Physicochemical studies of green phosphorescent light-emitting materials from cyclometalated heteroleptic iridium(III) complexes

A series of novel imidazole ligands were synthesized and characterized. Phosphorescence studies of series of heteroleptic cyclometalated iridium(III) complexes reveal that these complexes possess dominantly (3)MLCT and (3)π-π* excited states and the solvent shifts of these complexes are interpreted by Richardt-Dimroth and Marcus solvent functions. The results consistent with prior assignments on the absorption band to a metal-to-ligand charge transfer excited state associated with chelating ligand. Emission kinetic studies exploited that the radiative transition (k(r)), increases with increasing λ(em) and linear correlation exists between ln(k(nr)) and energy gap. Electronic transition theory is applied to study the effect of E(g) and ΔQ(e) on non-radiative transition (k(nr)). With a larger ΔQ(e), favouring vibrational overlap and leading to a larger value for k(nr).     

Physicochemical studies of molecular hyperpolarizability of imidazole derivatives

A series of substituted imidazoles have been synthesized in very good yield under solvent free condition by grinding 1,2-diketone, arylaldehyde, arylamine and ammonium acetate in the presence of molecular iodine as the catalyst. The short reaction time, good yield and easy workup make this protocol practically and economically attractive and the imidazoles are characterized by NMR spectra, X-ray, mass and CHN analysis. The push-pull character of series of imidazoles have been analyzed by the quotient of the occupations of the bonding (π) and anti-bonding (π*) orbitals of the central linking -N=C-C=C- unit. Excellent correlation of the push-pull parameter with the corresponding bond lengths d(CN) and d(CC) strongly recommend both the occupation quotients (π*/π) and the corresponding bond lengths are reasonable sensors for quantifying the push-pull character and for the molecular hyperpolarizability ?(0) of these compounds. To support the experimental results, theoretical calculations (heat of formation, NLO, NBO and vibrational analysis) were also made. Within this context, reasonable conclusions concerning the steric hindrance in the chromospheres, push-pull character, hyperpolarizability of the imidazoles and their application as NLO materials will be drawn.     

A high-throughput microfluidic assay to study neurite response to growth factor gradients

Studying neurite guidance by diffusible or substrate bound gradients is challenging with current techniques. In this study, we present the design, fabrication and utility of a microfluidic device to study neurite guidance under chemogradients. Experimental and computational studies demonstrated the establishment of a steep gradient of guidance cue within 30 min and stable for up to 48 h. The gradient was found to be insensitive to external perturbations such as media change and movement of device. The effects of netrin-1 (0.1-10 μg mL(-1)) and brain pulp (0.1 μL mL(-1)) were evaluated for their chemoattractive potential on neurite turning, while slit-2 (62.5 or 250 ng mL(-1)) was studied for its chemorepellant properties. Hippocampal or dorsal root ganglion (DRG) neurons were seeded into a micro-channel and packed onto the surface of a 3D collagen gel. Neurites grew into the matrix in three dimensions, and a gradient of guidance cue was created orthogonal to the direction of neurite growth to impact guidance. The average turning angle of each neurite was measured and averaged across multiple devices cultured under similar conditions to quantify the effect of guidance cue gradient. Significant positive turning towards gradient was measured in the presence of brain pulp and netrin-1 (1 μg mL(-1)), relative to control cultures which received no external guidance cue (p < 0.001). Netrin-1 released from transfected fibroblasts had the most positive turning effect of all the chemoattractive cues tested (p < 0.001). Slit-2 exhibited strong chemorepellant characteristics on both hippocampal and DRG neurite guidance at 250 ng mL(-1) concentration. Slit-2 also showed similar behavior on DRG neuron invasion into 3D collagen gel (p < 0.01 relative to control cultures). Taken together, the results suggest the utility of this microfluidic device to generate stable chemogradients for studying neurobiology, cell migration and proliferation, matrix remodeling and co-cultures with other cell lines, with potential applications in cancer biology, tissue engineering and regenerative medicine.     

Structural characteristics of some metallo-organic discogens

We have recently analysed the crystal and molecular structures of six metalloorganic discogens with substituted β-diketone ligands. The molecules consist of a rigid 11 atom core and a fringe made up of four phenyl rings substituted with alkyl/alkoxy chains. In complex (i), with four octyloxy chains, there are four oxygen atoms around the core. Complex (ii) is asymmetrically substituted with two heptyloxy and two heptyl chains and therefore has two oxygen atoms and complexes (iii) to (vi) have only alkyl chains and hence no oxygen atoms around the core. The metal atom used for coordination has been chosen as Cu/Pd/Ni. Determination of the crystal and molecular structures of these discogens has led to the identification of the following similarities: (1) All the six discogens crystallize in the triclinic space group P1. The recurrence of the space group may be correlated with the structural requirements for efficient packing of the molecules in the crystal lattice. (2) The coordination around the metal atom is square planar. (3) The 11 atom core is only nearly planar. (4) The phenyl rings and the chains are tilted with respect to the core. (5) The molecular conformation in the crystal confers a nearly rectangular shape to these discogens. (6) The chains are fully extended in an all trans conformation. (7) The molecular arrangement is tilted columnar except for the crystal structure of complex (ii).

In addition to the similarities, distinct differences in the crystal structural characteristics have also been observed. For example, when oxygen atoms are present in the fringe, the molecules have no crystallographic symmetry and they tend to pair. In the crystal structure of (i) where the repeat unit along the column is a molecular pair, the metal atoms are distributed in a zig-zag fashion. In the other crystals with columnar arrangement, the metal atoms are stacked one over another. Complex (ii) has a layer-like molecular arrangement in the crystalline phase.     

EPR and optical studies on transition metal doped LiRbB4O7 glasses

Electron paramagnetic resonance and optical investigations of copper and chromium doped LiRbB₄O₇ glasses are carried out. From the results and discussions, it is predicted that both the transition metal ions exhibit octahedral environment. In the case of Cr³⁺, the site symmetry is near octahedral, whereas in the case of Cu²⁺, it is tetragonally distorted octahedral environment. Crystal field, spin-Hamiltonian and bonding parameters are evaluated. The bonding parameters are suggesting ionic.     

High pressure synthesis of the mixed valent and nonsuperconducting ternary YbRhxSny compound

The ternary compound YbRh_1.4Sn_4.6 with the phase I structure (simple cubic) when subjected to a pressure of 40 kbar at 800°C is found to transform to phase III structure (f.c.c.) with the composition YbRh_1.1Sn₃. The latter compound has a lattice parameter of $\text{a = 13.735}\text{A}\text{?}$ which suggests that the Yb is in an intermediate valence state. The temperature dependence of magnetic susceptibility suggests that the Yb is in a homogeneously mixed valence state in the pressure synthesized product. In the phase I structure YbRh_1.1Sn_4.6 is superconducting at 8.6°K, but in the phase III structure the compound YbRh_1.1Sn₃ is not superconducting down to 0.9°K. It is suggested that superconductivity and mixed valence are incompatible.     

Electron transfer free radical mechanism in the reactions of arenediazonium cations with grignard reagents

The major pathway in the reactions of arenediazonium cations with certain Grignard reagents is found to involve an electron transfer from the latter to the π-system of the former reactant and radicals are the immediate precursors of the final products.     

Oxygen stability in YBa2Cu3O7−x

The highT _c superconductor YBa₂Cu₃O_7?x has been found to lose appreciable amount of oxygen in a surprisingly low temperature range (～425–630 K), which drastically degrades the superconducting property. Accompanying this oxygen loss, the resistance behaviour has also been found to change from metallic to semiconducting.