8238
J. Phys. Chem. B 2000, 104, 8238-8244
UV/Vis Spectroscopy of Metalloporphyrin and Metallophthalocyanine Monolayers Self-Assembled on Ultrathin Gold Films Gregory Kalyuzhny,† Alexander Vaskevich,† Gonen Ashkenasy,‡ Abraham Shanzer,*,‡ and Israel Rubinstein*,† Departments of Materials and Interfaces and Organic Chemistry, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: March 22, 2000; In Final Form: June 26, 2000
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 Å) 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.
Introduction The study of self-assembled monolayers (SAMs) and other ultrathin organic films on metal substrates requires advanced methods of characterization. X-ray photoelectron spectroscopy (XPS), ellipsometry, contact angle measurements, FTIR and Raman spectroscopies, scanning probe and diffraction techniques, as well as various electrochemical methods, have been used extensively for the characterization of SAMs.1 Nevertheless, a strong need exists for techniques that provide detailed chemical and structural information on SAMs adsorbed on metal surfaces. For SAMs containing chromophores, such as metalloporphyrins and metallophthalocyanines, transmission UV/vis spectroscopy may serve as an effective characterization tool. A major drawback in the use of this technique for the study of monolayers on gold is the intense absorption of the gold substrate. As shown here, the problem of the gold substrate can be minimized to a point where monolayer absorption can be resolved by using ultrathin gold films of e100 Å in thickness. To our knowledge, the first mention of UV/vis spectroscopy using ultrathin gold films was the study of RNase A labeled with eosin-5-isothiocyanate adsorbed on methyl- and hexaethyleneglycol-terminated SAMs.2,3 Later, a similar approach was utilized for the characterization of iron porphyrin-ferrocenethiol molecules, self-assembled on ultrathin gold film, demonstrating light-induced electron transfer.4 This method was also used to study the binding of metallo-macrocycle molecules to SAMs by axial coordination.5-7 Thicker gold films were utilized for the UV/vis study of Langmuir-Blodgett films of 5-(4-Noctadecylpyridyl)-10,15,20-tri-p-tolylporphyrin.8,9 The thickness of the gold films used for UV/vis transmittance measurements varied between 50 and 200 Å. * Corresponding author. E-mail:
[email protected]. Fax: 972-8-9344137. † Department of Materials and Interfaces. ‡ Department of Organic Chemistry.
The role of ultrathin gold films in the UV/vis spectroscopy of SAMs has not been previously studied, although it is known that the substrate may influence the electronic, and hence, the optical properties of the adsorbate, and vice versa. Such effects were studied mostly in colloid systems,10-14 although it is quite clear that the interaction may also be noticeable with ultrathin gold films, where the “island” structure resembles that of a colloid. However, in previous publications on UV/vis spectroscopy of ultrathin gold films,2-4,7-9 such effects were not mentioned. It is also clear that the influence of the gold underlayer depends on its structure, which is controlled by the preparation conditions. The influence of the evaporation parameters on the gold layer texture were extensively studied (see refs 15-20 and references therein). However, no systematic study of ultrathin (
