Abstract
The pure silica form of zeolite ZSM-12 was loaded with hyper-polarizable para-nitroaniline (pNA) molecules to form a composite material pNA/silica-ZSM-12. 7.6 wt.% of pNA is occluded in the straight non-intersecting channels (free diameter ≈6 Å) of the host material. The composite was characterized by optical microscopy, thermal analysis, NMR- and FTIR-spectroscopy, second harmonic generation (SHG) experiments, molecular modelling and X-ray powder diffraction. The pNA molecules are perfectly aligned with their long axis parallel to the channel direction. The molecules have specific docking sites in the channel, and each molecule uses the channel length of two unit cells. However, the overall symmetry of the composite is centrosymmetric. As indicated by FTIR spectroscopy and thermal analysis, the hydrogen bonds between the sorbate molecules inside the host lattice of silica-ZSM-12 (one-dimensional hydrogen bond system) are weaker as compared to pure pNA crystals (three-dimensional hydrogen bond system). The color of the composite material is yellow. The crystals are dichroitic and change their color in polarized light from colorless to bright yellow. pNA/silica-ZSM-12 shows hardly any SHG. This is in contrast to the composite pNA/AlPO 4-5 which contains the highly polarizable pNA molecules in a non-centrosymmetric arrangement yielding, therefore, strong SHG activity.
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