Nuttawisit Yasarawan

Dichroism in Dye-Doped Colloidal Liquid Crystals

Nematic liquid crystals were obtained in sterically stabilized suspensions of rodlike particles of sepiolite clay, with an average length up to 900 nm and aspect ratio up to 40. In agreement with computer simulations for hard spherocylinders, the isotropic-nematic transition shifted to lower volume fractions with increasing aspect ratio. However, the coexistence gap was broadened noticeably due to particle polydispersity. The sepiolite crystal structure includes channels filled with zeolitic water, which can be replaced by indigo dye molecules. The indigo molecules are constrained inside the zeolitic channels to be aligned along the long axes of the rods. As a result, the colloidal nematic phase showed a marked dichroism, with an order parameter up to 0.5 for magnetically aligned samples, similar to typical values for dye-doped thermotropic liquid crystals.

Arrested phase separation of colloidal rod–sphere mixtures

Sterically stabilised colloidal rod–sphere mixtures were prepared by mixing sepiolite rods of an average length L = 860 nm and aspect ratio L/D = 40 with silica spheres of average diameter d = 620 nm. Whereas most previous studies have addressed low or high L/d ratios, the present study has an intermediate ratio of 1.4. Samples were studied at rod concentrations from 3 to 9 wt%, in the isotropic–nematic coexistence region. No dramatic effects were seen on adding spheres, except for two samples at low rod concentrations where a rapid (local) phase separation resulted. This is ascribed to formation of nematic tactoids, separated by layers of spheres. Samples at higher rod concentrations did not show any rapid phase separation. Microscopy using fluorescent rods however showed a fine network of rods formed in this case. Macroscopic phase separation into sphere-rich and rod-rich phases was not observed in any sample, nor was any rapid clustering of spheres as seen previously in mixtures with L/d = 0.3. The late stage sediment density can be described well by approximating the osmotic pressure of the colloidal rods at second virial level. Whilst the absence of macroscopic phase separation suggests that these mixtures do not reach chemical equilibrium and instead remain stuck in a range of long-lived metastable states, the observations on the sediment density show that nevertheless local mechanical equilibrium is attained.

Exploring molecular structures, orbital interactions, intramolecular proton-transfer reaction kinetics, electronic transitions and complexation of 3-hydroxycoumarin species using DFT methods.

Optimal structures and electronic properties of various species of 3-hydroxycoumarin (3-HCou) have been explored using density functional theory (DFT) methods under polarizable continuum model (PCM) of solvation. Electron transfer from pyrone to benzene moieties is enhanced upon deprotonation. Anionic and radical species have similar orbital-interaction characteristics but the charges in the former are distributed more uniformly. The rate of intramolecular proton transfer for the neutral species increases many folds upon excitation. The HOMO-LUMO transition with π→π* character mainly accounts for the UV absorption of most 3-HCou species in solution. The wavelengths of maximal absorption predicted using TD-DFT method are in agreement with the previous experiment. For the charged species, calculations with the range-corrected functional yield better agreement with the previous experiment. Anionic 3-HCou species shows high degrees of complexation with chromium(III) and copper(II) compared with oxovanadium(IV) and zinc(II). Either oxovanadium(IV) or zinc(II) prefers forming two isomeric complexes with comparable degrees of formation.

Substituent effect on the proton-related phenomena and chelation behavior of hydroxypicolinic compounds: a DFT investigation

Comparative study on kinetics and thermodynamics of proton-related reactions of hydroxypicolinic acids has been carried out using density functional theory associated with polarizable continuum model of solvation. Mechanisms for such reactions have been established. Both 3- and 4-hydroxypicolinic acid prefer zwitterionic forms to normal forms. For 6-hydroxypicolinic acid, keto forms are highly preferred. The pKa values and UV/Vis bands predicted for some picolinic compounds agree with the experiment. 5-Hydroxypicolinate shows the highest preference for complexation with copper(II) but 6-hydroxypolinate gives rise to the most stable complex. Kinetic stability of the trans-isomer relative to the cis-isomer of the complexes has been evaluated. UV/Vis spectral data predicted for some picolinate complexes are also in agreement with the previous experiment.

Complexation reactions, electronic properties, and reactivity descriptors of cysteamine-based ligands in aqueous solution: a PCM/DFT study

Computational approaches based on density functional theory (DFT) combined with polarizable continuum model (PCM) of solvation have been used to probe the likelihood of complexation in water between oxo-vanadium(IV) and various medicinal cysteamine-based ligands. The experimental electronic spectra of a complex formed by oxo-vanadium(IV) and penicillamine in water agree well with the theoretical spectra based on the time-dependent density functional theory (TD-DFT) calculations. Among all density functionals adopted, CAM-B3LYP outperforms the others in predicting both structural and spectroscopic properties of oxo-transition metal complexes of cysteamine-based ligands. A variety of chelation behaviors have been found for the ligands tested, depending on the choice of substituent added to the cysteamine backbone. Solvation has a great impact on the thermodynamic driving force for cysteine and its derivatives to undergo complexation. In all cases, the thiolate sulfur atom forms stronger coordination bond than either the amine nitrogen or carboxylate oxygen atoms. Based on the thermodynamic and nucleophilicity index calculations, penicillamine has the highest potential to form complex with oxo-vanadium(IV).