Using nonlinear optical properties in conjunction with chemical synthesis to understand defects in self-assembled molecular films

The potential utility of self-assembled monolayer and multilayer (SAM) thin films in applications such as optical information storage, photorefractives, and solid state microelectronics is generating a great deal of attention in the chemistry and materials science communities. The interest in these materials arises because of the ease of growth, the mild conditions for the formation of the self-assembled layers, and the structural and thermal stability of these assemblies, once they are formed. Because of the Zr-bisphosphonate interlayer linking chemistry, these films are self-terminating at each layer, allowing for the identity of individual layers to be controlled by macroscopic means. The zirconium-phosphate/phosphonate linkage is used because it is a thermally stable inorganic crystalline structure. The bulk properties of these films are currently being studied, but one issue that remains to be addressed is the quantification and characterization of defects in the formed layers. A novel approach to measuring defects on a molecular scale is to utilize the functional chemistry for orientational control in the growth of these films. By constructing these films with control over the orientation of each layer, a multilayer film can be assembled with either centrosymmetric or noncentrosymmetric bulk ordering. Theoretically, the χ(2) nonlinear response of an opposing bilayer structure should be zero because there is a center of inversion in the macroscopic assembly, and any surface second harmonic generation (SHG) signal observed from this film should be the direct result of defects.; Katz and coworkers developed a nonlinear chromophore for use in SAM assemblies. This chromophore (1) is a rigid azo dye with a large molecular hyperpolarizability giving rise to a large second harmonic signal from each monolayer. Functionalizing only one terminus with a phosphonate group controls the orientation of this chromophore. When this molecule is exposed to a zirconated surface, it adsorbs with the phosphonate group directed toward the surface. The exposed hydroxyl terminus of the chromophore is then phosphorylated. The second chromophore (2) is identical to the first except the functional groups at each end are exchanged. By synthesizing these chromophores so that they are functionalized selectively at each terminus, we can control the direction of the dipole moment for each formed layer. By assembling one layer each of chromophores 1 and 2, we can construct a bilayer with a center of inversion. Quantifying the intensity of the SHG signal, which is present against a small background, we can work toward gaining an understanding of defect properties in ZP SAMs.