Browsing by Author "Mustefa, Ahmed (PhD)"

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    Computational and Experimental Study on the Dual Fluorescence Phenomenon of 4-(N, N-dimethylamino)- benzonitrile and Related Compounds
    (Addis Ababa Universty, 2006-07) Gebremedhn, Elias; Mustefa, Ahmed (PhD)
    The behavior of the ground and the first two excited states of the compound 4-(N,N-dimethylamino)-benzonitrile (4DMABN) have been studied using both experimental and theoretical methods. For comparison purpose the ground and the first two excited states of the compounds 3-(N,N-dimethyl amino)-benzonitrile (3DMABN) and 2,3,6,7- tetrahydro-1H,5H-pyridol[3,2,1-ij]quinoline-9-carbonitrile (TMABN) have been studied theoretically. The experimentally achieved absorption spectrum of 4DMABN has shown two distinct bands. And the emission spectrum has also two distinct bands, the behavior of which is dependent on the wavelength of the exciting light. This is attributed to the presence of two stable conformers of the compound in the ground state. Excited state calculations using TDDFT method have shown that the first two excited states of the compound 4DMABN have different properties in the gaseous and the solvated system. This is in agreement with the experimental result that the compound shows two emission bands in the solvent acetonitrile and one band in the gaseous state. The same theoretical calculation has shown higher excitation energy to the S2 state at the perpendicular conformation of the compound 3DMABN. This agrees with the absence of a CT band in experiment. For the compound TMABN the same calculation has shown for the existence of only one emitting excited electronic state at a considerably twisted conformation. This is in agreement with the experimentally found single CT band.
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    Computational Investigation of Conducting Polythiophenes and Substituted Polythiophenes
    (Addis Ababa Universty, 2007-06) Beyene, Taye; Mustefa, Ahmed (PhD)
    Thiophene oligomers up to eight thiophene units and 3-(2,5-dialkoxyphenyl) thiophenes have been investigated using HF/STO-3G, DFT/STO-3G, DFT/3-21G*, and DFT/6- 31G* computational levels. The structures are optimized to calculate the thermodynamic properties, UV-Vis absorption wavelength, and band gap of the oligomers. Increase in the number of thiophene rings increases the stability of both the doped and undoped states. The band gap decreases with increases in the number of thiophene rings. Up on oxidation, the band gap became further narrower compared with the neutral forms. Introducing dialkoxy phenyl-substituent at the 3-position of thiophene enhanced the stability and band gap of the oligomers. Longer alkoxy chains on the phenyl ring made the oligomers to be more stable and to have narrower band gap. Key words: Thiophenes, density functional theory, band gap, excitation energy, and thermodynamic properties
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    Computational Studies of First row Transition metal Complexes of Schiff Base Derived from Ninhydrin and α, L-alanine
    (2009-07) G/mariam, Tesfalem; Mustefa, Ahmed (PhD)
    A Density functional theory and semi empirical calculation have been carried out on a first row transition metal complexes, Mn(II), Fe(III), Co(II), Ni(II), Zn(II) to predict molecular properties of the metal complexes chelated to the intermediate Schiff base, IDIPA, derived from ninhydrin and α, L-alanine in their octahedral structure. The hybrid B3LYP method has been used to predict the geometry and IR spectra of the metal complexes. Geometry and infrared spectra of the metal complexes, Mn(II), Fe(II), Co(II), Ni(II), and Zn(II) were calculated with B3LYP method using 6-31G, 3-21G(d), 6-31G(d), 3-21G(d), and 3-21G(d) basis set, respectively, and compared with their experimental data. The electronic spectra of the ligand and metal complexes were also performed with ZINDO method. The geometry of the metal complexes were predicted and the ligand were characterized as tridentate and monobasic potential ligand for the metals in their octahedral structure. The electronic spectral calculation of the metal complexes were clearly indicative of a coordination of six in which the number of ligands, IDIPA, coordinated to the metal vary for the first two metal complexes, Mn(II), Fe(III). The computational prediction of IR spectra and electronic spectra using B3LYP and ZINDO, respectively, are good models reproducing results, which are in good agreement to the experiment
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    Computational Study on Hydrogen Bonded Complexes formed by Anthyridone and 2,6-diaminopyridine-3,5-dialdehyde and their Derivatives
    (Addis Ababa Universty, 2009-07) Assefa, Teshome; Mustefa, Ahmed (PhD)
    Computational study was performed on a hydrogen bonded complex formed by antyridone and 2,6-diaminopyridine-3,5-dialdehyde. Six other complexes were designed based on the structure of the complex by changing the R- group of monomer A in to C6H5, p-toluene, p-phenol, OH and OCH3, the X group of monomer B in to chlorine. The binding energies of the complexes were calculated based on their optimized geometry using DFT/B3LYP at 6-31 G level. The electronic spectra for the complexes were calculated with ZINDO and IR spectra of the monomers and complex 1 were computed using DFT/B3LYP at 6-31 G level. It was indicated that all complexes could be formed, via hydrogen bonds. Complexes 2 - 4 were hindered due to steric effect. Electron donating nature was the stability factor. The first absorptions in the electronic spectra of the complexes were red shifted compared with those of the monomers. The stretching vibrations of the N – H bonds were increased with the formation of the hydrogen bond. Key Words: antyridone, 2,6-diaminopyridine-3,5-dialdehyde, DFT and hydrogen bond
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    Computational study on the Thieno[3,4-b]pyrazines and its derivatives based conducting polymers
    (Addis Ababa Universty, 2014-06) Menberu, Wasihun; Mustefa, Ahmed (PhD)
    The research in the (thieno[3,4-b]pyrazine, TP) π -conjugated molecules has become one of the most interesting topics in the fields of chemistry. The use of low band gap materials is a viable method for better harvesting of the solar spectrum and increasing its efficiency. The control of the band gap of these materials is a research issue of ongoing interest. In this work, the geometrical and electronic properties of (Thieno[3,4- b]pyrazine, [TP]) based donor-acceptor conjugated oligomers were studied by the density functional theory (DFT) at the B3LYP level with 6-31G (d,p) basis set from the geometry optimization the inter-rings bonds are longer than normal double bonds but shorter than the single bonds these indicate that the emerging of a quinoidal like distortions as a result of oxidation. A low band gap will be expected in polymers containing donor-acceptor (D - A) repeating units. The absorption spectra of a designed low-band gap conjugated polymer has been studied using the TDDFT/B3LYP/6-31G. Two main absorption peaks can be seen, the one largest in wavelength corresponding to a HOMO to LUMO transition, except compound C1 and one involving higher order excitations. The thermodynamic property calculations show that up on polymerization the change in thermodynamic properties decreases. This decrease in change in thermodynamic properties shows that the oxidized forms of the compound (C1) is unstable with compare to other compounds. Keyword/phrases: Thieno[3,4-b]pyrazine(TP), donor-acceptor, low-band gap, optoelectronic, DFT, geometrical and electronic properties
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    Experimental and Computational Investigation on the Origin of Inverted Solvatochromism in Cyanine and Cyanine Similar Compounds
    (Addis Ababa University, 2013-06-06) Hayire, Genne; Redi, Mesfin (PhD); Mustefa, Ahmed (PhD)
    Photophysical properties of the of methyl red, methyl orange, methyl violate and brilliant green molecules have been studied in solvents of varying polarity using steady-state fluorescence and UV-Vis spectroscopy. In addition computational technique has been applied to explain and verify the experimental result. The investigated molecules showed different solvatochromic shits. Attempt has been made to explain the different solvatochromisms, i.e. negative, positive and inverted solvatochromism. In all the studied compounds a strong solvent polarity dependent shift in absorption position was observed, except methyl violate, which did not show a significant solvent dependence. In methyl red a different situation was observed: in non polar to polar solvents a shift to lower λmax (positive solvatochromism), and in contrast, in polar to strong polar solvents negative solvatochromism was observed. This apparent reversal of solvatochromic behavior, which is commonly known as inverted solvatochromism, is against the theory of solvatochromism. In this work, the observed anomality is explained based on the existence of equilibrium structures each responsible to the resulting positive and negative solvatochromism. The computational results also support the existence of two mesomeric structures, namely polyene and zwitterionic structures.
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    Theoretical and Experimental Studies on Synthesis and Characterization of Nickel(Ii), Copper(Ii) and Zinc(Ii) Complexes Derived From New 1,10-Phenanthroline Dione Derivatives.
    (Addis Ababa University, 2015-06-06) Abera, Soressa; Mustefa, Ahmed (PhD)
    Theoretical calculations using DFT/B3LYP methods were undertaken to evaluate the structural and spectral characteristics of two chelating ligands 1,10- Phenanthroline 5,6- dione dihydrozone (PDDH ) and 1,10-Phenanthroline 5,6-diiminoethanol (PDDIE), in order to assess their potentiality towards metal complex formation. Encouraged by the conclusions from these studies, attempts were made to prepare these ligands and their Ni(II), Cu(II) and Zn(II) complexes. Prompted by the fai1ure to synthesize the ligand PDDH, by the reaction between 1,10- Phenanthroline 5,6- dione (PD) and hydrazine; preparation of Ni(II), Cu(II) and Zn(II) complexes of PDDH was successfully achieved through template synthesis in which each metal complex of PD (in 2:1 mole ratio) in alcoholic solution was treated with hydrazine( in one mole ratio). Synthesis of the second ligand, PDIE and its complexes with Ni(II), Cu(II) and Zn(II) was fruitfully completed. Characterization of the PDDH- metal complexes, PDDIE and its metal complexes by physical, analytical(C, H. N, Cl and metal), conductance, spectral (IR, 1HNMR, 13CNMR, UV-Vis-NIR, AAS) and magnetic susceptibility studies was done. Satisfactory correlation of theoretical and experimental studies was attempted. Polymeric structures with mixed geometries and mixed chelating sequences (NNNN and NN for PDDH and NNOO and NN for PDDIE) were proposed for the metal complexes on the basis of the observed subnormal magnetic susceptibilities/ moments which are in tune with analytical and spectral studies.
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    Theoretical Studies of Naturally Occuring Homoisoflavonoids
    (Addis Ababa Universty, 2009-07) Abrha, Meressa; Mustefa, Ahmed (PhD)
    Nowadays molecular modelling becomes an alternative way for sophisticated and costly experimental techniques. In this project the structures of five naturally occurring homoisoflavonoids namely; 5, 7-dihydroxy-6-methoxy-3-(4’-hydroxybenzyl) chroman-4-one (A), 7-hydroxy-3-(4’-hydroxybenzyl) flavone (B), (E)-7-hydroxy-3-(4’-methoxybenzylidene) chroman-4-one (C), Brazilin (7, 11b-dihydrobenz[b]indeno [1, 2-d] pyran-3, 6a, 9, 10, (6H)-tetrol) (D) and 5, 7-hydroxy-3’, 4’-dimethoxyspiro {2H-1-benzopyran-3-bicyclo [4.2.0] octa [1’.3’.5’]-trien}-4-one (E) are determined by geometry optimization using Density Functional Theory. Comparison of optimized structural parameters like bond lengths, bond angles and torsional (dihedral) angles are extracted. Computation of molecular properties such as NMR shielding tensors (1H and 13C chemical shifts with respect to TMS), infrared harmonic vibrational frequency (scaled with 0.9614) and UV-absorption transitions of these natural homoisoflavonoids are reported here. Calculated results are found to be in good agreement with the experimentally observed values. Though IR absorption frequency spectra and UV absorption spectra were done in gas-phase, the obtained results are in good agreement with the experimental values.