Self-Assembly of Dipoles

When mixed SAMs of NMB and DMAMB were prepared in toluene, the surface NO2 concentration, as determined by external reflection FTIR spectroscopy, displays a plateau at about 40%. If one assumes that the equilibrium concentration of the two components in the mixed SAM, in a nonpolar solvent, is driven by the formation of a two-dimensional assembly with zero net dipole moment, the results can be explained by using the Hammett equation. 

Such control of surface functionahties, surface chemical potential, and surface dipole is not possible in mixed SAMs of cofunctionalized saturated -alkanethiols, since dipolar interaction of surface functionahties wiU result in surface reorganization [95, 99-101]. 

As already indicated in Fig. 9.9 and 9.12, the induced molecular dipole by 4 -subtituent should have a significant impact upon the tilt angle of the bonded molecule. This was examined in great detail using ER-FTIR spectroscopy of SAMs of several 4 -subtituted-4-mercaptobiphenyls on Au(lll) and Ag(lll) [10]. 

The impact of the choice of the substrate to be either gold or silver upon a different molecular orientation of rigid moieties within a SAM was recently demonstrated by Somashekarappa and Sampath [107]. They studied the impact of the different orientation of 2,9,6,23-tetraamino cobalt phtalocyanine bound as a SAM onto silver or gold upon their behavior in electrocatalysis. It was found that the different tilt of the phtalocyanine macrocyles and the consequently different accessibility of the metal surfaces and catalytic center results in a different reaction pathway and oxidation products. 

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