UV/Vis spectroscopy of metalloporphyrin and metallophthalocyanine monolayers self-assembled on ultrathin gold films

The use of evaporated ultrathin gold films on mica to obtain transmission UV/vis spectra of monomolecular overlayers containing chromophores is demonstrated. The gold substrates (thickness, 13-100 A) were studied by scanning force microscopy, UV/vis spectroscopy, and X-ray diffraction and found to exhibit an array of {111} textured islands, whose sizes and optical properties can be controlled by the evaporation conditions and subsequent annealing. The use of gold substrates of this kind enables one to clearly resolve UV/vis absorption spectra of (sub)monolayers. This methodology is applied to the study of the binding of cobalt and iron tetraphenylporphyrins (CoTPP and FeTPPCl, respectively) and cobalt phthalocyanine (CoPc) to various N-donor ligand monolayers, self-assembled on ultrathin gold films. Enhancement of the gold surface plasmon absorption, resulting from molecular self-assembly on the gold island film, is used to semiquantitatively follow porphyrin binding to the monolayer.