Electron Donor-Acceptor Complexes of the Fullerenes C60 and C70 with Amines

Based on electronic absorption spectroscopy, C60 is found to form complexes with aromatic amines with an enthalpy of association in the range 9-16 kJ mol-1. Interaction of C70 with the amines is negligible. Cyclic voltammetric measurements confirm these observations.

Molecular structure of electron donor-acceptor complexes of metallotetraphenylporphyrins with trinitrobenzene

The metallotetraphenylporphyrins, MTPPs, where M=Co(II), Cu(II) and Ag(II) form one to one molecular complexes in solution with 1,3,5-trinitrobenzene (TNB). The crystal structure of CoTPP.TNB.2CH3OH revealed that the mean separation between the porphyrin and TNB planes is 3·27 Å and the centre of the aryl ring of TNB is displaced by 0·90 Å from the centre of the porphyrin plane. Extended Huckel molecular orbital calculations suggest that acceptor orbitals are predominantly nitro-group based rather than an aryl π framework. The contribution of the metal orbitals in the donation is also seen as predicted by the structural data. Electrochemical redox measurements in solution provide evidence for charge-transfer stabilisation in these complexes.

Folding of a Donor-Containing Ionene by Intercalation with an Acceptor

Cationic ionenes that bear electron-rich 1,5-dialkoxynaphthalene (DAN) units within the alkylene segment were allowed to interact with different types of electron-deficient, acceptor-containing molecules in an effort to realize intercalation-induced folding of the ionenes; the collapse of the chains was expected to occur in such a way that the donor and acceptor units become arranged in an alternating fashion. Several acceptor-bearing molecules were prepared by the derivatization of pyromellitic dianhydride and naphthalene tetracarboxylic dianhydride with two different oligoethylene glycol monomethyl ether monoamines. This yielded acceptor molecules with different water solubility and allowed the examination of solvophobic effects in the folding process. UV/Vis spectroscopic studies were carried out by using a 1:1 mixture of the DAN-ionenes and different acceptor molecules in water/DMSO solvent mixtures. The intensity of the charge-transfer (CT) band was seen to increase with the water content in the solvent mixture, thereby suggesting that the intercalation is indeed aided by solvophobic effects. The naphthalene diimide (NDI) bearing acceptor molecules consistently formed significantly stronger CT complexes when compared to the pyromellitic diimide (PDI) bearing acceptor molecules, which is a reflection of the stronger pi-stacking tendency of the former. AFM studies of drop-cast films of different ionene-acceptor combinations revealed that compact folded structures are formed most effectively under conditions in which the strongest CT complex is formed.

Pulsed laser deposition film of a donor-acceptor-donor polymer as possible active layer in devices

A molecule having a ketone group between two thiophene groups was synthesized. Presence of alternating electron donating and accepting moieties gives this material a donor-acceptor-donor (DAD) architecture. PolyDAD was synthesized from DAD monomer by oxidative polymerization. Device quality films of polyDAD were fabricated using pulsed laser deposition technique. X-ray photoelectron spectroscopy (XPS) and fourier transform infrared spectra (FTIR) data of both as synthesized and film indicate the material does not degrade during ablation. Optical band gap was determined to be about 1.45 eV. Four orders of magnitude increase in conductivity was observed from as synthesized to pulsed laser deposition (PLD) fabricated film of polyDAD. Annealing of polyDAD films increase conductivity, indicating better ordering of the molecules upon heating. Rectifying devices were fabricated from polyDAD, and preliminary results are discussed.

Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study

In this paper, we have computed the quadratic nonlinear optical (NLO) properties of a class of weak charge transfer (CT) complexes. These weak complexes are formed when the methyl substituted benzenes (donors) are added to strong acceptors like chloranil (CHL) or di-chloro-di-cyano benzoquinone (DDQ) in chloroform or in dichloromethane. The formation of such complexes is manifested by the presence of a broad absorption maximum in the visible range of the spectrum where neither the donor nor the acceptor absorbs. The appearance of this visible band is due to CT interactions, which result in strong NLO responses. We have employed the semiempirical intermediate neglect of differential overlap (INDO/S) Hamiltonian to calculate the energy levels of these CT complexes using single and double configuration interaction (SDCI). The solvent effects are taken into account by using the self-consistent reaction field (SCRF) scheme. The geometry of the complex is obtained by exploring different relative molecular geometries by rotating the acceptor with respect to the fixed donor about three different axes. The theoretical geometry that best fits the experimental energy gaps, beta(HRS) and macroscopic depolarization ratios is taken to be the most probable geometry of the complex. Our studies show that the most probable geometry of these complexes in solution is the parallel displaced structure with a significant twist in some cases. (C) 2011 American Institute of Physics. doi:10.1063/1.3526748]

Characterization and Acceptor Preference of a Soluble Meningococcal Group C Polysialyltransferase

Vaccines against Neisseria meningitidis group C are based on its alpha-2,9-linked polysialic acid capsular polysaccharide. This polysialic acid expressed on the surface of N. meningitidis and in the absence of specific antibody serves to evade host defense mechanisms. The polysialyltransferase (PST) that forms the group C polysialic acid (NmC PST) is located in the cytoplasmic membrane. Until recently, detailed characterization of bacterial polysialyltransferases has been hampered by a lack of availability of soluble enzyme preparations. We have constructed chimeras of the group C polysialyltransferase that catalyzes the formation alpha-2,9-polysialic acid as a soluble enzyme. We used site-directed mutagenesis to determine the region of the enzyme necessary for synthesis of the alpha-2,9 linkage. A chimera of NmB and NmC PSTs containing only amino acids 1 to 107 of the NmB polysialyltransferase catalyzed the synthesis of alpha-2,8-polysialic acid. The NmC polysialyltransferase requires an exogenous acceptor for catalytic activity. While it requires a minimum of a disialylated oligosaccharide to catalyze transfer, it can form high-molecular-weight alpha-2,9-polysialic acid in a nonprocessive fashion when initiated with an alpha-2,8-polysialic acid acceptor. De novo synthesis in vivo requires an endogenous acceptor. We attempted to reconstitute de novo activity of the soluble group C polysialyltransferase with membrane components. We found that an acapsular mutant with a defect in the polysialyltransferase produces outer membrane vesicles containing an acceptor for the alpha-2,9-polysialyltransferase. This acceptor is an amphipathic molecule and can be elongated to produce polysialic acid that is reactive with group C-specific antibody.

Photoinduced electron transfer in zinc porphyrin-crown ether supramolecule with Eu3+ electron acceptor in crown void

Photoinduced electron transfer (ET) reactions in the zinc porphyrin-crown ether (ZnPCE) supramolecule, in which one crown ether moiety containing Eu3+ as electron acceptor is covalently linked to zinc porphyrin (ZnP), were studied by flash photolysis. In methanol solutions, highly efficient charge separation occurs via intramolecular ET from (ZnP)-Zn-3 to Eu3+ encapsulated in the crown ether void (k(1) = (3 +/- 1) X 10(3) s(-1)) and intramolecular ET from 3ZnP to uncomplexed Eu2+ (k(2) = (2.5 +/- 0.5) X 10(3) s(-1)). Intermolecular ET from Eu2+ escaped from the crown ether void to ZnP.+ (k(tau) = (4.3 +/- 0.7) X 10(8) M(-1) s(-1)) seems to be the main pathway of charge recombination.

Electron donor-acceptor complexes of I-2 with diethyl ether and diethyl sulphide. An ab initio MO study

The charge-transfer complexes of I-2 with the n-donors diethyl ether and diethyl sulfide were studied at the Hartree-Fock and MP2 levels. The structures were fully optimized using the 3-21G((*)) basis set as well as with effective core potentials. The calculations consistently yield a C-2v structure for the ether-I-2 complex, but an unsymmetrical form for the sulfide-I-2 complex. A natural bond orbital analysis and the BSSE-corrected complexation energies reveal stronger interactions in the sulfide complex. The computed orbital energies of the monomers and complexes reproduce the trends in experimentally observed vertical ionization potentials.

Electron donor-acceptor complex of ICl with diethyl ether - He I photoelectron spectroscopy and ab initio molecular orbital study

The He I photoelectron spectrum of the diethyl ether-ICl complex has been obtained. The oxygen orbitals are shifted to higher binding energies and that of ICl to lower binding energies owing to complex formation. Ab initio molecular orbital (MO) calculations of the complex molecule showed that the bonding is between the sigma-type lone pair of oxygen and the I atom and that the complex has C-2v symmetry. The binding energy of the complex is computed to be 8.06 kcal mol(-1) at the MP2/3-21G* level. The orbital energies obtained from the photoelectron spectra of the complex are compared and assigned with orbital energies obtained by MO calculations. Natural bond orbital analysis (NBO) shows that charge transfer is from the sigma-type oxygen lone pair to the iodine atom and the magnitude of charge transfer is 0.0744 e.

Di- and tri-metallic complexes containing two bridging cyclodiphosphazane ligands: the crystal structure of [MO2(CO)8{µ-cis-[PhNP(OC6H4Me-p)]2}2]

The reactions of the mononuclear cyclodiphosphazane complexes, cis-[Mo(CO)(4){cis-[PhNP(OR)](2)}(2)] with [Mo(CO)(4)(nbd)] (nbd = norbornadiene). [Mo(CO)(4)(NHC5H10)(2)] or [MCl(2)(cod)] (cod = cycloocta-1,5-diene) afforded the homobimetallic complexes; [Mo-2(CO)(8){mu-cis-[PhNP(OR)](2)}(2)] (R = C(5)H(4)Me-p 5 or CH2CF3 6) or the heterobimetallic complexes. [Mo-2(CO)(8){mu-cis-[PhNP(OE)](2)}(2)MCl(2)] (R = C(6)H(4)Me-p; M = Pd 7 or Pt 8). In all the above complexes, the two metal moieties are bridged by two cyclodiphosphazane ligands. The reactions of the mononuclear complexes, cis-[M(CO)(4)(A){cis-[PhNP(OC(6)H(4)Me-p)](2)}] with (M'Cl-2(cod)] afforded the trinuclear complexes, cis-[M'Cl-2[M(CO)(4)(A){cis-[PhNP(OC(6)H(4)Me-p)](2)}](2)] (M' = Pd, M = Mo, A = P(OMe)(3) 10; M' = Pt, M = Mo. A = P(OMe)(3) 11; M' = Pd. M = W. A = NHC5H10 12; M' = Pt, M = W. A = NHC5H10 13). The structure of the complex 5 has been determined by single-crystal X-ray crystallography.

UVPES and ab initio molecular orbital studies on the electron donor-acceptor complexes of bromine with methylamines

The He I photoelectron spectra of bromine, methylamine, and their complex have been obtained, and the spectra show that lone-pair orbital energy of nitrogen in methylamine is stabilized by 1.8 eV and the bromine orbital energies are destabilized by about 0.5 eV due to complexation. Ab initio calculations have been performed on the charge-transfer complexes of Br-2 with ammonia and methyl-, dimethyl-, and trimethylamines at the 3-21G*, 6-311G, and 6-311G* levels and also with effective core potentials. Calculations predict donor and acceptor orbital energy shifts upon complexation, and there is a reasonable agreement between the calculated and experimental results. Complexation energies have been corrected for BSSE. Frequency analysis has confirmed that ammonia and trimethylamine form complexes with C-3v symmetry and methylamine and dimethylamine with C-s symmetry. Calculations reveal that the lone-pair orbital of nitrogen in amine and the sigma* orbital of Br-2 are involved in the charge-transfer interaction. LANL1DZ basis seems to be consistent and give a reliable estimate of the complexation energy. The computed complexation energies, orbital energy shifts, and natural bond orbital analysis show that the strength of the complex gradually increases from ammonia to trimethylamine.

N-3-benzoyl-2 ',3 '-di-O-benzoyluridine

The structure of N-3-benzoyl-2',3'-di-O-benzoyluridine, C30H24N2O9, has two molecules in the asymmetric unit. The uracil bases of both the molecules are in the anti conformation with respect to the ribose moiety and the furanosyl rings adopt a C3'-endo conformation. The orientation about the C4'-C5' bond is gauche-gauche. The two crystallographically independent molecules are linked through several C-H ... O hydrogen bonds. The nucleoside molecules pack as columns along the a axis and these columns repeat along the c axis.

Synthesis and Self-Assembly of Donor-Acceptor-Donor Based Oligothiophenes and Their Optoelectronic Properties

In this work, the synthesis of an oligothiophene having a donor acceptor donor (D-A-D) chromophore with hydrogen bonding groups is described. The D-A-D molecule was demonstrated to self-organize via intermolecular H-bonding between barbituric acid units. Interactions between the oligothiophene subunits were also found to be important, affording nanoribbons that could be observed by atomic force and transmission electron microscopy. The applicability of the oligothiophene for organic electronic applications was investigated by fabricating organic field-effect transistors (OFETs) and organic photovoltaic devices. The OFET measurements yielded p-type mobility of 7 x 10(-7) cm(2)/(Vs), and when blended with C(60)-PCBM, the photovoltaic efficiency was observed to be 0.18%.

Conformational basis for substrate recognition and regulation of catalytic activity in Staphylococcus aureus nucleoside di-phosphate kinase

Nucleoside diphosphate kinases (NDK) are characterized by high catalytic turnover rates and diverse substrate specificity. These features make this enzyme an effective activator of a pro-drug an application that has been actively pursued for a variety of therapeutic strategies. The catalytic mechanism of this enzyme is governed by a conserved histidine that coordinates a magnesium ion at the active site. Despite substantial structural and biochemical information on NDK, the mechanistic feature of the phospho-transfer that leads to auto-phosphorylation remains unclear. While the role of the histidine residue is well documented, the other active site residues, in particular the conserved serine remains poorly characterized. Studies on some homologues suggest no role for the serine residue at the active site, while others suggest a crucial role for this serine in the regulation and quaternary association of this enzyme in some species. Here we report the biochemical features of the Staphylococcus aureus NDK and the mutant enzymes. We also describe the crystal structures of the apo-NDK, as a transition state mimic with vanadate and in complex with different nucleotide substrates. These structures formed the basis for molecular dynamics simulations to understand the broad substrate specificity of this enzyme and the role of active site residues in the phospho-transfer mechanism and oligomerization. Put together, these data suggest that concerted changes in the conformation of specific residues facilitate the stabilization of nucleotide complexes thereby enabling the steps involved in the ping-pong reaction mechanism without large changes to the overall structure of this enzyme. (C) 2011 Elsevier B.V. All rights reserved.