X-ray scattering studies of mixed langmuir monolayers and Langmuir-Blodgett films of a noncentrosymmetric porphyrin with cadmium arachidate

The structures of mixed Langmuir (floating) monolayers and Langmuir-Blodget t (LB) films of a phenanthroline-porphyrin with cadmium arachidate (PhenPor + CdAr) have been investigated by synchrotron X-ray grazing incidence diff raction (GIXD) and specular X-ray reflectivity (SXR). GIXD measurements of the floating monolayers showed only one peak, arising from the CdAr domains in the films, at a scattering angle of 21.5 degrees. This is consistent wi th a hexagonal structure (alpha = 4.77 Angstrom). The correlation length in these domains is 250 Angstrom. GMD measurements of the LB films, however, show two sets of diffraction features: one arises from CdAr domains with a rectangular in-plane structure (alpha = 7.44 Angstrom and b = 4.90 Angstrom ) and a correlation length of 85 Angstrom; the other is from porphyrin doma ins with an oblique in-plane structure (alpha (p) 15.2 Angstrom, b(p) = 8.8 6 Angstrom, and gamma (p) = 80 degrees) and a correlation length of 105 Ang strom. These dimensions are consistent with the surface pressure-area isoth erm measurements and indicate that the two components are immiscible. The t hickness of the bilayer is 57 Angstrom, and there is no correlation between the bilayers. Introduction of a trigger compound does not alter the struct ure of the films but slightly increases the bilayer thickness. The SXR meas urements of the floating monolayers also support the suggested immiscibilit y of the two components in the films.