Ling-Siu Choi

Enhancement of the molecular hyperpolarizability by a supramolecular amylose–dye inclusion complex, studied by hyper-Rayleigh scattering with fluorescence suppression

Abstract The first hyperpolarizability β of a free hemicyanine dye and a homologue dye included in a supramolecular complex have been determined by hyper-Rayleigh scattering. Since the inclusion complex is fluorescent, high-frequency demodulation of the time-delayed multiphoton fluorescence has been used to retrieve a fluorescence-free inherent value for its first hyperpolarizability. The free dye does not exhibit fluorescence; the inclusion induces fluorescence with a lifetime of 4.8±0.1 ns; and the inclusion complex has a fluorescence-free value for its dispersion-free first hyperpolarizability β0 of approximately twice that for the free dye ((200±5)×10−30 vs. (100±10)×10−30 esu)). The enhanced polar orientation of this complex in thin films, and better thermal and mechanical stability, together with this increase in molecular nonlinearity confirm inclusion as a way to engineer efficient macroscopic arrangements for nonlinear optics.

A novel supramolecular self-assembly thin film with spontaneous polar order

A novel supramolecular thin film in which a non-linear optical (NLO) chromophore is uniaxially included in the helical cavity of amylose, revealed a spontaneous self-assembly with a long-range polar order of the chromophores perpendicular to the film surface. This was manifested by a significantly large second harmonic generation (SHG) observed from solution-cast solid supramolecular thin films on a glass substrate without any external field. Such self-assembly/self-poling and the long-term polar stability of the supramolecular chromophores are thought to be due to the helical rigid-rod structure, their dipolar and/or chiral recognition in the ordering process following the polar initiation through the interface interaction of the chromophore with the glass substrate, and H-bonding among the ordered supramolecules.

Electrical conductivity of ladder-type polymeric transition-metal complexes derived from 2,5-diamino-1,4-benzenedithiol

Abstract Ladder-type polymeric transition metal(II) complexes based on 2,5-diamino-1,4-benzenedithiol (DADT) are synthesized. DADT is a unique ligand among S,N-donor ligand systems in that its metal complexes have a high conductivity, particularly the Co(II) complex ( σ RT =1.6 S/cm); reaction with atmospheric oxygen decreases the conductivity. These complexes are parametric with g values of 2 at room temperature, but their spin densities vary markedly depending upon the complexed metal. The conductivities of these metal(II) complexes are discussed in relation to oxygenation and magnetic properties.

Fluorescence Probe Studies of Poly(acrylic acid) Interchain Complexation Induced by High Shear Flow and Influence of Cationic Surfactants on the Complexation

A dilute solution of poly(acrylic acid) was found to undergo a stable interchain complexation (through H-bonding) in a high shear flow. This requires the PAA to be ultra-high molecular weight, low concentration (< 5 x 10−4 M) and at a pH range of 6–8. The mode and strength of the PAA complexation was strongly influenced by the presence of salts and cationic surfactants. Addition of salts to the shearing PAA solution disrupted the complexation, while the modification of the PAA with the surfactant enhanced the complexation but only below a critical concentration that depends on the surfactant chain length and pH employed. The chain morphology developed by such shear-induced complexation is a local rigidity formation in the PAA chains. Conformational transition toward the PAA rigidity was monitored by fluorescence probe techniques using auramine O. Discussion is made with respect to conformational transition and chain morphology developed in the PAA by shear in relation to molecular environmental changes.