Optical anisotropy of porphyrin Langmuir–Blodgett films

Langmuir–Blodgett (LB) films of 5,10,15,20-tetrakis-[4-(1-Heptyloxy)phenyl]porphyrin (H2THOPP) have been deposited onto quartz and silver substrates. The deposited layers (20 monolayers thick) have been characterised by reflectance anisotropy spectroscopy, a technique that recently has been successfully used to study the optical anisotropies of organic materials. For porphyrin LB films, the experimental results show that a large anisotropy signal is measured in the Soret band energy region. We show that this anisotropy is clearly connected to the derivative of the dielectric function of the layer. We propose an interpretation of the derivative-like line shape as due to either strain or similar perturbations on the otherwise symmetric top-layer molecules induced by the lower-lying tilted molecules aligned along a direction related to the growth.