Optical anisotropy of Langmuir-Blodgett sapphyrin films

The polarization dependence of the optical reflectivity for sapphyrin layers deposited by the Langmuir-Blodgett technique onto a gold substrate has been measured. The experimental results show that characteristic reflectance anisotropy spectroscopy (RAS) spectra are related to layers of different thicknesses. In order to interpret the measured spectral features, the anisotropy of the optical properties of the sapphyrin molecule has been evaluated by using a semi-empirical quantum chemistry approach. The results show clearly that the main RAS structures are related to the electronic properties of the sapphyrin molecules. In particular, two different regimes are observed. Below one (true) monolayer, the optical signal remains very low and structureless, in agreement with the sapphyrin molecules being stacked with their planes almost perpendicular to the substrate plane. Above one monolayer, instead, a strong RAS signal related to the Soret band develops, suggesting that sapphyrin molecules lay more parallel to the substrate surface, in an ordered fashion. (C) 2000 American Institute of Physics. [S0003-6951(00)00446-0].