Three relaxation processes from an electric-field-induced polar structure in a columnar liquid crystalline urea derivative.

We have studied the polarization relaxation process after removing an applied electric field in the columnar hexagonal phase of N,N'-bis(3,4,5-trihexadecyloxyphenyl)urea by means of second-harmonic generation (SHG), switching current, and texture observation. Transient SHG and current associated by sudden field change show polarization switching associated with a molecular reorientation of about a few ms depending on the field strength. Furthermore, the texture observation reveals a column undulation process during 10 s to 1 min. The size of the polar domains was estimated to be 160 molecules by analyzing the field dependence of the SHG signal intensity on the basis of an Ising model.

Electro-optic Kerr effect in the isotropic phase above the columnar phase of a urea derivative.

The authors have measured the electro-optic Kerr effect in the isotropic phase of a urea derivative. Electric-field-induced birefringence Deltan was observed in the isotropic phase even 30 degrees C above the isotropic-columnar phase transition temperature. The induced birefringence is inversely proportional to temperature, as predicted by the Landau-de Gennes theory. Two distinct regions are identified from the proportionality constants in the isotropic phase; optical second-harmonic generation (SHG) is easily observable in the low-temperature region on applying an electric field, whereas SHG activity does not emerge in the higher-temperature region. The structure of molecular assemblies is discussed based on these experimental results.

Photoresponsive self-assembly and self-organization of hydrogen-bonded supramolecular tapes.

Self-assembling building blocks that are readily functionalizable and capable of achieving programmed hierarchical organization have enabled us to create various functional nanomaterials. We have previously demonstrated that N,N'-disubstituted 4,6-diaminopyrimidin-2(1 H)-one (DAP), a guanine-cytosine hybridized molecule, is a versatile building block for the creation of tapelike supramolecular polymer species in solution. In the current study, DAP was functionalized with azobenzene side chains. 1H NMR, UV/Vis, and dynamic light scattering studies confirmed the presence of nanometer-scale tapelike supramolecular polymers in alkane solvents at micromolar regimes. At higher concentrations (millimolar regimes), the supramolecular polymers hierarchically organized into lamellar superstructures to form organogels, as shown by X-ray diffraction and polarized optical microscopy. Remarkably, the azobenzene side chains are photoisomerizable even in the supramolecular polymers, owing to their loosely packed state supported by the rigid hydrogen-bonded scaffold, enabling us to establish photocontrollable supramolecular polymerization and higher order organization of the tapelike supramolecular polymers into lamellar superstructures.

Polarization switching in a columnar liquid crystalline urea as studied by optical second-harmonic generation interferometry.

The polar order and its switching characteristics have been investigated by means of optical second-harmonic generation interferometry in a compound N, N'-bis(3,4,5-trialkoxylphenyl)urea (R=n- C16 H33) being connected by intermolecular hydrogen bondings to form a columnar liquid crystalline phase. The polar structure is formed along the column by applying an electric field and is cooperatively switched by reversing the field. The polar order is relaxed to a nonpolar state within a few milliseconds by terminating the field. No macroscopic polar order exists at least in a range of a visible wavelength scale in the absence of a field.

A ferroelectrically switchable columnar liquid crystal phase with achiral molecules: superstructures and properties of liquid crystalline ureas.

Novel columnar liquid crystalline compounds N,N'-bis(3,4,5-trialkoxylphenyl)ureas 1a-c (R = n-C(8)H(17), n-C(12)H(25), and n-C(16)H(33)) were synthesized, and their phase transitions were measured by differential scanning calorimetery. The superstructures were investigated by X-ray diffraction, polarized light optical microscopy, and IR spectroscopy. The compounds exhibited both rectangular and hexagonal columnar phases in which the urea molecules in each column were stacked in one direction with strong hydrogen bonds. To confirm the ferroelectric switching, optoelectronic experiments were carried out, and the hexagonal columnar phases of 1b and 1c gave a sharp peak of spontaneous polarization in response to an applied triangular wave electric field (0.1-18 Hz). This is the first example of ferroelectrically switchable columnar liquid crystal phases generated by achiral molecules.