Structure-dependent Optical Anisotropy of Porphyrin Langmuir-Schaefer Films

We have studied the optical anisotropy of Langmuir-Schaefer layers of PdC(10)OAP porphyrin, deposited onto gold substrates with thickness in the range 0-16 monolayers (ML). Deposition has been carried out at two values of the surface pressure 17, corresponding to different layer structures. In one case (Pi = 30 mN/m), molecules are well ordered in stacks oriented edge-on with respect to the substrate. In the other (Pi = 10 mN/m), a complex reorganization of the system happens several days after deposition, to form a mesoscopic two-dimensional lattice. The spectra measured by reflectance anisotropy spectroscopy (RAS) in two cases are clearly characterized. In the former, the line shape is dominated by a characteristic, large structure appearing in coincidence with the Soret band of the molecule, the development of which from a "peak-like" to a "derivative-like" appearance occurs at a well-defined critical thickness Theta(c) (8 ML). In the latter, the line shape is always "peak-like". We explain both line shapes in terms of morphological characteristics of the layer, occurring at different thickness values. The present results clearly show the potential of RAS to characterize efficiently the deposition of organic materials, and suggest that in short time it will be used as an in situ and real time spectroscopy, as already done in inorganic growth.