Optical characterization of amphiphilic, metal-free, nonperipheral phthalocyanine in Langmuir-Blodgett films

Langmuir-Blodgett (LB) films of the amphiphilic, metal free, phthalocyanine 1,4-di(4-hydroxybutyl)-8,11,15,18,22,25- isopentyllphthalocyanine (A40iso5H₂) have been deposited onto a hydrophobically treated glass side. Extrapolation of the pressure-area isotherm shows the area per molecule to be approximately 161 angstroms², which is consistent with similar phthalocyanines. The relationship between absorption and film thickness was studied by measuring the absorption spectra of increasing numbers of monolayers. A mathematical program was used to fit the absorption peaks and hence allow calculation of the phthalocyanine molecule angle on the substrate surface in the characteristic herringbone structure. Measurement of the diffuse reflectance spectra enabled a calculation of the refractive index and extinction coefficient to be made across a range of wavelengths. The step structure of the films was analyzed using a scanning electron microscope to investigate film homogeneity at the boundary and across the film surface. The films were deposited onto a layer of gold in order to excite surface plasmons with a 632.8 nm He-Ne laser. The optical constants of the LB films were determined at this wavelength from the surface plasmon curves using a fitting procedure. The results obtained at 632.8 nm were compared with the values calculated from spectroscopic data, and found to be in good agreement.