Surfaces, solvatochromic probes

Another well-represented category was that of self-assembled monolayers (SAMS) and other supramolecular systems. The experiments on the SAMS included studies of the surface pKa of one system (110), the kinetics and thermodynamics of the self-assembly process (111), and the characterization of the SAM surface by study of solution contact angles (112). The experiments on supramolecular systems included studies on chemical equilibria in such systems (113, 114, 115), the kinetics of inclusion phenomena (116), and the use of solvatochromic probes in studying inclusion phenomena (117). 

Aniline derivatives have also been employed as solvatochromic probes to characterize the solvation abilities of various polymers and polymer surfaces. Thin films of various polymers were prepared by Paley and coworkers171 that incorporated the probe 7 for studying the polarity/polarizability and the probe pair 8/7 for studying the HBA/EPD... 

Other in-situ spectroscopies have been employed to study different aspects of adsorbed species. Among them it is possible to mention fluorescence spectroscopy [125], diffuse reflectance of solvatochromic probes [126], Raman spectroscopy [127], and surface-enhanced Raman spectroscopy [128]. 

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

Other related kinds of medium-dependent behavior have been observed, such as second-sphere coordination effects (e.g., with crown ethers, cyclodextrins ), and solvatochromic medes have been used as probes of their environment in polymers, micelles, " zeolites, inorganic glasses, and surfaces, etc. They may be used in this way either by virtue of their response to the electric field experienced in a particular environment, or because of specific interactions between the probe and the environment, e.g., hydrogen bonding. Solvatochromism is also a useful predictor of nonlinear optical behavior (see Chapter 9.14), because the same properties which give rise to strong solvatochromism are also necessary for large, second-order, nonlinear optical coefficients (/3). ... 

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