Effect of Side Chain Length Variation on the Optical Properties of PPE-PPV Hybrid Polymers

New 4-fold alkoxy-substituted poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) polymers (PPE-PPV) have been synthesized in order to elucidate the previously observed effect of side chains on the thin film properties (i.e., color variation) of PPE-PPVs. The length of the side chains attached to the PPE segments, CnH2n+1, has been varied from n = 12 to 19. The side chains attached to the PPV segments have been kept fixed to C8H17. Polymers with n <16 are yellow in color, whereas those with n = 16 are orange. Differential scanning calorimetry and nanoindentation analyses reveal side chain crystallization above room temperature in samples with longer side chains (n = 16). Reorganization of the longer side chains attached to the PPE units seems to support stronger p-p overlap between the chain backbones. The red shift for n = 16 was confirmed by photoluminescence (PL) and electroluminescence (EL) spectra obtained for inkjet printed and spin-coated thin films. However, the obtained orange-red EL emission colors are unstable upon increase of the applied voltage. A blue shift of up to 100 nm was observed. All the polymers exhibited very high relative and absolute PL quantum yields in solution (~70%). Their solid-state absolute PL quantum yield was found to be between 10 and 20%. Polymeric light emitting diodes (PLED) with the following structure ITO//PEDOT:PSS//Pn/8//Ca//Ag were fabricated. The devices were fully characterized and showed low turn-on voltages.