively charged ions within the void spaces of the framework. A popular PPV synthesis route involves a sulfonium precursor cation, (α,α’-bis-(tetrahydrothiophenio)-p-xylene), which can be readily exchanged into the zeolite. The precursor is a monomer unit of the polymer and under the right conditions will undergo a ‘ship-in-a-bottle’ synthesis to form encapsulated PPV. The inherent basicity of alkali exchanged faujasite X zeolites (NaX, KX, and CsX) as well as Linde type A zeolite, was utilized to induce the polymerization of the sulfonium precursor, followed by thermal elimination under vacuum at 220°C to form fully conjugated PPV.
Formation of PPV was monitored before and after heating of the samples via electronic and vibrational spectroscopy. Characteristic spectra have been obtained for both types of samples, thereby showing the successful formation of PPV oligomers. However, the emission spectra of post-heat samples are more vibronic in nature, lower in intensity, and red-shifted compared to pre-heat samples. X-Ray diffraction shows that the zeolite maintains its crystalline framework, and is not destroyed by the polymerization. As predicted it appears that encapsulation improves the overall optical efficiency of the polymer, shown by increased quantum yields of the PPV-Zeolite composites when compared to pure polymer thin films.
Differences between effective conjugation length and photoluminescence quenching abilities of PPVs formed within X and A suggests that the building schemes of the two are unique. We hypothesize that the polymer chains while contained within the void space of zeolite A, they also extend out onto the surface and block pore openings. Alternatively in zeolite X, PPV is distributed throughout and allows for entry of other molecules into the framework. The framework organization of X also limits the polymer chain length, whereas A allows for extended linear growth.