Studies of various properties of different derivatives of asymmetric poly(paraphenylenevinylene) (PPV) polymers for light emitting applications

Abstract

In this study various poly(paraphenylenevinylene) (PPV) derivatives with different asymmetric alkoxy side-chain lengths were synthesized by a modified Gilch polymerization to improve solubilities and to investigate photoluminescence for light-emitting performance. The optical properties of the monomers and polymers with different alkoxy groups, bonded to the PPV main chain, were examined by UV–Vis and photoluminescence spectroscopy. The UV–Vis spectra showed increases in the intensities of the π-π* transition peaks of asymmetric PPV polymers compared to those of the asymmetric monomers. The excitation and emission wavelengths in the solution states were 467–472 nm and 571–579 nm, respectively. The photoluminescence quantum efficiencies of asymmetric polymers were also investigated and found as being 31–38%, with long excited-state lifetimes of 2.94–3.53 ns. The photoluminescence quantum efficiencies of the asymmetric monomers and polymers were compared. The band gap energy value with respect to emission energies were calculated to be in the range 2.17–2.13 eV for the asymmetric polymers. It was noted that these polymers demonstrated good thermal stabilities and optical and photoluminescence properties, showing great potential in optoelectronic applications. The thermal properties and heat changes in the polymer structures were studied by TGA and DSC. All polymers were amorphous, being stable up to 380 °C. The pre-decay temperature for asymmetric dialkyl monomers was around 169 °C and around 380 °C for the asymmetric dialkyl polymers. Most of the mass losses for asymmetric dialkyl monomers occurred in the range 185–475 °C and 396–512 °C for asymmetric dialkyl polymers. The DSC graphs of these polymers revealed the glass transition temperatures (Tg) 76–113 °C. Thus the PPV derivatives having longer flexible alkyl or alkoxy substituents revealed low Tg or low morphological stabilities. The asymmetric dialkyl PPV polymers showed enthalpies 58.79–123.15 cal/g and entropies 0.176–0.343 cal/g oC.