The role of spacer layers on the orientation of a hemicyanine dye in Langmuir–Blodgett films

Abstract

The differences in orientation and order are investigated, via polarized IR spectroscopy, between Langmuir–Blodgett (LB) films of only hemicyanine and films of hemicyanine interleaved with spacer layers of either arachidic acid or cadmium arachidate. The hemicyanine layers of LB films without spacer layers (Z‐type configuration) lose their orientation with the surface of the substrate after only a few layers are deposited. The hemicyanine layers interleaved with spacer layers (Y‐type configuration) maintain an approximately fixed orientation with respect to the surface. The spacer layers are fully deuterated which permits IR differentiation of the hydrocarbon tails of the dye from the tails of the fatty acid or salt so that the orientation of each species can be assessed. The IR results explain the second harmonic generation (SHG) response from these noncentrosymmetric films. In addition, the interleaved LB structures with cadmium arachidate have an increased temperature stability compared to LB films of purely hemicyanine dye.