Molecular order in self-assembled multilayer films measured by optical Second Harmonic

CATALINA VON BILDERLING; MARIO TAGLIAZUCCHI; ERNESTO J. CALVO; ANDREA V. BRAGAS

San Carlos de Bariloche, Argentina

Workshop; US-Argentina Workshop on Nanomaterials; 2009

Second Harmonic Generation (SHG) has become a simple, specific and versatile tool for surface and interface studies in the last two decades. It has been successfully used in biology to measure membrane potentials at specific cellular sites1 and to study real time conformational changes of proteins2. SHG also provides a direct measurement of the average molecular orientation, which is invaluable information to study the growth of multilayer polymer films3.In this work we present measurements of the optical SHG in self assembled multilayer films of PAZO/PAH. Noncentrosymmetrically orientated azobenzene chromophores, present in the AZO groups of PAZO, are expected to give a detectable SHG signal.The polarization dependence of the SHG allows extracting the average tilt angle of the chromophores away from the surface normal. The experiments were performed in transmission, using a modelocked Ti:Sa laser. Regular film growth during deposition cycles was verified by UV-vis absorption spectroscopy, and films with different number of bilayers were studied. We found that the first bilayer in contact with the substrate, present a broad orientation distribution of the molecules. Increasing the number of bilayers (6, 9, 12) produces the ordering of the molecules giving a small tilt angle, meaning that the molecules are orientated normal to the surface. This result, together with an increase of the SHG signal with the number of layers, indicates that the chromophores are orderly adsorbed.