Optical anisotropy and gas sensing properties of ordered porphyrin films

Ordered films of porphyrins are routinely obtained by Langmuir-Blodgett technique, with a thickness control at the level of a single monolayer. The study of their optical anisotropy by reflectance anisotropy spectroscopy (RAS) has evidenced an interesting modification of the spectral line shape around the Soret band of the porphyrin layer after a certain critical thickness has been deposited onto the substrate. This finding has been connected to a structural modification of the film, although a detailed model was lacking until now. Recently, a new theoretical study based on the local field interaction has proposed to explain the RAS spectra in terms of the orientation of the molecules in a layer, that would change from layer to layer as the system is assembled. In order to test the model, we have collected new experimental data studying the modification of the optical anisotropy during exposure of the film to a gas (ethanol), mimicking the application for gas sensing analysis. The results show that the model is adequate to describe the real system: reordering of the layer during or after growth originates the anomalous spectra.