Determination of surface coverage for tetraphenylporphyrin monolayers using ultraviolet visible absorption and x-ray photoelectron spectroscopies

The surface coverage of thiolated tetraphenylporphyrin derivatives assembled on gold has been determined using ultraviolet/visible absorption spectroscopy (UV/Vis) and x-ray photoelectron spectroscopy (XPS). Ultraviolet/visible absorption spectroscopy was used to calculate the surface concentration of porphyrin molecules through two independent approaches: the first measured the porphyrin concentration in solution after the displacement of a porphyrin monolayer from the gold surface; and the second directly exploited the absorptivity of the monolayers. Furthermore, we determined experimentally the extinction coefficients for the porphyrin monolayers and verified that the extinction coefficients are in agreement with a simple theoretical model that is frequently applied to porphyrin monolayers. Two separate models, both based on a uniform overlayer, were applied to the XPS data to determine the surface concentration of the porphyrin monolayers. Finally, for comparison of the UV/Vis and XPS results, a model of an ordered tetraphenylporphyrin arrangement on a surface was utilized to calculate coverage. The results of these two spectroscopic techniques differed by as much as a factor of 4, but they were generally comparable to those for related porphyrin systems. Determination of the surface coverage of monolayers is often challenging, thus the comparison of two independent experimental techniques allows for a more accurate estimation of monolayer coverage, and demonstrates the often unrealized potential to calculate the coverage of organic monolayers using XPS. Copyright (C) 2002 John Wiley Sons, Ltd.