Anwar Usman

Spectral studies and structure of a 2-hydroxyacetophenone 3-hexamethyleneiminyl thiosemicarbazonate(-2) copper(II) complex containing 1,10-phenanthroline

The spectral studies and structure of a ternary complex of copper(II) with 2-hydroxyacetophenone 3-hexamethyliminylthiosemicarbazonate (L(2-)) and 1,10-phenanthroline (phen) are reported. The thiosemicarbazone binds to the metal as a dianionic ONS-donor (L(2-)) ligand, and forms a complex of the stoichiometry [CuLphen]. The copper(II) complex was characterized by IR and UV/Vis spectroscopies, as well as by solid state room-temperature magnetic susceptibility. Spin Hamiltonian and bonding parameters of the compound are calculated from the EPR spectra. Computer simulation of EPR spectrum in DMF at 77 K aided the calculation of magnetic and bonding parameters of the compound. The structure of the compound is solved by single crystal X-ray diffraction. The geometry around copper is distorted square pyramidal.

Clear Ag-Ag bonds in three silver(I) carboxylate complexes with high cytotoxicity properties

Abstract The reaction of Ag2O and carboxylate ligands in ammonium solution results in three coordination polymers, [Ag(fbc)]n 1, [Ag2(cpd)]n 2 and [Ag2(idc)]n 3, where fbcH is 4-fluorobenzoic acid, cpdH2 is cyclopentane-1,1-dicarboxylic acid and idcH2 is iminodiacetic acid. The X-ray crystal structural analysis indicates that compounds 1 and 2 are two-dimensional frameworks and 3 is a three-dimensional framework. All the three complexes show clear Ag–Ag bonds and high cytotoxicity properties to normal cells and carcinoma cells.

Characterization of two members of the cryptochrome/photolyase family from Ostreococcus tauri provides insights into the origin and evolution of cryptochromes.

Cryptochromes (Crys) are blue light receptors believed to have evolved from the DNA photolyase protein family, implying that light control and light protection share a common ancient origin. In this paper, we report the identification of five genes of the Cry/photolyase family (CPF) in two green algae of the Ostreococcus genus. Phylogenetic analyses were used to confidently assign three of these sequences to cyclobutane pyrimidine dimer (CPD) photolyases, one of them to a DASH-type Cry, and a third CPF gene has high homology with the recently described diatom CPF1 that displays a bifunctional activity. Both purified OtCPF1 and OtCPF2 proteins show non-covalent binding to flavin adenine dinucleotide (FAD), and additionally to 5,10-methenyl-tetrahydrofolate (MTHF) for OtCPF2. Expression analyses revealed that all five CPF members of Ostreococcus tauri are regulated by light. Furthermore, we show that OtCPF1 and OtCPF2 display photolyase activity and that OtCPF1 is able to interact with the CLOCK:BMAL heterodimer, transcription factors regulating circadian clock function in other organisms. Finally, we provide evidence for the involvement of OtCPF1 in the maintenance of the Ostreococcus circadian clock. This work improves our understanding of the evolutionary transition between photolyases and Crys.

Copper(I)–azoimidazoles: a comparative account on the structure and electronic properties of copper(I) complexes of 1-methyl-2-(phenylazo)imidazole and 1-alkyl-2-(naphthyl-(α/β)-azo)imidazoles

Abstract Bis-[1-alkyl-2-(naphthyl-(α/β)azo)imidazole]copper(I) perchlorate derivatives, [Cu(α-NaiR)2(ClO4)] and Cu(β-NaiR)2](ClO4), have been characterised by spectral and electrochemical studies. The single crystal X-ray structure of bis-[1-ethyl-2-(naphthyl-α-azo)imidazole]copper(I) perchlorate shows strong bonding with two imidazole-N atoms, two azo-N donors interact weakly and the structure is described as having a [2+2] distorted linear geometry. A structural comparison has been done with the X-ray structure of bis-[1-methyl-2-(phenylazo)imidazole]copper(I) perchlorate which is Td symmetric. Solution electronic spectra and redox properties are compared and have been correlated with EHMO calculation.

Spectro-temporal characterization of the photoactivation mechanism of two new oxidized cryptochrome/photolyase photoreceptors.

The photoactivation dynamics of two new flavoproteins (OtCPF1 and OtCPF2) of the cryptochrome photolyase family (CPF), belonging to the green alga Ostreococcus tauri , was studied by broadband UV-vis femtosecond absorption spectroscopy. Upon excitation of the protein chromophoric cofactor, flavin adenine dinucleotide in its oxidized form (FAD(ox)), we observed in both cases the ultrafast photoreduction of FAD(ox): in 390 fs for OtCPF1 and 590 fs for OtCPF2. Although such ultrafast electron transfer has already been reported for other flavoproteins and CPF members, the present result is the first demonstration with full spectral characterization of the mechanism. Analysis of the photoproduct spectra allowed identifying tryptophan as the primary electron donor. This residue is found to be oxidized to its protonated radical cation form (WH(*+)), while FAD(ox) is reduced to FAD(*-). Subsequent kinetics were observed in the picosecond and subnanosecond regime, mostly described by a biexponential partial decay of the photoproduct transient signal (9 and 81 ps for OtCPF1, and 13 and 340 ps for OtCPF2), with reduced spectral changes, while a long-lived photoproduct remains in the nanosecond time scale. We interpret these observations within the model proposed by the groups of Brettel and Vos, which describes the photoreduction of FADH(*) within E. coli CPD photolyase (EcCPD) as a sequential electron transfer along a chain of three tryptophan residues, although in that case the rate limiting step was the primary photoreduction in 30 ps. In the present study, excitation of FAD(ox) permitted to reveal the following steps and spectroscopically assign them to the hole-hopping process along the tryptophan chain, accompanied by partial charge recombination at each step. In addition, structural analysis performed by homology modeling allowed us to propose a tentative structure of the relative orientations of FAD and the conserved tryptophan triad. The results of preliminary transient anisotropy measurements performed on OtCPF2 finally showed good compatibility with the oxidation of the distal tryptophan residue (WH(351)) in 340 ps, hence, with the overall Brettel-Vos mechanism.

Generation of Multiple Excitons in Ag2S Quantum Dots: Single High-Energy versus Multiple-Photon Excitation

We explored biexciton generation via carrier multiplication (or multiple-exciton generation) by high-energy photons and by multiple-photon absorption in Ag2S quantum dots (QDs) using femtosecond broad-band transient absorption spectroscopy. Irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S QDs, two distinct characteristic time constants of 9.6-10.2 and 135-175 ps are obtained for the nonradiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely, tightly bound and weakly bound excitons. More importantly, the lifetimes of multiple excitons in Ag2S QDs were about 1 and 2 orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices.

Synthesis, spectral studies and structure of 2-hydroxyacetophenone nicotinic acid hydrazone

Abstract 2-Hydroxyacetophenone nicotinic acid hydrazone (H2ApNH) was synthesized as a part of our work, in search for non-linear optical crystal based on hydrazones, and studied spectroscopically. Complete NMR assignments for the hydrazone was made using COSY homonuclear and HMQC heteronuclear correlation techniques. Solid state reflectance was also studied in order to understand the electronic structure of the synthesized compound. The crystal and molecular structures of H2ApNH were determined. The compound crystallizes into an orthorhombic lattice with a non-centrosymmetric space group Pca21 with two crystallographically unique molecules of in an asymmetric unit. The geometry reveals quasi co planarity in the whole molecular skeleton with localization of the double bonds in the CN–N–CO with an E-configuration.

Glycine Crystallization in Solution by CW Laser-Induced Microbubble on Gold Thin Film Surface

We have developed a novel laser-induced crystallization method utilizing local heat-induced bubble/water interface. Continuous laser beam of 1064 nm is focused on a gold nanoparticles thin film surface covered with glycine supersaturated aqueous solution. Light absorption of the film due to localized plasmon resonance caused local heating at the focal position and produced a single thermal vapor microbubble, which generated thermal gradient followed by convection flow around the bubble and eventually induced glycine crystallization and growth. The crystallization mechanism is discussed by considering gathering and accumulating molecules around the bubble/water interface assisted by convection flow and temperature jump.

Zn(II) and Cd(II) N-carbazolylacetates with strong fluorescence

N-carbazolylacetic acid was synthesized using a new method in a high yield and two carbazolylacetato complexes [Zn(Cabo)-(phen)2ClO4]·0.5C2H5OH and [Cd(Cabo)(phen)2-ClO4] (Cabo=N-carbazolylacetate, phen=1,10-phenanthroline) were synthesized and characterized by elemental analysis, IR, UV–Vis spectra, thermal analyses, photoluminescence measurement and X-ray diffraction studies. Both of the complexes exhibit strong blue emission in solid state as well as high thermal stability and solvent-resistant properties.

A Layer‐by‐Layer ZnO Nanoparticle–PbS Quantum Dot Self‐Assembly Platform for Ultrafast Interfacial Electron Injection

Absorbent layers of semiconductor quantum dots (QDs) are now used as material platforms for low-cost, high-performance solar cells. The semiconductor metal oxide nanoparticles as an acceptor layer have become an integral part of the next generation solar cell. To achieve sufficient electron transfer and subsequently high conversion efficiency in these solar cells, however, energy-level alignment and interfacial contact between the donor and the acceptor units are needed. Here, the layer-by-layer (LbL) technique is used to assemble ZnO nanoparticles (NPs), providing adequate PbS QD uptake to achieve greater interfacial contact compared with traditional sputtering methods. Electron injection at the PbS QD and ZnO NP interface is investigated using broadband transient absorption spectroscopy with 120 femtosecond temporal resolution. The results indicate that electron injection from photoexcited PbS QDs to ZnO NPs occurs on a time scale of a few hundred femtoseconds. This observation is supported by the interfacial electronic-energy alignment between the donor and acceptor moieties. Finally, due to the combination of large interfacial contact and ultrafast electron injection, this proposed platform of assembled thin films holds promise for a variety of solar cell architectures and other settings that principally rely on interfacial contact, such as photocatalysis.