Gaëtan Couderc

Alignment and relaxation dynamics of dye molecules in host-guest inclusion compounds as probed by dielectric spectroscopy.

The alignment and relaxation dynamics of a polar dye molecule, N,N-dimethyl-4(4-nitrophenylazo)aniline (DNAA), in zeolite L and perhydrotriphenylene (PHTP) channels were investigated by means of a combination of optical, dielectric, and quantum-chemical methods. Both the zeolite L and PHTP channels enable the dye molecules to align along the channel axis. An amplified net dipole moment of DNAA in PHTP is observed and attributed to enhanced 1D close alignment of dye molecules. In zeolite L channels, a concentration gradient is found with aggregation at the channel entrances. The dynamics of the dye in zeolite L channels reveals localized conical rotational fluctuation modes following Arrhenius-type activation with energy of 0.31 eV, which we assign to small noninteracting fluctuating polar units of the dyes being loosely aligned or isolated. Unlike zeolite L, relaxations in PHTP are characterized by cooperative wobbling motions interpreted as increased intermolecular dipole interaction due to a closely packed one-dimensional array. Temperature-dependent activation energies of 0.25 eV below 0 degrees C and 0.37 eV at ambient temperature reflect the role of the soft channel walls in the activation process.

Polymorphism, polar morphology and absolute structure determination of 4-iodo-4′-nitrobiphenyl (INBP)

4-Iodo-4′-nitrobiphenyl (INBP), a typical nonlinear optical molecule, features three polymorphs: Fdd2 as known before and two new forms, P2/c and P2. Monopolar crystals of the Fdd2 structure express a polar morphology and allowed an absolute structure determination. Iodo substituents cover the (001) and related faces. A negative pyroelectric coefficient was observed. The P2/c form was obtained from sublimation. Heating INBP (Fdd2) in the range of 150 to 180 °C ended in a second polar form (P2). DSC and dielectric measurements allowed us to assume a diffuse type of phase change from Fdd2 into P2. Lattice energy calculations for Fdd2 and P2/c yielded very similar energies. The twinning state observed for crystals not expressing polar symmetry is explained as a 50 : 50 % intergrowth of Fdd2 and P2/c. Periodic fragments allowed the close packing mode of all three polymorphs to be understood. A Hirshfeld surface analysis makes evident that apart from a strong linear synthon (I⋯NO2) other interactions in all three forms are very similar.

Polarity formation in molecular crystals studied by scanning pyroelectric microscopy

Scanning pyroelectric microscopy (SPEM) has successfully been applied to study spatial polarisation distributions in channel-type inclusion crystals and zeolite based host-guest systems, which show macroscopic polarity as the result of polar growth or oriented sorption of hyperpolarisable molecules.