Nanomechanical properties of ordered phthalocyanine Langmuir-Blodgett layers

The mechanical properties of monolayer and multilayer structures of several symmetrically substituted alkoxy copper phthalocyanine, CuPc(OR)(8), Langmuir-Blodgett (LB) films were evaluated. One-dimensional compliance of the monolayers was determined from the slopes of the surface pressure-area isotherms. Multilayers were examined using a SPM and a commercial nanoindentation system. The nanomechanical studies show a distinct relationship between the position and length of alkoxy substituents on the Pc macrocycle and the LB film elasticity and hardness. The phthalocyanine complexes with peripheral octabutoxy and octaoctyloxy substituents form stiff LB monolayers and multilayers. The nonperipheral alkoxy derivetized CuPc films were less stiff. For monolayer films, it appears that the extent of pi-pi interaction determines the strength of the film. In multilayers, significant additional stability can be imparted through interdigitation of long paraffinic chains which play a significant role in determining the interlayer structure of the LB layers.