Laser irradiation effects on the optical properties of layer-by-layer PPV/Congo Red thin films

In this paper we investigate the optical properties of thin films produced through the layer-by-layer (LbL) technique with a poly (p-phenylene vinylene) (PPV) precursor and the Congo Red (CR) azodye. Some significant changes are observed when the PPV/CR film is irradiated with polarized laser. An increase of about 10 times in the PPV emission intensity was observed, with an irradiation time of 100 min. Photoinduced birefringence measurements revealed that a high birefringence was produced in the region of the film submitted to photo-irradiation, while a photobleaching phenomenon was observed in the same region. However, such phenomena were not detected in measurements performed at low temperatures. The results indicate that a degradation process of the CR molecules is occurring with photo-irradiation, and the changes observed in the photoluminescence spectra are correlated with the combined effects of degradation and energy transfer between the PPV light-emitting conjugated segments and CR azochromophores.