Effect of the deposition type on the structure of terthiophene-vinylbenzoate Langmuir–Blodgett films

The effect of the deposition type on the resulting overall structure of multilayer Langmuir–Blodgett (LB) films of terthiophene-vinylbenzoate (TSe) embedded in an octadecyl amine (ODA) matrix was studied. In UV–Vis polarized absorption spectra a large hypsochromic shift is observed for the LB-films compared with the solution indicating that TSe exists predominantly as aggregates in the films. In second harmonic generation (SHG) measurements the signal from the Y-type film was very weak as would be expected for an essentially centrosymmetric system. For X- and Z-type films, similar SHG signals were expected since the orientation of the active molecules should be uniform although opposite in these films. However, the SHG signal from the Z-type films was spatially inhomogeneous and much lower than that from the X-type film. The reason for these differences lies in the details of the film deposition process. In the preparation of Z-type films, the long time exposure of the hydrophobic part of the film to the aqueous subphase induces turning over of the molecules. Thus, the monolayer structure achieved at the air-subphase interface breaks down resulting in only partly organized films with decreased SHG intensity. For X-type films no such exposure takes place during the deposition and all TSe molecules remain in the original orientation.