Raman studies of solutions of single-wall carbon nanotube salts

Polyelectrolyte solutions of Na-doped single-wall carbon nanotube (SWNT) salts are studied by Raman spectroscopy. Their Raman signature is first compared to undoped SWNT suspensions and dry alkali-doped SWNT powders, and the results indicate that the nanotube solutions consist of heavily doped (charged) SWNT. Raman signature of doping is then used to monitor in situ the oxidation reaction of the nanotube salt solutions upon exposure to air and to an acceptor molecule (benzoquinone). The results indicate a direct charge-transfer reaction from the acceptor molecule to the SWNT, leading to their gradual charge neutralization and eventual precipitation in solution. The results are consistent with a simple redox titration process occurring at the thermodynamical equilibrium.     

Optical sensor for the visual detection of mercury using mesoporous silica anchoring porphyrin moiety

A low cost, solid optical sensor for the rapid detection of low concentrations of Hg2+ in aqueous media was prepared by the monolayer functionalization of mesoporous silica with 5,10,15,20-tetraphenylporphinetetrasulfonic acid (TPPS), anchored by N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAC). The detection is based on the color change of TPPS from orange to green as a result of the formation of a charge-transfer complex with Hg2+. The intensity of the charge-transfer band varies linearly with Hg2+ in the concentration range from zero to 2.5 x 10(-7) mol dm(-3). The lower detection limit observed for Hg2+ concentration is 1.75 x 10(-8) mol dm(-3). The material exhibits good chemical and mechanical stability, and did not show any degradation of TPPS for a period of eight months. The sensor was applied for the analysis of various environmental samples. The effects of pH, sample volume, reaction time, amount of material, and the presence of foreign ions on the detection method are discussed.     

Use of UV-vis reflection spectroscopy for determining the organization of viologen and viologen tetracyanoquinodimethanide monolayers

UV-vis reflection spectroscopy has been used for proving in situ the organization of pure viologen and hybrid viologen tetracyanoquinodimethanide monolayers at the air-water interface. Other more classical measurements concerning Langmuir monolayers, including surface pressure-area and surface potential-area isotherms, are also provided. The organization of the viologen in the Langmuir monolayer was investigated upon the different states of compression, and the tilt angle of the viologen moieties with respect to the water surface was determined. A gradual transition of the viologen molecules from a flat orientation in the gas phase to a more tilted position with respect to the water surface in the condensed phases occurs. The addition of a tetracyanoquinodimethane (TCNQ) salt in the subphase leads to the penetration of TCNQ anions into the positively charged viologen monolayer forming a hybrid viologen tetracyanoquinodimethanide film where a charge-transfer interaction between the two moieties is observed. From a quantitative analysis of the reflection spectra, an organization model of these hybrid monolayers at the air-water interface is proposed, suggesting a parallel arrangement of viologen and TCNQ units with a 1:2 stoichiometry.     

Covalent attachment and hybridization of DNA oligonucleotides on patterned single-walled carbon nanotube films

DNA oligonucleotides were covalently immobilized to prepatterned single-walled carbon nanotube (SWNT) multilayer films by amidation. SWNT multilayer films were constructed via consecutive condensation reactions creating stacks of functionalized SWNT layers linked together by 4,4'-oxydianiline. Aminated- or carboxylated-DNA oligonucleotides were covalently immobilized to the respective carboxylated or aminated SWNT multilayer films through amide bond formation using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. UV-vis-NIR spectroscopic analysis indicated that the SWNT film surface density increased uniformly according to the number of reaction cycles. Scanning electron microscopy and contact angle measurements of the SWNT multilayer film revealed a uniform coverage over the substrate surface. The covalent attachment of DNA oligonucleotides to the SWNT multilayer films and their subsequent hybridization with complementary oligonucleotides were verified using X-ray photoelectron spectroscopy and fluorescence-based measurements. This is the first report demonstrating that DNA oligonucleotides can be covalently attached to immobilized SWNT multilayer films. The anchored DNA oligonucleotides were shown to exhibit excellent specificity, realizing their potential in future biosensor applications.     

Luminescence quenching by DNA-bound viologens: effect of reactant identity on efficiency and dynamics of electron transfer in DNA

Photoinduced electron transfer from two intercalating photoactive donors, Ru(phen)2dppz2+ and ethidium, to intercalating viologen acceptors of the N,N'-dialkyl-6-(2'-pyridiniumyl)phenanthridinium family has been investigated through steady-state and time-resolved luminescence quenching measurements. Efficient quenching of the emission from these donors bound to DNA is observed at low concentrations of acceptor (1-10 eq.), and in time-resolved emission experiments it is determined that electron transfer occurs on the nanosecond time scale. Furthermore, transient absorption measurements confirm that the quenching is the result of a charge-transfer process; upon photoreaction of intercalated Ru(phen)2dppz2+ with a viologen acceptor, an intermediate with spectral properties resembling the expected charge-separated pair is observed. The quenching yields and kinetics obtained with this quencher are in marked contrast to those observed with these same donors paired with Rh(phi)2bpy3+ as an acceptor. The differing efficiencies of electron transfer for these donor/acceptor pairs bound to DNA as compared to others previously described are discussed qualitatively in terms of the structural and electronic properties of the different reactants.     

A heterowheel [3]pseudorotaxane by integrating β-cyclodextrin and cucurbit[8]uril inclusion complexes

A heterowheel [3]pseudorotaxane was prepared by integrating two binary inclusion complexes of β-cyclodextrin-hydroxynaphthalene (β-CD·3) with a cucurbit[8]uril-viologen derivative (CB[8]·2), in which simultaneous molecular recognition of the adamantine moiety in 2 by β-CD and the charge-transfer interaction of 3 with the viologen nucleus of 2 in the cavity of CB[8] are two crucial factors for the formation of the quaternary complex.     

Photochemical reactions of fac-[Mn(CO)3(phen)imidazole]+: evidence for long-lived radical species intermediates

The electronic absorption spectrum of fac-[Mn(CO)(3)(phen)imH](+), fac-1 in CH(2)Cl(2) is characterized by a strong absorption band at 378 nm (epsilon(max) = 3200 mol(-1) L cm(-1)). On the basis of quantum mechanical calculations, the visible absorption band has been assigned to ligand-to-ligand charge-transfer (LLCT, im-->phen) and metal-to-ligand charge-transfer (MLCT, Mn-->phen) charge transfer transition. When fac-1 in CH(2)Cl(2) is irradiated with 350 nm continuous light, the absorption features are gradually shifted to represent those of the meridional complex mer-[Mn(CO)(3)(phen)imH](+), mer-1 (lambda(max) = 556 nm). The net photoreaction under these conditions is a photoisomerization, although, the presence of the long-lived radical species was also detected by (1)H NMR and FTIR spectroscopy. 355 nm continuous photolysis of fac-1 in CH(3)CN solution also gives the long-lived intermediate which is readily trapped by metylviologen (MV(2+)) giving rise to the formation of the one-electron reduced methyl viologen (MV(*+)). The UV-vis spectra monitored during the slow (45 min) thermal back reaction exhibited isosbestic conversion at 426 nm. On the basis of spectroscopic techniques and quantum mechanical calculations, the role of the radicals produced is analyzed.     

Zeolite-mediated photochemical charge separation using a surface-entrapped ruthenium-polypyridyl complex

Employing the strategy of quaternization of the 2,2' N atoms of the conjugated bipyridine ligand 1,4-bis[2-(4'-methyl-2,2'-bipyrid-4-yl)ethenyl]benzene (L), a polypyridyl complex of ruthenium(II) was tethered on the surface of zeolite Y. Electrochemical and spectroscopic properties of the complex suggest that, upon visible photoexcitation of the MLCT band, the electron is localized on the conjugated ligand rather than the bipyridines. Electron transfer from the surface complex to bipyridinium ions (methyl viologen) within the zeolite was observed. Visible light photolysis of the ruthenium-zeolite solid ion-exchanged with diquat and suspended in a propyl viologen sulfonate solution led to permanent formation of the blue propyl viologen sulfonate radical ion in solution. The model that is proposed involves intrazeolitic charge transfer to ion-exchanged diquat followed by interfacial (zeolite to solution) electron transfer to propyl viologen sulfonate in solution. Because of the slow intramolecular back-electron-transfer reaction and the forward electron propagation via the ion-exchanged diquat, Ru(III) is formed. This Ru(III) complex formed on the zeolite is proposed to react rapidly with water in the presence of light, followed by reaction with the propyl viologen sulfonate, to form pyridones and regeneration of Ru(II), which then continues the photochemical process.     

Spectrophotometric study of the charge-transfer complexes of iodine with antipyrine in organic solvents

The charge-transfer complex formation of iodine with antipyrine has been studied spectrophotometrically in chloroform, dichloromethane (DCM) and 1,2-dichloroethane (DCE) solutions at 25 degrees C. The results indicate the formation of 1:1 charge-transfer complexes. The observed time dependence of the charge-transfer band and subsequent formation of I(3)(-) in solution were related to the slow transformation of the initially formed 1:1 antipyrine:I(2) outer complex to an inner electron donor-acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion. The values of the equilibrium constant, K, are calculated for each complex and the influence of the solvent properties on the formation of EDA complexes and the rates of subsequent reaction is evaluated.     

Viologen-calix[6]arene pseudorotaxanes. Ion-pair recognition and threading/dethreading molecular motions

A calix[6]arene wheel, whose cavity has been extended and rigidified by N-phenylureido groups on the upper rim, forms pseudorotaxane species with molecular axles containing the viologen (4,4'-bipyridinium) unit in CH(2)Cl(2) solution. In these conditions, the self-assembly process is very efficient, with associated DeltaG degrees values of around -8 kcal mol(-1). The counteranions of the bipyridinium-based threads play indeed an important role in the formation of the complex. The use of either tosylate or hexafluorophosphate salts of the guests affects both the stability of the complexes and the rate of the threading process. Such effects have been interpreted in terms of ion-pair recognition, suggesting that coordination of the counteranions of the viologen thread by the ureido groups of the calixarene wheel is crucial for the breaking of tight ion pairs prior to threading. The rate constants of the threading/dethreading reactions coupled with the redox processes of the viologen unit of the axle have been obtained by means of cyclic voltammetry. The pseudorotaxane species undergo fast dethreading (submicrosecond time scale) on electrochemical reduction of the guest. The heterogeneous electron-transfer kinetics for the reduction of the viologen unit is slowed upon encapsulation into the calixarene cavity.     

Self-inclusion behavior and circular dichroism of aliphatic chain-linked beta-cyclodextrin-viologen compounds and their reduced forms depending on the side of modification

[Reaction: see text]. The self-inclusion behavior and induced circular dichroism (ICD) characteristics of two beta-cyclodextrin (beta-CD) derivatives, in which a 1-methyl-4,4'-bipyridinium (viologen) group is connected by an octamethylene chain to either the primary (2(2+)) or secondary (3(2+)) side of beta-CD, and of their reduced forms, are investigated. 1H NMR studies showed that 2(2+) forms an intramolecular self-inclusion complex with K(in) = 3.1 +/- 0.4, whereas 3(2+) forms a head-to-head type of dimer with K(D) = 65 +/- 10 M(-1) at 25 degrees C. 2(2+) and 3(2+) form [2]pseudorotaxanes with alpha-CD, with the secondary side of the alpha-CD facing the viologen moiety. The ICD characteristics of mono-6-[4-(1-methyl-4-pyridinio)-1-pyridinio]-beta-CD (1(2+)), 2(2+), 3(2+), and methyloctyl viologen-beta-CD complexes were obtained for the oxidized and reduced states of the viologen units. The results indicated dimer formation for 1 degrees , and intramolecular complexation for 2*+ and 2 degrees in which the reduced viologen units are outside the beta-CD cavity. The results also indicated intramolecular complexation for 3*+ and 3 degrees, but with reduced viologen units inside the cavity. This work provides unequivocal evidence of the preference of the secondary side of cyclodextrins for viologen groups, regardless of their oxidation states, and the dependence of ICD of the viologen chromophores on their location with respect to the CD cavity.     

Spectroscopic studies of the reaction of iodine with 2,3-diaminopyridine

The charge-transfer interaction of 2,3-diaminopyridine (DAPY) and iodine has been investigated spectrophotometrically in the solvents chloroform and dichloromethane at room temperature. The results indicate the formation of 1:2 charge-transfer complex in each solvent with the observation of the two characteristic absorptions for triiodide ion around 355 and 295 nm. The iodine complex is formulated as [(DAPY)I]+.I3-. The formation of the triiodide ion, I3-, is further confirmed by the observation of the characteristic bands for non-linear I3- ion with C2v symmetry at 151 and 132 cm(-1) assigned to nu(as)(I-I) and nu(s)(I-I) of the I-I bonds and at 61 cm(-1) due to bending delta(I3-). The mid infrared spectra of (DAPY) and triiodide complex are also obtained and assigned.     

Elektronendonator-Akzeptor-Komplexe von Diazonium-Ionen und die Struktur von stabilisierten Diazonium-Salzen. 20. Mitteilung zur Kenntnis der Azokupplungsreaktion

1. In 0.1N HCl/H₂O, o- and p-nitrobenzenediazonium ions rapidly form a complex with 2-naphthol-6, 8-disulphonic acid anions. Visible and NMR, spectra show that it has the structure of a charge-transfer complex (π-complex). The latter is probably an intermediate in the electrophilic aromatic substitution (diazo coupling reaction). 2. Diazonium ions form charge-transfer complexes with naphthalene, 1-methylnaphthalene, naphthalene-1-sulphonic acid, 1-naphthyl-methanesulphonic acid and also 2-naphthol-1-sulphonic acid. The equilibrium constants of all these complexes have been determined. 3. The stabilisation of diazonium salts by arylsulphonic acids with regard to decomposition is due to charge-transfer complex formation and not to formation of diazosulphonates as assumed by former investigators. The sulphonic group is not essential for the stabilisation. 4. Charge-transfer complex formation decreases the electrophilicity of the diazonium ion (rate of diazo coupling reaction) only slightly.     

Control of Photoinduced Electron Transfer from Zinc-Porphyrin to Methyl Viologen by Supramolecular Formation between Monoclonal Antibody and Zinc-Porphyrin

Photoinduced electron transfer from tetrakis(4-carboxy-phenyl porphyrin)-zinc complex (Zn-TCPP) to an acceptor molecule (methyl viologen; MV2+) has been found to be controlled by the complex formation of monoclonal antibody 03-1 for the porphyrin (TCPP) and Zn-TCPP. Although there are no ground-state interactions between Zn-TCPP and MV2+ for a 2:1 complex of antibody 03-1 and Zn-TCPP, the fluorescence of Zn-TCPP is quenched by the addition of MV2+. The Stern-Volmer plots and emission lifetime studies show that there is a long-range electron transfer through the antibody 03-1.     

Complex formation of methyl viologen and diquat with macrocyclic ether dibenzo-24-crown-8

The complex formation of a coplanar N,N'-ethylene-bipyridinium dication and dibenzo-24-crown-8 in methanol was detected by electrochemical techniques. It is demonstrated here that a coplanar ring arrangement is not required for an efficient charge-transfer interaction between bipyridinium and crown. The much stronger complexation of methyl viologen was investigated by both electrochemical and spectral techniques. The presence of a benzene ring in the crown molecule is a condition for complex formation. The described complex exerts catalytic activity for the reduction of oxygen.     

Synthesis,Structure and Photoluminescence of （CH_3-4,4＇-H_2bipy）-（HgCl_4） with CH_3-4,4＇-H_2bipy Generated in Situ

A novel viologen(4,4'bipyridinium)-based complex (CH3-4,4'-H2bipy)(HgCl4) (1),in which the CH3-4,4'-H2bipy (MQ2+) was generated in situ,has been synthesized via hydrothermal reaction and structurally characterized. Complex 1 crystallizes in the space group P21/c of monoclinic system with four formula units in a cell:a = 8.1848(6),b = 21.809(2),c = 9.0285(6) ,β = 107.377(1)°,V = 1538.0(2) 3,C11H11Cl4HgN2,Mr = 513.61,Dc = 2.218 g/cm3,S = 1.009,μ(MoKα) = 10.685 mm-1,F(000) = 956,R = 0.0360 and wR = 0.0812. The crystal structure analysis reveals that the title complex features an isolated structure based on a CH3-4,4'-H2bipy moiety and a mercury atom terminally bound by four chlorine atoms. Photoluminescence investigation reveals a strong emission in blue region,which may originate from π→π* charge-transfer interaction of the CH3-4,4'-H2bipy moiety.     

Synthesis, Structure and Photoluminescence of(CH3-4, 4'-H2bipy)-(HgCl4)with CH3-4, 4'-H2bipy Generated in Situ

A novel viologen(4,4'bipyridinium)-based complex(CH3-4,4'-H2bipy)(HgCl4)(1),in which the CH3-4,4'-H2bipy(MQ2+)was generated in situ,has been synthesized via hydrothermal reaction and structurally characterized.Complex 1 crystallizes in the space group P21/c of monoclinic system with four formula units in a cell:a = 8.1848(6),b = 21.809(2),c = 9.0285(6)(A),β =.107.377(1)°,V= 1538.0(2)(A)3,C11H11Cl4HgN2,Mr = 513.61,Dc = 2.218 g/cm3,S = 1.009,μ(MoKα)= 10.685 mm-1,F(000)= 956,R = 0.0360 and wR = 0.0812.The crystal structure analysis reveals that the title complex features an isolated structure based on a CH3-4,4'-H2bipy moiety and a mercury atom terminally bound by four chlorine atoms.Photoluminescence investigation reveals a strong emission in blue region,which may originate from π→π* charge-transfer interaction of the CH3-4,4'-H2bipy moiety.     

Synthesis and supramolecular properties of molecular clips with anthracene sidewalls

Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.     

Interaction of single-walled carbon nanotubes with poly(propyl ether imine) dendrimers

We study the complexation of nontoxic, native poly(propyl ether imine) dendrimers with single-walled carbon nanotubes (SWNTs). The interaction was monitored by measuring the quenching of inherent fluorescence of the dendrimer. The dendrimer-nanotube binding also resulted in the increased electrical resistance of the hole doped SWNT, due to charge-transfer interaction between dendrimer and nanotube. This charge-transfer interaction was further corroborated by observing a shift in frequency of the tangential Raman modes of SWNT. We also report the effect of acidic and neutral pH conditions on the binding affinities. Experimental studies were supplemented by all atom molecular dynamics simulations to provide a microscopic picture of the dendrimer-nanotube complex. The complexation was achieved through charge transfer and hydrophobic interactions, aided by multitude of oxygen, nitrogen, and n-propyl moieties of the dendrimer.     

Nanowires of metal-organic complex by photocrystallization: a system to achieve addressable electrically bistable devices and memory elements

A new method has been achieved to form a Cu:benzoquinone derivative (DDQ) charge-transfer complex by the photoexcitation of [Cu(DDQ)2(CH 3COO)2] ( 1) that has been synthesized by the reaction of DDQ and hydrated cupric acetate in acetonitrile. Photoexcitation of coordinated complex 1 leads to the formation of charge-transfer complex Cu2+(DDQ(.-)2 ( 2). The charge transfer complex 2, when spun on solid substrates, forms nanowires. Sandwich structures of 2 exhibit electrical bistability associated with memory phenomenon. Read-only and random-access memory phenomena are evidenced in nanowires of 2 providing a route to attend the issues pertaining to the addressibility of organic memory devices.