Thickness Dependence of the Optical Anisotropy for Porphyrin Octaester Langmuir-Schaefer Films

We have studied the optical anisotropy of porphyrin layers with different thicknesses deposited onto gold substrates by the Langmuir-Schaefer technique. In coincidence with the Soret band of the molecule, the optical spectral line shape as determined by reflectance anisotropy spectroscopy exhibits a characteristic, large structure, which changes from a "peaklike" to a "derivative-like" shape at a well-defined thickness (8-10 monolayers). We interpret this result in the framework of electronic effects due to structural changes in the layers, yielding solid-state effects originating from the coupling of the pi orbitals of the porphyrin macrocycles.