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

Abstract

The polar domain structure and dynamics of hostlguest materials which include polar rodlike molecules were investigated by pyroelectric and dielectric techniques. Using scanning pyroelectric microscopy (SPEM), spatially resolved information on a p m scale was obtained of the 3-D domain structure of the polarization caused by an acentric host lattice andlor the polar arrangement of dipolar guest molecules. Two classes of pyroelectric host/guest materials were examined with SPEM: (1) an inorganic host (zeolite AIP04-5) which was loaded with pnitroaniline (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) ("9. Despite great differences in the chemical composition and morphology, both types of hostlguest systems revealed 180" macro-domains, of which the particular shape and sign of the polarization were in accordance with models about crystal growth andlor 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 AlP04-5 crystals, which were assigned to local and cooperative relaxation modes of molecular chains of the hydrogen-bonded PNA guest molecules.

An Elementary Picture of Dielectric Spectroscopy in Solids: Physical Basis

“…We have given some relevant examples among various fields of current interest in materials characterization, where DS and IS have found strong applications and success. The growing ability and creativity of DS and IS users are expected to increase even further the spectrum of application of these spectroscopic techniques, in particular for adsorption studies and monitoring host-guest interactions where there are already some initial successes (63).…”