Effect of chalcogen atoms on the electronic band gaps of the quinoxaline containing donor-acceptor-donor type semiconducting polymers: a systematic DFT investigation.

CONTEXT: Due to the widely known positive contributions of the quinoxaline group in organic semiconductors, we conducted a fully computational study using quantum mechanical methods to investigate the effect of quinoxaline in the electron acceptor unit with the combination of different chalcogen atoms on the band gap of a series of donor-acceptor-donor type conjugated polymers. Using density functional theory, we mainly calculated the electronic band gap values of the structures containing four different chalcogen atoms (O, S, Se, and Te) in the electron donor and acceptor units. While chalcogendiazoloquinoxaline groups were used as the electron acceptor units, furan, thiophene, selenophene, and tellurophene were used as the donor units. Our theoretical results showed that the use of heavy chalcogen atoms in both donor and acceptor units resulted in a low band gap. Besides this, the effect of heavy chalcogen atoms used in the electron donor units is much more pronounced compared to the ones used in the acceptor units. More importantly, our findings proved that the inclusion of the chalcogendiazoloquinoxaline group instead of benzochalcogenadiazole as the acceptor unit significantly decreases the electronic band gap of the conjugated polymer. The lowest band gap was found to be 0.10 eV for the 4,9-di(tellurophen-2-yl)-[1,2,5]telluradiazolo[3,4-g]quinoxaline polymer. METHODS: Conformational analysis of the monomers and their corresponding oligomers was performed at the B3LYP/LANL2DZ level of theory. Then, long-range corrected hybrid functional LC-BLYP in a combination with the LANL2DZ basis set was utilized for the calculation of electronic properties and HOMO and LUMO energy gaps of monomers and oligomers through the reoptimization of the lowest energy conformers obtained from the B3LYP/LANL2DZ calculations in the previous step. All energy minimum structures were confirmed through vibrational frequency analysis at both calculation levels. The Gaussian 09 rev. D.01 software was used for all calculations, and GaussView 5.0.9 for visualizations.

Antiproliferative activity of platinum(II) and copper(II) complexes containing novel biquinoxaline ligands.

Nowadays, cancer represents one of the major causes of death in humans worldwide, which renders the quest for new and improved antineoplastic agents to become an urgent issue in the field of biomedicine and human health. The present research focuses on the synthesis of 2,3,2',3'-tetra(pyridin-2-yl)-6,6'-biquinoxaline) and (2,3,2',3'-tetra(thiophen-2-yl)-6,6'-biquinoxaline) containing copper(II) and platinum(II) compounds as prodrug candidates. The binding interaction of these compounds with calf thymus DNA (CT-DNA) and human serum albumin were assessed with UV titration, thermal decomposition, viscometric, and fluorometric methods. The thermodynamical parameters and the temperature-dependent binding constant (K'b) values point out to spontaneous interactions between the complexes and CT-DNA via the van der Waals interactions and/or hydrogen bonding, except Cu(ttbq)Cl2 for which electrostatic interaction was proposed. The antitumor activity of the complexes against several human glioblastomata, lung, breast, cervix, and prostate cell lines were investigated by examining cell viability, oxidative stress, apoptosis-terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, in vitro migration and invasion, in vitro-comet DNA damage, and plasmid DNA interaction assays. The U87 and HeLa cells were investigated as the cancer cells most sensitive to our complexes. The exerted cytotoxic effect of complexes was attributed to the formation of the reactive oxygen species in vitro. It is clearly demonstrated that Cu(ttbq)Cl2, Pt(ttbq)Cl2, and Pt(tpbq)Cl2 have the highest DNA degradation potential and anticancer effect among the tested complexes by leading apoptosis. The wound healing and invasion analysis results also supported the higher anticancer activity of these two compounds.

Effect of chalcogen atoms on the electronic band gaps of donor-acceptor-donor type semiconducting polymers: a systematic DFT investigation.

We systematically investigated and compared the electronic band gaps of 16 different donor-acceptor-donor type semiconducting polymer systems that included different chalcogen atoms in their donor and acceptor units. The five-membered heterocyclic rings furan, thiophene, selenophene, and tellurophene were considered as electron donor units, whereas benzochalcogenadiazole groups, i.e., benzoxadiazole, benzothiadiazole, benzoselenadiazole, and benzotelluradiazole, were used as electron acceptor units. Our findings from B3LYP/6-31G(d) and B3LYP/LANL2DZ calculations performed with and without the polarizable continuum model indicated that the size of the chalcogen atom used as a heteroatom in the donor units plays a more important role than the size of the chalcogen atom in the benzochalcogenadiazole acceptor unit does. On the other hand, our results also suggest that the best way to modify and narrow the electronic band gap is to use heavy chalcogen atoms in both donor and acceptor units.

A computational study on 4,7-di(furan-2-yl)benzo[c][1,2,5]thiadiazole monomer and its oligomers.

The energy gap, Eg, between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels that determines the electronic and optical properties of 4,7-di(furan-2-yl)benzo[c][1,2,5]thiadiazole (FSF) polymer is calculated by performing quantum chemical calculations. First, we theoretically investigated the most stable conformers of FSF monomer and its corresponding oligomers at the B3LYP/6-31G(d) and B3LYP/LANL2DZ levels of theory. We reveal the theoretical molecular structure of this very recently synthesized novel monomer and its oligomers for the first time in the literature. Our results from the B3LYP/6-31G(d) calculations indicated that FSF polymer has a low HOMO-LUMO gap of 1.55 eV to be in good agreement with the experiments. Experimental design and synthesis of novel conjugated polymers require time-consuming and expensive procedures. The findings from this study are promising for the use of computational methods in the design of the novel conjugated polymers, and help to narrow the materials to be used in design and synthesis of conjugated polymers with desired properties.

Conformational behaviors of trans-2,3- and trans-2,5-dihalo-1,4-diselenanes. A complete basis set, hybrid-density functional theory study and natural bond orbital interpretations.

Complete basis set CBS-4, hybrid-density functional theory (hybrid-DFT: B3LYP/6-311+G**) based methods and natural bond orbital (NBO) interpretations have been used to examine the contributions of the hyperconjugative, electrostatic, and steric effects on the conformational behaviors of trans-2,3-dihalo-1,4-diselenane [halo = F (1), Cl (2), Br (3)] and trans-2,5-dihalo-1,4-diselenane [halo = F (4), Cl (5), Br (6)]. Both levels of theory showed that the axial conformation stability, compared to its corresponding equatorial conformation, decreases from compounds 1 → 3 and 4 → 6. Based on the results obtained from the NBO analysis, there are significant anomeric effects for compounds 1-6. The anomeric effect associated with the electron delocalization is in favor of the axial conformation and increases from compounds 1 → 3 and 4 → 6. On the other hand, dipole moment differences between the axial and equatorial conformations [Δ(µ(eq)-µ(ax)] decrease from compounds 1 → 3. Although Δ(µ(eq)-µ(ax)) parameter decreases from compound 1 to compound 3, the dipole moment values of the axial conformations are smaller than those of their corresponding equatorial conformations. Therefore, the anomeric effect associated with the electron delocalizations (for halogen-C-Se segments) and the electrostatic model associated with the dipole-dipole interactions fail to account for the increase of the equatorial conformations stability on going from compound 1 to compound 3. Since there is no dipole moment for the axial and equatorial conformations of compounds 4-6, consequently, the conformational preferences in compounds 1-6 is in general dictated by the steric hindrance factor associated with the 1,3-syn-axial repulsions. Importantly, the CBS-4 results show that the entropy difference (∆S) between the equatorial axial conformations increases from compounds 1 → 3 and 4 → 6. This fact can be explained by the anomeric effect associated with the electron delocalization which affects the C2-Se bond orders and increase the rigidity of the corresponding rings. The Gibbs free energy difference values between the axial and equatorial conformations (i.e. ΔG(ax-ax) and ΔG(eq-eq)) of compounds 1 and 4, 2 and 5 and also 3 and 6 have been calculated. The correlations between the anomeric effect, electrostatic model, ΔG(eq-ax), ΔG(ax-ax), ΔG(eq-eq), bond orders, dipole-dipole interactions, structural parameters and conformational behaviors of compounds 1-6 have been investigated.

Natural bond orbital, nuclear magnetic resonance analysis and hybrid-density functional theory study of σ-aromaticity in Al2F6, Al2Cl6, Al2Br6 and Al2I6.

Natural bond orbital (NBO), nuclear magnetic resonance (NMR) analysis and hybrid-density functional theory based method (B3LYP/Def2-TZVPP) were used to investigate the correlation between the nucleus-independent chemical shifts [NICS, as an aromaticity criterion], σ Al(1)-X2(b) → σ*Al(3)-X4(b) electron delocalizations and the dissociation energies of Al2F6, Al2Cl6, Al2Br6 and Al2I6 to 2AlX3 (X = F, Cl, Br, I). The results obtained showed that the dissociation energies of Al2F6, Al2Cl6, Al2Br6 and Al2I6 decrease from Al2F6 to Al2I6. Like aromatic molecules, these compounds have relatively significant negative NICSiso(0) values. Clearly, based on magnetic criteria, they exhibit aromatic character and make it possible to consider them as σ-delocalized aromatic species, such as Möbius σ-aromatic species. The σ-aromatic character which is demonstrated by their NICSiso(0) values decreases from Al2F6 to Al2I6. The NICSiso values are dominated by the in-plane σ22 (i.e., σyy, the plane containing halogen atoms bridged) chemical shift components. The increase of the NICSiso values explains significantly the decrease of the corresponding dissociation energies of Al2F6, Al2Cl6, Al2Br6 and Al2I6. Importantly, the NBO results suggest that in these compounds the dissociation energies are controlled by the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations. The decrease of the stabilization energies associated with σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations is in accordance with the variation of the calculated NICSiso values. The correlations between the dissociation energies of Al2F6, Al2Cl6, Al2Br6 and Al2I6, σ Al(1)-X2(b) →σ*Al(3)-X4(b) electron delocalizations, natural atomic orbitals (NAOs) and NICSiso values have been investigated.

A theoretical investigation of the relative stability of hydrated glycine and methylcarbamic acid--from water clusters to interstellar ices.

We have theoretically investigated how the low-energy conformers of the neutral and the zwitterionic forms of glycine as well as methylcarbamic acid are stabilized by the presence water. The MP2/6-311++G(d,p) method was utilized to conduct calculations on glycine and methylcarbamic acid in both isolated clusters and in clusters embedded in the conductor-like polarizable continuum model (C-PCM), where the clusters explicitly contain between one and ten water molecules. The neutral forms of glycine and methylcarbamic acid were found to have similar hydration energies, whereas the neutral methylcarbamic acid was determined to be approximately 32 kJ mol(-1) more stable than the neutral glycine in the isolated clusters and 30 kJ mol(-1) more stable in the C-PCM embedded clusters. Both the number and strength of the hydrogen bonding interactions between water and the zwitterions drive the stability. This lowers the relative energy of the glycine zwitterion from 50 kJ mol(-1) above neutral glycine, when there are two water molecules in the clusters to 11 kJ mol(-1) below for the clusters containing ten water molecules. For the methylcarbamic acid clusters with two water molecules, the zwitterion is 51 kJ mol(-1) higher in energy than the neutral form, but it remains 13 kJ mol(-1) above the neutral methylcarbamic acid in the clusters containing ten water molecules. When the bulk water environment is simulated by the C-PCM calculations, we find both the methylcarbamic acid and glycine zwitterionic forms have similar energies at 20 kJ mol(-1) above the neutral methylcarbamic acid energy and 10 kJ mol(-1) lower than the neutral glycine energy. Although neither methylcarbamic acid nor glycine have been detected in the interstellar medium yet, our findings indicate that methylcarbamic acid is the more stable product from methylamine and carbon dioxide reactions in a water ice. This suggests that methylcarbamic acid likely plays a role in the intermediate steps if glycine is formed in the interstellar medium.

A theoretical investigation of the low energy conformers of the isomers glycine and methylcarbamic acid and their role in the interstellar medium.

We have theoretically investigated the low energy conformers of neutral glycine (NH(2)CH(2)COOH) and its isomer methylcarbamic acid (CH(3)NHCOOH) in the gas phase. A total of 16 different levels of the theory, including CCSD(T), MP2 and B3LYP methods with various Pople and Dunning type basis sets with and without polarization and diffuse functions were used. We found eight low energy glycine conformers, where the heavy atoms in three have a planar backbone, and four low energy methylcarbamic acid conformers all with non-planar backbones. Interestingly at all levels of theory, we found that the most stable methylcarbamic acid conformer is significantly lower in energy than the lowest energy glycine conformer. The MP2 level and single point CCSD(T) calculations show the lowest energy methylcarbamic acid conformer to be between 31 to 37 kJ mol(-1) lower in energy than the lowest energy glycine conformer. These calculations suggest that methylcarbamic acid might serve as a precursor to glycine formation in the Interstellar Medium (ISM). We also report the theoretical harmonic vibrational frequencies, infrared intensities, moment of inertia, rotational constants and dipole moments for all of the conformers. In order to understand how glycine or methylcarbamic acid might be formed in the ISM, larger calculations which model glycine or its isomer interacting with several surrounding molecules, such as water, are needed. We demonstrate that B3LYP method should provide a reliable and computationally practical approach to modeling these larger systems.

A computational study on the structures of methylamine-carbon dioxide-water clusters: evidence for the barrier free formation of the methylcarbamic acid zwitterion (CH3NH2+COO-) in interstellar water ices.

We investigated theoretically the interaction between methylamine (CH(3)NH(2)) and carbon dioxide (CO(2)) in the presence of water (H(2)O) molecules thus simulating the geometries of various methylamine-carbon dioxide complexes (CH(3)NH(2)/CO(2)) relevant to the chemical processing of icy grains in the interstellar medium (ISM). Two approaches were followed. In the amorphous water phase approach, structures of methylamine-carbon dioxide-water [CH(3)NH(2)/CO(2)/(H(2)O)(n)] clusters (n = 0-20) were studied using density functional theory (DFT). In the crystalline water approach, we simulated methylamine and carbon dioxide interactions on a fragment of the crystalline water ice surface in the presence of additional water molecules in the CH(3)NH(2)/CO(2) environment using DFT and effective fragment potentials (EFP). Both the geometry optimization and vibrational frequency analysis results obtained from these two approaches suggested that the surrounding water molecules which form hydrogen bonds with the CH(3)NH(2)/CO(2) complex draw the carbon dioxide closer to the methylamine. This enables, when two or more water molecules are present, an electron transfer from methylamine to carbon dioxide to form the methylcarbamic acid zwitterion, CH(3)NH(2)(+)CO(2)(-), in which the carbon dioxide is bent. Our calculations show that the zwitterion is formed without involving any electronic excitation on the ground state surface; this structure is only stable in the presence of water, i.e. in a methyl amine-carbon dioxide-water ice. Notably, in the vibrational frequency calculations on the methylcarbamic acid zwitterion and two water molecules we find the carbon dioxide asymmetric stretch is drastically red shifted by 435 cm(-1) to 1989 cm(-1) and the carbon dioxide symmetric stretch becomes strongly infrared active. We discuss how the methylcarbamic acid zwitterion CH(3)NH(2)(+)CO(2)(-) might be experimentally and astronomically identified by its asymmetric CO(2) stretching mode using infrared spectroscopy.

AM1 parameters for palladium and silver.

We report the parameterization of AM1 for the elements palladium and silver. The basis sets for both metals contain one set each of s-, p- and d-orbitals. AM1 parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Zr, Mo, Pd, Ag, I and Au. The performance and typical errors of AM1 are discussed for Pd and Ag and compared with the PM6 Hamiltonian.

AM1* parameters for gold.

We report the parameterization of AM1* for gold. The basis set for gold contains one set each of s-, p- and d-orbitals. AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Zr, Mo, I and Au. The performance and typical errors of AM1* for gold are discussed.

AM1* parameters for manganese and iron.

We report the parameterization of AM1* for the elements manganese and iron. The basis sets for both metals contain one set each of s-, p- and d-orbitals. AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Zr, Mo, I and Au. The performance and typical errors of AM1* are discussed for Mn and Fe, and are compared with available NDDO Hamiltonians.

AM1 parameters for cobalt and nickel.

We report the parameterization of AM1 for the elements Co and Ni. The basis sets for both metals contain one set each of s-, p- and d-orbitals. AM1 parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Co, Ni, Cu, Zn, Br, Zr, Mo and I. The performance and typical errors of AM1 are discussed for Co and Ni and compared with available NDDO Hamiltonians.

AM1* parameters for vanadium and chromium.

Our extension of the AM1 semiempirical molecular orbital technique, AM1*, has been parameterized for the elements V and Cr. The basis sets for both metals contain one set each of s-, p- and d-orbitals. AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, V, Cr, Cu, Zn, Br, Zr, Mo and I. The performance and typical errors of AM1* are discussed for V and Cr and compared with available NDDO Hamiltonians.

AM1* parameters for bromine and iodine.

Our extension of the AM1 semiempirical molecular orbital technique, AM1*, has been parameterized for the elements Br and I. The basis sets for both halogens contain a set of d-orbitals as polarization functions. AM1* performs as well as other MNDO-like methods that use d-orbitals in the basis, and better than those that rely on an sp-basis. Thus, AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, Cu, Zn, Br, Zr, Mo and I.

AM1* parameters for copper and zinc.

Our extension of the AM1 semiempirical molecular orbital technique, AM1*, has been parameterized for the elements Cu and Zn. The basis sets for both metals contain a set of d-orbitals. The zinc parameterization uses a filled d-shell to give 12 valence electrons. Thus, AM1* parameters are now available for H, C, N, O and F (which use the original AM1 parameters), Al, Si, P, S, Cl, Ti, Cu, Zn, Zr and Mo. The performance and typical errors of AM1* are discussed for the newly parameterized elements.

Prediction of lower critical solution temperature of N-isopropylacrylamide-acrylic acid copolymer by an artificial neural network model.

In this paper, we have investigated the lower critical solution temperature (LCST) of N-isopropylacrylamide-acrylic acid (NIPAAm-AAc) copolymer as a function of chain-transfer agent/initiator mole ratio, acrylic acid content of copolymer, concentration, pH and ionic strength of aqueous copolymer solution. Aqueous solutions with the desired properties were prepared from previously purified polymers, synthesized at 65 degrees C by solution polymerization using ethanol. The effects of each parameter on the LCST were examined experimentally. In addition, an artificial neural network model that is able to predict the lower critical solution temperature was developed. The predictions from this model compare well against both training and test data sets with an average error less than 2.53%.