Various orientations of pendant dye attached to polyamic acid Langmuir and Langmuir-Blodgett films studied by an optical second-harmonic generation interferometry technique

The pendant dye attached to the polyamic acid backbone in the monolayer Langmuir film is directed toward the air side from the polymer main chain, so that the monolayer has a second-harmonic (SH) active C_∞V symmetry where the symmetry axis is along the surface normal. The orientation is maintained in the monolayer Langmuir-Blodgett (LB) film obtained by vertical deposition. Since the counter ammonium ion with long alkyl chain functions as a pillar to support the dye moieties, the structure is changed owing to the removal of the counter ion by imidization: the dye moieties collapse and lie along the surface in the polyimide monolayer LB film in the air. However, the dye moiety attached to polyimide maintains the SH-active C_∞V symmetry in benzene solution, since the dye orientation is supported by the interaction with the solution.