LANGMUIR FILM-FORMING AND 2ND-HARMONIC GENERATION PROPERTIES OF LANTHANIDE COMPLEXES

A series of amphiphilic lanthanide complexes were designed and synthesized, in which the lanthanide complex anions act both as the counterions of hemicyanine and as the spacer within a Langmuir-Blodgett (LB) film. Studies on the surface pressure-area (pi-A) isotherms of these complexes show that the film-forming properties can be clearly improved if appropriate beta-diketone ligands were chosen. The effects of molecular structures of the complexes, including the variation of lanthanide central ions, the structures of beta-diketone ligands, and the length of alkyl chains in hemicyanines, on the film-forming properties of the materials are discussed. From second-harmonic generation experiments, the largely enhanced second-order molecular hyperpolarizability of lanthanide complexes with good film-forming properties were obtained compared with the hemicyanine iodide. This effect may be due in part to the local field effect but primarily molecular ordering and ordered segregation of hemicyanine chromophores by the bulky lanthanide complex anions. The charge separation of the hemicyanine chromophores was supported by the crystal structure of a model complex. The homogeneity of the film was verified by low angle X-ray diffraction and also by the linear relationship of the absorbance vs the number of layers of the LB films of a selected complex.