Spontaneous polarization and orientational dynamics of polar rod-like molecules in host/guest materials

The polar domain structure and dynamics of host/guest materials which include polar rodlike molecules were investigated by pyroelectric and dielectric techniques. Using scanning pyroelectric microscopy (SPEM), spatially resolved information on a μm scale was obtained of the 3-D domain structure of the polarization caused by an acentric host lattice and/or the polar arrangement of dipolar guest molecules. Two classes of pyroelectric host/guest materials were examined with SPEM: (1) an inorganic host (zeolite AlPO₄-5) which was loaded with p-nitroaniline (PNA) guest molecules by an adsorption process, and (2) an organic channel-type inclusion material, which establishes its macroscopic polarity by cocrystallization of perhydrotriphenylene (PHTP) with the polar guest 1-(4-nitrophenyl)piperazine) (NPP). Despite great differences in the chemical composition and morphology, both types of host/guest systems revealed 180° macro-domains, of which the particular shape and sign of the polarization were in accordance with models about crystal growth and/or directed adsorption of polar molecules. In order to assess the molecular dynamics of the included guest molecules, frequency dependent local pyroelectric measurement, based on SPEM, and broad-band dielectric relaxation spectroscopy were performed. Whereas guest molecules included in PHTP channels show no rotational dynamics, we found several relaxation processes in PNA-loaded AlPO₄-5 crystals, which were assigned to local and cooperative relaxation modes of molecular chains of the hydrogen-bonded PNA guest molecules