Optical properties of organic dye monolayers by surface plasmon spectroscopy

Abstract

The spectroscopic properties of dye monolayers on silver films were determined by using the technique of attenuated total reflection (ATR). The dyes N,N′-di(methyloctadecyl) squarylium and N,N′- dioctadecyloxacarbocyanine were deposited on silver films by the Langmuir-Blodgett monolayer technique. From a least squares fit of the experimental ATR curves to the exact reflection formula, the dielectric function of the dye monolayers was determined in the visible wavelength region. It was found that squarylium forms ordered layers with a regular array of molecules and a film thickness equal to the length of the molecule (2.66 nm). These films were characterized by a strong absorption band at λs = 530 nm (dimer band) and were highly anisotropic, the transition dipole moment lying in the plane of the monolayer film. Undiluted cyanine dye films on silver, on the other hand, showed a thickness less than that expected from the length of the molecule and a high degree of disorder. Increasing regularity in arrangement and growing thickness were observed when the cyanine dye was diluted with cadmium arachidate. For both the pure and diluted cyanine dye monolayers, two absorption bands have been measured in the visible region (a monomer band at λs = 500 nm and a dimer band at λs = 464 nm) and it was confirmed that their relative intensities depended on the mixing ratio of cyanine dye to Cd-arachidate. © 1978 American Institute of Physics.