Correlação entre transferência de energia e a polarização da luz absorvida ou emitida em polímeros luminescentes

Abstract

In present work we correlated the energy transfer and the light polarization of emission or absorption of luminescent polymers. The correlation between the analyzed light polarization and energy transfer effects were supported by emission ellipsometry technique and Stokes´ theory to electromagnetic waves. Studying the emission of Layer-by-Layer Poly(p-phenylene vinylene)- dodecylbenzene sulfonic (PPV/DBS) films, we observed that thinner films have higher emission polarization degree with higher order. The Atomic Force Microscopy (AFM) analysis corroborate the behavior data showing the surface nanostructure. Controlling the film thickness and the results of Photoluminescence Excitation (PLE) and AFM experiments, it was possible to determine the thickness of interface polymer/substrate. Moreover, comparing the absorbance and PLE spectra we founded that the thinner films presented vertical Frank-Condon transitions. In the last case, the energy transfer probability via Förster processes decrease substatially due to the reduction of the active electronic-vibrational states. Thus, it was observed, first time, that the substrate/polymer interface region is ~40-50nm thick. Another system studied were the polymer poly(3-octhylthiophene) (P3OT). P3OT films were grown by chronocoulometry technique varying the dopant. The doping ion in a polymeric film favors polarized emission, when it was excited with linearly polarized light. In principle, films grown by electrochemistry are isotropic, however, due to the doping molecules the P3OT films provide up to ~30% of emission polarization. This fact apparently dubious is explained in terms of energy transfer, where the dopant acts as a barrier for excited carriers migration. As a result, we found that the best synthesis condition occurs at 0.1 M of supporting electrolyte concentration in the electrolytic solution; which it is in concordance of the literature. Finally, we studied the structure Acceptor-Donor (AD), poly[9,9 - octyl - 2,7 fluoreneylenevinylene - alt - 4 ,7 - (di - 2,5 - thienyl) - 2 ,1 ,3 benzothiadiazole] (LaPPS37), where energy transfer processes are fundamental important to understand the inter-and intramolecular mechanisms. It was observed that the photo-excited carriers form excitons in donor species and transfer energy to the acceptors in random directions. The percentage was evaluated the minimal of 81%. For the first time, we propose a quantify photophysical model to explain the energy transfer processes for AD structures. The information discussed in present work should be considered to develop new devices mainly due to energy transfer studies and their correlation with the polarization of the emitted light, determined by a single experiment, ellipsometry emission and the Stokes observable (or parameters).