Gunasekaran Velmurugan

Highly Emissive Luminogens Based on Imidazo[1,2‐a]pyridine for Electroluminescent Applications

A search for novel organic luminogens led us to design and synthesize some N-fused imidazole derivatives based on imidazo[1,2-a]pyridine as the core and arylamine and imidazole as the peripheral groups. The fluorophores were synthesized through a multicomponent cascade reaction (A(3) coupling) of a heterocyclic azine with an aldehyde and alkyne, followed by Suzuki coupling and a multicomponent cyclization reaction. All of the compounds exhibited interesting photophysical responses, especially arylamine-containing derivatives, which displayed strong positive solvatochromism in the emission spectra that indicated a more polar excited state owing to an efficient charge migration from the donor arylamine to the imidazo[1,2-a]pyridine acceptor. The quantum yields ranged from 0.2 to 0.7 and depended on the substitution pattern, most notably that based on the donor group at the C2 position. Moreover, the influence of general and specific solvent effects on the photophysical properties of the fluorophores was discussed with four-parameter Catalán and Kamlet-Taft solvent scales. The excellent thermal, electrochemical, and morphological stability of the compounds was explored by cyclic voltammetry, thermogravimetric analysis, and AFM methods. Furthermore, to understand the structure, bonding, and band gap of the molecules, DFT calculations were performed. The performance of the electroluminescence behavior of the imidazo[1,2-a]pyridine derivative was investigated by fabricating a multilayer organic light-emitting diode with a configuration of ITO/NPB (60 nm)/EML (40 nm)/BCP (15 nm)/Alq3 (20 nm)/LiF (0.5 nm)/Al(100 nm) (ITO=indium tin oxide, EML=emissive layer, BCP=2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, Alq3 =tris(8-hydroxyquinolinato)aluminum), which exhibited white emission with a turn-on voltage of 8 V and a brightness of 22 cd m(-2).

Computational evaluation of optoelectronic and photophysical properties of unsymmetrical distyrylbiphenyls

Unsymmetrical distyrylbiphenyls (UDSBs) have been evaluated for their suitability for optoelectronic applications. Totally 14 UDSBs including four already reported have been investigated using DFT/TD-DFT calculations. The computed results reveal that the UDSB 1–12 can be used as good hole transport materials and UDSB 13 and 14 can be used as good electron transport materials. The newly designed UDSBs show promising optoelectronic properties and they can be used as a ‘trifunctional materials’ (emitter, hole and electron transport) in OLEDs. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials, electron affinities, reorganization energies and exciton binding energies for these complexes are affected by different donor and acceptor groups. The photophysical characterization of the UDSBs show that, except UDSB 11–14 the absorption and emission spectra of all other molecules have π → π* character as revealed by natural transition orbital (NTO) analysis. The results obtained confirm that the optical properties of UDSBs can be significantly tuned by suitable substitution and these compounds can be used to make efficient OLEDs.

The nature of Pd–carbene and Pd–halogen bonds in (bisNHC)PdX2 type catalysts: insights from density functional theory

Methylene bridged palladium biscarbene halide complexes have proven to be excellent catalysts in cross coupling reactions and in activation of alkanes. These types of complexes are widely used in decomposition and reductive elimination reactions because of their more stable carbene–Pd bond. The electronic structure and bonding of such methylene bridged palladium biscarbene complexes (LnPdX2) (Ln = NHC) have been investigated using density functional theory. The calculated results reveal that the nature of the substituents as well as the coordinating halide ion determine the strength of the Pd–Ccarbene and Pd–X bonds. These two bonds can be fine-tuned to achieve better catalytic activity through proper substituents. In effect, this can be done by making the Pd–Ccarbene bond much stronger through σ-donation and π-back donation and also by making the Pd–X bond weaker by choosing a suitable halide (X).

Structural elucidation and physicochemical properties of mononuclear Uranyl(VI) complexes incorporating dianionic units.

Two derivatives of organouranyl mononuclear complexes [UO2(L)THF] (1) and [UO2(L)Alc] (2), where L = (2,2′-(1E,1′E)-(2,2-dimethylpropane-1,3-dyl)bis(azanylylidene, THF = Tetrahydrofuran, Alc = Alcohol), have been prepared. These complexes have been determined by elemental analyses, single crystal X-ray crystallography and various spectroscopic studies. Moreover, the structure of these complexes have also been studied by DFT and time dependent DFT measurements showing that both the complexes have distorted pentagonal bipyramidal environment around uranyl ion. TD-DFT results indicate that the complex 1 displays an intense band at 458.7 nm which is mainly associated to the uranyl centered LMCT, where complex 2 shows a band at 461.8 nm that have significant LMCT character. The bonding has been further analyzed by EDA and NBO. The photocatalytic activity of complexes 1 and 2 for the degradation of rhodamine-B (RhB) and methylene blue (MB) under the irradiation of 500W Xe lamp has been explored, and found more efficient in presence of complex 1 than complex 2 for both dyes. In addition, dye adsorption and photoluminescence properties have also been discussed for both complexes.

Noncovalent interactions between the second coordination sphere and the active site of [NiFeSe] hydrogenase

QM/MM studies on seven truncated models of the oxidized as-isolated state of the [NiFeSe] Hases have been undertaken in order to find out the influence of the residues on the second coordination sphere on the active site. The major interactions of the second coordination sphere with that of the active site concerns hydrogen bonds between the CN ligand and Pro420 and Ala421 residues, weak S⋯N interactions between the Cys(brdg) and the Sec with the His82 and Arg422 residues. Five types of weak noncovalent interactions between the ligands and the residues in the second coordination sphere are classified based on RGD and QTAIM methods. These residues have been found to affect the electronic structure of the active site as evidenced by the computed IR spectral features. The study reveals that the inclusion of all the residues in the second and the third coordination sphere of the enzyme may not be necessary for an accurate description of the active site features. The non-covalent interactions have been found to be stabilizing the Sec in the conformers.