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Polar Solvents (polar + solvent)
Selected AbstractsChlorophyll a Self-assembly in Polar Solvent,Water Mixtures ,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 1 2000Radka Vladkova ABSTRACT The conversion of chlorophyll a (Chl a) monomers into large aggregates in six polar solvents upon addition of water has been studied by means of absorption, fluorescence spectroscopy and fluorescence lifetime measurements for the purpose of elucidating the various environmental factors promoting Chl a self-assembly and determining the type of its organization. Two empirical solvent parameter scales were used for quantitative characterization of the different solvation properties of the solvents and their mixtures with water. The mole fractions of water f1/2 giving rise to the midpoint values of the relative fluorescence quantum yield were determined for each solvent, and then various solvent,water mixture parameters for the f1/2 values were compared. On the basis of their comparison, it is concluded that the hydrogen-bonding ability and the dipole,dipole interactions (function of the dielectric constant) of the solvent,water mixtures are those that promote Chl a self-assembly. The influence of the different nature of the nonaqueous solvents on the Chl aggregation is manifested by both the different water contents required to induce Chl monomer , aggregate transition and the formation of two types of aggregates at the completion of the transition: species absorbing at 740,760 nm (in methanol, ethanol, acetonitrile, acetone) and at 667,670 nm (in pyridine and tetrahydrofuran). It is concluded that the type of Chl organization depends on the coordination ability and the polarizability (function of the index of refraction) of the organic solvent. The ordering of the solvents with respect to the f1/2 values,methanol < ethanol < acetonitrile < acetone < pyridine < tetrahydrofuran,yielded a typical lyotropic (Hofmeister) series. On the basis of this solvent ordering and the disparate effects of the two groups of solvents on the Chl a aggregate organization, it is pointed out that the mechanism of Chl a self-assembly in aqueous media can be considered a manifestation of the Hofmeister effect, as displayed in the lipid-phase behavior (Koynova et al., Eur. J. Biophys. 25, 261,274, 1997). It relates to the solvent ability to modify the bulk structure and to distribute unevenly between the Chl,water interface and bulk liquid. [source] Formation of New Phosphates from Aldehydes by a DBU-Catalyzed Phospha-Brook Rearrangement in a Polar Solvent.CHEMINFORM, Issue 5 2006Laurent El Kaim No abstract is available for this article. [source] Enol Forms of 1,3-Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion-Assisted InterconversionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2010Mark Sigalov Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecularO,H···N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O,H···N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O,···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto,enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods. [source] Monocyclopentadienyl Phenoxido,Amino and Phenoxido,Amido Titanium Complexes: Synthesis, Characterisation, and Reactivity of Asymmetric Metal Centre DerivativesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2008Giuseppe Alesso Abstract Reduction of phenol,imine derivatives R,N=CH(3,5-R2C6H2 -2-OH) (R = tBu; R, = C6H51a, p -MeC6H41b, Cy 1c, tBu 1d, 2,6-Me2C6H31e; R = H; R, = p -MeC6H41f; Cy = cyclohexyl) with MBH4 (M = Li, Na) or AlLiH4 in ethyl ether or thf at room temperature affords the phenol,amine compounds R,NHCH2(3,5-R2C6H2 -2-OH) 2a,c and 2e,f. The N -R-[2,4-di- tert -butyl]benzo-1-oxa-3-azine species (R = tBu 2d1, 2,6-Me2C6H32e1) are obtained by Mannich reaction of 2,4-di- tert -butylphenol with RNH2 in refluxing methanol. Intermediate 2d1 is converted in ethanol at room temperature into N - tert -butyl[2-hydroxy-3,5-di- tert -butyl]benzylamine (2d), whereas 2e is not obtained from 2e1 by using this procedure.N -alkyl,N - tert -butyl[2-hydroxy-3,5-di- tert -butyl]benzylaminecompounds tBuN(R)CH2(3,5- tBu2C6H2 -2-OH) (R = Me 2g, Et 2h, nPr 2i, CH2Ph 2j) are also prepared by the appropriate synthetic method. Treatment of 2a,c with 1 equiv. of TiCpCl3 in the presence of 2.5 equiv. of NEt3 in hexane at room temperature gives the monocyclopentadienyl phenoxido,amido monochloride complexes TiCp[R,NCH2(3,5- tBu2C6H2 -2-O)]Cl (R, = C6H53a, R, = p -MeC6H43b, R, = Cy 3c). The analogous complex Ti(,5 -C5H4SiMe2Cl)[C6H5NCH2(3,5- tBu2C6H2 -2-O)]Cl (4a) results from the reaction of 2a with Ti(,5 -C5H4SiMe2Cl)Cl3. Nevertheless, 2d reacts with TiCpCl3 in hexane in the presence of NEt3 at room temperature yielding the monocyclopentadienyl phenoxido dichloride compound TiCp[tBuNHCH2(3,5- tBu2C6H2 -2-O)]Cl2 (5), whereas in ethyl ether and in the absence of NEt3 adduct 5·HCl is obtained, which is further converted into TiCp[tBuNCH2(3,5- tBu2C6H2 -2-O)]Cl (3d) by addition of a NEt3/ethyl ether solution. The reaction of TiCpCl3 with 2a in the presence of 2.5 equiv. of NEt3 in a polar solvent (thf, CH2Cl2 or toluene) at room temperature affords TiCp[Ph(H)NCH2(3,5- tBu2C6H2 -2-O)]Cl (6a) as a mixture of two stereoisomers. All the reported compounds were characterised by the usual analytical and spectroscopic methods and the molecular structures of 2a, 2d, 2e and 3d were determined by X-ray diffraction analysis from suitable single crystals. Preliminary studies of catalytic activity for ethylene polymerisation by using solid methylaluminoxane as cocatalyst were performed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Palladium-Catalyzed Preparation of Propargylic or Allenylic Sulfides from Propargyl Halides or Mesylate and ThiolsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 3 2004Ken Tsutsumi Abstract In the presence of a catalytic amount of Pd0 -dppe complex [dppe: 1,2-bis(diphenylphosphanyl)ethane], generated in situ from [Pd2(dba)3·CHCl3] and dppe, propargylic bromide 1a reacted with an equimolar amount of propanethiol at 60 °C in DMF to afford propargylic sulfide 2 in an excellent yield. The reaction occurs readily when carried out in the presence of the weak base triethylamine. The choice of both the phosphane, which is employed as the palladium atom's ligand, and the solvent have a remarkable effect on this reaction. We found that the optimum conditions for the reaction are those using a bidentate phosphane ligand (dppe) in a polar solvent (DMF). Compound 1a reacted smoothly with both aromatic (PhSH) and secondary thiols (CySH) in high yields. The reactions with thiols bearing functional groups (OH or Cl) proceeded selectively in good to moderate yields. Primary chlorides 1b,e were readily converted into their corresponding propargylic sulfides 7,10 in high yields. The Pd0 -dppe catalyst was ineffective in the reaction of the bromide 1g bearing a tBu group at the propargylic position, but the reaction of the corresponding mesylate 1h using the Pd0 -DIOP catalyst [DIOP = O -isopropylidene-2,3-dihydroxy-1,4-bis(diphenylphosphanyl)butane] at 100 °C afforded the product 11 in good yield. Allenylic sulfides were obtained from 1g,i. We suggest that a cationic ,3 -type complex may be a more reactive intermediate in this catalytic reaction than neutral ,1 - or ,3 -allenyl/propargylpalladium complexes. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Ab initio QM/MM study of excited state electron transfer between pyrene and 4,4,-bis(dimethylamino)-diphenylmethane with different solvent systems: Role of hydrogen bonding within solvent moleculesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2005Kakali Sen Abstract The exciplex is a charge transfer species formed in the process of electron transfer between an electron donor and an electron acceptor and hence is very sensitive to solvent polarity. In order to understand the role of solvent in exciplex formation between pyrene (PY) and 4,4,-bis(dimethylamino)diphenylmethane (DMDPM), we used two types of solvent approximations: an implicit solvent model and an explicit solvent model. The difference in energies between the excited and the meta-stable Frank,Condon state (,E) of the structures were assumed to correspond to the emission maximum of the exciplex in different solvents. The ,E values show the trend of stabilization of the exciplex with an increase in solvent polarity. This trend in stabilization is substantially more prominent in the explicit solvent model than that with the implicit solvent model. The ,E value obtained in methanol reflects equal stabilization compared to that in a more polar solvent, N,N-dimethylformamide. This extra stabilization of the exciplex may be explained on the basis of the H-bonding capability of the protic solvent, methanol. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene,tetracyanoethylene complexINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2003Hai-Bo Yi Abstract Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge-balanced electron donor,acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor,acceptor complex was obtained by the second-order Møller,Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor,acceptor complex in this work reveals that the S1 and S2 states of the electron donor,acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S0 , S1 and S0 , S2 transitions arise from ,,,* excitations. The charge recombination in the ion pair complex will produce the charge-balanced ground state or excited triplet state. According to the generalized Mulliken,Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2-dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23,35, 2003 [source] Character of long-chain branching in highly purified natural rubberJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2010Sureerut Amnuaypornsri Abstract The nature of long-chain branching in natural rubber (NR) from Hevea brasiliensis was analyzed for NR purified by enzymatic deproteinization in the latex state followed by acetone extraction in the solid state to remove the proteins and neutral lipids, respectively. The treatment of purified NR in a toluene solution with a polar solvent, such as methanol or acetic acid, resulted in a clear decrease in the molecular weight, gel content, and Huggins' constant; this was caused by the decomposition of branch points in the purified rubber. This finding clearly showed that long-chain branching in the purified NR was mainly derived from the association of phospholipids linked with both terminal groups in the rubber chain via hydrogen bonds. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010 [source] Reactions of 3-benzylindole-2-carbohydrazides: Synthesis of new 10-Benzyl-1,2-dihydro-1-oxo-1,2,4-triazino[4,5- a]indoles and 3-Benzyl-2-(1,3,4-oxadiazol-2-yl)indolesJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 1 2004Shambabu Joseph Maddirala 3-Benzylindole-2-carbohydrazides (4) on reaction with triethylorthoformate in a polar solvent like DMF yielded only 10-benzyl-1,2-dihydro-1-oxo-1,2,4-triazino[4,5- a]indoles (5) while (4) on reaction with triethylorthoacetate in DMF yielded both 10-benzyl-4-methyl-1,2-dihydro-1-oxo-1,2,4-triazino[4,5- a]indoles (5) and 3-benzyl-2-(5-methyl-1,3,4-oxadiazol-2-yl)indoles (6) instead of only the triazinoindoles as expected. The oxadiazolylindoles (6) were also synthesized by refluxing (4) with excess of orthoesters. The structures of the compounds formed were characterized by their analytical and spectral data. [source] Collagen structure: The molecular source of the tendon magic angle effectJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 2 2007Gary D. Fullerton PhD Abstract This review of tendon/collagen structure shows that the orientational variation in MRI signals from tendon, which is referred to as the "magic angle" (MA) effect, is caused by irreducible separation of charges on the main chain of the collagen molecule. These charges are held apart in a vacuum by stereotactic restriction of protein folding due in large part to a high concentration of hydroxyproline ring residues in the amino acids of mammalian collagen. The elevated protein electrostatic energy is reduced in water by the large dielectric constant of the highly polar solvent (, , 80). The water molecules serve as dielectric molecules that are bound by an energy that is nearly equivalent to the electrostatic energy between the neighboring positive and negative charge pairs in a vacuum. These highly immobilized water molecules and secondary molecules in the hydrogen-bonded water network are confined to the transverse plane of the tendon. Orientational restriction causes residual dipole coupling, which is directly responsible for the frequency and phase shifts observed in orientational MRI (OMRI) described by the MA effect. Reference to a wide range of biophysical measurements shows that native hydration is a monolayer on collagen hm = 1.6 g/g, which divides into two components consisting of primary hydration on polar surfaces hpp = 0.8 g/g and secondary hydration hs = 0.8 g/g bridging over hydrophobic surface regions. Primary hydration further divides into side-chain hydration hpsc = 0.54 g/g and main-chain hydration hpmc = 0.263 g/g. The main-chain fraction consists of water that bridges between charges on the main chain and is responsible for almost all of the enthalpy of melting ,H = 70 J/g-dry mass. Main-chain water bridges consist of one extremely immobilized Ramachandran water bridge per tripeptide hRa = 0.0658 g/g and one double water bridge per tripeptide hdwb = 0.1974 g/g, with three water molecules that are sufficiently slowed to act as the spin-lattice relaxation sink for the entire tendon. J. Magn. Reson. Imaging 2007. © 2007 Wiley-Liss, Inc. [source] Non-aqueous reverse micelles media for the SNAr reaction between 1-fluoro-2,4-dinitrobenzene and piperidine,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2006N. Mariano Correa Abstract The kinetics of the nucleophilic aromatic substitution (SNAr) reaction between 1-fluoro-2,4- dinitrobenzene (FDNB) and piperidine (PIP) in ethylene glycol (EG)/ sodium bis (2-ethyl-1-hexyl) sulfosuccinate (AOT)/n -heptane and dimethylformamide (DMF)/AOT/n -heptane non-aqueous reverse micelle systems is reported. EG and DMF were used as models for hydrogen bond donor (HBD) and non-hydrogen bond donor (non-HBD) polar solvents, respectively. The reaction was found not to be base catalyzed in these media. A mechanism to rationalize the kinetic results is proposed in which both reactants may be distributed between the two environments. The distribution constants of FDNB between the organic and each micellar pseudophases were determined by an independent fluorescence method. These results were used to evaluate the amine distribution constant and the intrinsic second-order rate coefficient of the SNAr reaction in the interface. The reaction was also studied in the pure solvents EG and DMF for comparison. The results in EG/AOT/n -heptane at Ws,=,2 give similar kinetic profiles than in water/AOT/n -hexane at W,=,10. With these HBD solvents, the interface saturation by the substrate is reached at around the same value of [AOT] and the intrinsic second-order rate coefficient in the interface, k,b, has comparable values. On the other hand, when DMF is used as a polar non-HBD solvent, the intrinsic second-order rate constant increases by a factor of about 200 as compared to the values obtained using HBD solvents as a polar core. It is concluded that higher catalytic power is obtained when non-HBD solvents are used as polar solvent in the micelle interior. Copyright © 2006 John Wiley & Sons, Ltd. [source] Fluorescence and photoisomerization studies of p -nitrophenyl-substituted ethenylindolesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 1 2006Anil K. Singh Abstract The synthesis, electronic absorption, fluorescence (,f, ,ex, ,f, ,f) and photoisomerization (,t,c, photostationary state composition) properties of 3-(4-nitrophenylethenyl- E)-NH-indole (1), 3-(4-nitrophenylethenyl- E)- N -ethylindole (2) and 3-(4-nitrophenyl ethenyl- E)- N -benzenesulfonylindole (3) in organic solvents of varying polarity are reported. The absorption maximum of these compounds undergoes a moderate red shift with increasing solvent polarity. However, the fluorescence maximum becomes highly red shifted with increasing solvent polarity. Whereas 1 and 2 show broad fluorescence bands, 3 exhibits dual fluorescence. Further, 1 and 2 fluoresce much more efficiently than 3. Correlation of the Stokes shift with solvent polarity parameters such as ,f and ET(30) and excited-state dipole moment indicate a highly polar excited state for 1,3. Time-resolved fluorescence studies show that the fluorescence decays are single- and multi-exponential type, depending on the solvent polarity. Further, 1 and 2 do not show photoisomerization on irradiation. However, 3 is photoactive and shows efficient photoisomerization in non-polar heptane. The sensitivity (,) of the photoreaction is determined in various solvent in terms of the Hammett plot, which showed that the excited states involved are electron deficient in nature and consequently stabilized more by an electron sufficient polar solvent and electron donating substituent. These results led us to suggest the existence of three types of excited states, namely the locally excited state, the intramolecular charge-transfer excited state and the conformationally relaxed intramolecular charge-transfer excited state in the photoprocesses of these compounds. Copyright © 2005 John Wiley & Sons, Ltd. [source] Factors affecting the sensitivity to acid inhibition in novel acrylates characterized by secondary functionalitiesJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2007Harini Kilambi Abstract Here we demonstrate that acrylates exhibit significant rate reductions in the presence of small concentrations of protic acids (0.1,0.5 wt %) compared with the bulk monomer concentration. Dramatically different sensitivities to acid inhibition, differing by up to 2 orders of magnitude, are exhibited for various acrylates. This study examines the various factors that cause enhanced sensitivity toward acid inhibition in novel acrylates characterized by carbamate and cyclic carbonate secondary functionalities. Acid inhibition studies conducted in the presence of a highly polar solvent, such as propylene carbonate, have been performed to determine the impact of overall medium polarity and the extent of acid dissociation on the sensitivity to acid inhibition. The studies depict only a twofold increase in the parameters associated with acid inhibition, upon the addition of 70 wt % propylene carbonate, in comparison with an increase of 2 orders of magnitude for the novel acrylates. These studies indicate that the susceptibility to acid inhibition is primarily determined by the stability of the hypothesized radical,acid complex as well as its propensity to terminate with other species in the system and not by the extent of acid dissociation in the system. Furthermore, it is implied that the stability of the radical,acid complex and its propensity to terminate with other species in the system are dominated by intramolecular interactions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1287,1295, 2007 [source] Synthesis of 5- tert -butyl-1-(3- tert -butyldimethylsiloxy)phenyl-4,4-dimethyl-2,6,7-trioxabicyclo[3.2.0]heptanes and their fluoride-induced chemiluminescent decomposition: effect of a phenolic electron donor on the CIEEL decay rate in aprotic polar solventLUMINESCENCE: THE JOURNAL OF BIOLOGICAL AND CHEMICAL LUMINESCENCE, Issue 5 2002Masakatsu Matsumoto Abstract Four bicyclic dioxetanes bearing a phenolic substituent, 3- tert -butyldimethylsiloxy-4-chlorophenyl (3a), 5- tert -butyldimethylsiloxy-4-chloro-2-ethylphenyl (3b), 5- tert -butyldimethylsiloxy-2-ethylphenyl (3c), and 3- tert -butyldimethylsiloxy-4-ethylphenyl (3d), were synthesized. All dioxetanes 3a,3d gave intense blue light on treatment with tetrabutylammonium fluoride (TBAF) in DMSO or acetonitrile. Kinetic study on the fluoride-induced CIEEL decay of these dioxetanes 3a,3d and the parent dioxetane 2b revealed that the para -substitution with chlorine on the phenolic moiety of dioxetane increases free energy of activation (,G,), while the para -substitution with ethyl on the aryl decreases ,G,. On the other hand, substitution with an ethyl at the ortho -position instead of the para -position was found to increase ,G, and to suppress the CIEEL decay. This fact is attributed to the steric factor of the ortho -ethyl group which would prevent the aromatic ring from rotating freely around the axis joined to the peroxide ring, and supports the suggestion for a CIEEL-active dioxetane bearing a phenolic moiety that an intramolecular electron transfer occurs preferentially from the phenolic donor to O,O of the dioxetane ring, when the aromatic ring lies in a certain conformation(s). Copyright © 2002 John Wiley & Sons, Ltd. [source] Liquid Chromatographic Separation of Olefin Oligomers and its Relation to Separation of Polyolefins , an OverviewMACROMOLECULAR SYMPOSIA, Issue 1 2009Tibor Macko Abstract Summary: Linear and branched alkanes are oligomers of polyethylene. Alkanes with higher molar masses are called waxes. These substances are widely used as fuels, oils, lubricants, etc. and for these reasons many groups have tried to analyse, separate and characterise alkanes by various methods, including liquid chromatography. Alkanes may be separated according to their size in solution by SEC. In addition to chromatographic systems separating in the SEC mode, various sorbent-solvent systems have been published, where alkanes have been separated one from another by adsorption and/or precipitation mechanism. The mobile phase is either a non-polar solvent or a polar solvent or a mixture of a solvent and a non-solvent for alkanes. Even near critical conditions, which have several advantages for applications of HPLC in polymer analysis, have been identified for alkanes. Moreover, selective separations of branched alkanes according to their structure have been published. In the majority of these published studies, solvents with low boiling points have been used as the mobile phases, which do not allow dissolution of crystalline polyolefins at atmospheric pressure. However, taking into account experiences with the separation of alkanes, new HPLC systems for the separation of polyolefins may be developed. This is a major challenge and first results are presented in this contribution. [source] Chlorin,Bacteriochlorin Energy-transfer Dyads as Prototypes for Near-infrared Molecular Imaging Probes: Controlling Charge-transfer and Fluorescence Properties in Polar MediaPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 4 2009Hooi Ling Kee The photophysical properties of two energy-transfer dyads that are potential candidates for near-infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC-FbB and ZnC-FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2-dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of ,(5,10 ps),1 and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (,f = 0.19) and singlet excited-state lifetimes (,,5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited-state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC-FbB than for ZnC-FbB in a given solvent. For example, the ,f and , values for FbC-FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC-FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge-transfer states, as assessed by ground-state redox potentials and supported by molecular-orbital energies derived from density functional theory calculations. Controlling the extent of excited-state quenching in polar media will allow the favorable photophysical properties of the chlorin,bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC-FbB, 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (,f = 760 nm), long bacteriochlorin excited-state lifetime (,5.5 ns), and narrow (,20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity- and lifetime-imaging techniques. [source] Behavior of quaternary ammonium salt photobase generators on initiating free radical photo-polymerizationPOLYMER INTERNATIONAL, Issue 6 2006Rong Zhong Abstract Series of quaternary ammonium tetraphenyl borates salt photobase generators (PBGs) were synthesized using p -methoxyphenacylmethylene and 2-naphthoymethylene as chromophores, and triethylene diamine, pyridine or 3-methyl pyridine as tertiary amine. The kinetics for polymerization of trimethylolpropane triacrylate (TMPTA) monomer using PBGs as free radical photo-initiator was monitored by differential photo-calorimeter (DPC). It was found that all the quaternary ammonium tetraphenyl borate salt photobase generators synthesized could initiate free radical polymerization of the acrylate monomer TMPTA by exposure to UV irradiation, but the activity was relatively low. Addition of a small amount of polar solvent to the system could largely increase the polymerization rate and final conversion. Photo-polymerization was also improved by increasing light intensity or raising reaction temperature. PBGs with p -methoxyphenacylmethylene as chromophore had higher absorbance at around 280 nm and showed higher activity in initiating photo-polymerization than those with N -(2-naphthoylmethyl) as chromophore. Copyright © 2006 Society of Chemical Industry [source] Azide,Tetrazole Ring-Chain Isomerism in Polyazido-1,3,5-triazines, Triazido- s -heptazine, and DiazidotetrazinesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2006Anton Hammerl Abstract The azide,tetrazole isomerism in several polyazido-1,3,5-triazines, triazido- sym -heptazine, and some diazido-1,2,4,5-tetrazines was investigated by ab initio quantum chemical methods in order to determine whether the polyazides are suitable starting materials for the synthesis of the isomeric tetrazoles. The effects of solvation in CCl4, DMSO and water on this isomerism were included using the self consistent reaction field (SCRF) method. The effect of amino- and nitrosubstituents on the azide,tetrazole isomerism was also examined. In the gas phase all investigated polyazidoheterocycles do not cyclize to form tetrazoles. An electron-donating amino group favors the ring closure to tetrazoles, whereas an electron-withdrawing nitro group favors the azides. Solvation in polar solvents favors the formation of a tetrazole ring system due to higher charge separation in the tetrazole ring system, but for all polyazido-1,3,5-triazines, including triazido- s -heptazine, the effects of solvation are not strong enough to shift the equilibrium to the tetrazole side, which explains why several attempts to detect these compounds have failed. The monotetetrazoles of diazidotetrazine and bis(azido)azo-1,2,4,5-tetrazine and the ditetrazole of bis(azido)hydrazo-1,2,4,5-tetrazine are the minimum energy species in DMSO and water. Thus we predict that the diazidoazo- and hydrazotetrazines will readily cyclize to the tetrazoles in polar solvents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Kinetic Studies of the Oxidative Addition and Transmetallation Steps Involved in the Cross-Coupling of Alkynyl Stannanes with Aryl Iodides Catalysed by ,2 -(Dimethyl fumarate)(iminophosphane)palladium(0) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2004Bruno Crociani Abstract The complexes [Pd(,2 -dmfu)(P,N)] {dmfu = dimethyl fumarate; P,N = 2-(PPh2)C6H4,1-CH=NR, R = C6H4OMe-4 (1a), CHMe2 (2a), C6H3Me2 -2,6 (3a), C6H3(CHMe2)2 -2,6 (4a)} undergo dynamic processes in solution which consist of a P,N ligand site exchange through initial rupture of the Pd,N bond at lower energy and an olefin dissociation-association at higher energy. According to equilibrium constant values for olefin replacement, the complex [Pd(,2 -fn)(P,N)] (fn = fumaronitrile, 1b) has a greater thermodynamic stability than its dmfu analogue 1a. The kinetics of the oxidative addition of ArI (Ar = C6H4CF3 -4) to 1a and 2a lead to the products [PdI(Ar)(P,N)] (1c, 2c) and obey the rate law, kobs = k1A + k2A[ArI]. The k1A step involves oxidative addition to a reactive species [Pd(solvent)(P,N)] formed from dmfu dissociation. The k2A step is better interpreted in terms of oxidative addition to a species [Pd(,2 -dmfu)(solvent)(,1 -P,N)] formed in a pre-equilibrium step from Pd,N bond breaking. The complexes 1c and 2c react with PhC,CSnBu3 in the presence of an activated olefin (ol = dmfu, fn) to yield the palladium(0) derivatives [Pd(,2 -ol)(P,N)] along with ISnBu3 and PhC,CAr. The kinetics of the transmetallation step, which is rate-determining for the overall reaction, obey the rate law: kobs = k2T[PhC,CSnBu3]. The k2T values are markedly enhanced in more polar solvents such as CH3CN and DMF. The solvent effect and the activation parameters suggest an associative SE2 mechanism with substantial charge separation in the transition state. The kinetic data of the above reactions in various solvents indicate that, for the cross-coupling of PhC,CSnBu3 with ArI catalysed by 1a or 2a, the rate-determining step is represented by the oxidative addition and that CH3CN is the solvent in which the highest rates are observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] (Neocuproin)zinc Thiolates: Attempts at Modeling Cobalamin-Independent Methionine SynthaseEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2004Jan Seebacher Abstract Several new complexes [(neo)Zn(SR)2] [neo = neocuproin (2,9-dimethylphenanthroline)] have been synthesized and structurally characterised. They react in a stepwise fashion with the alkylating agents CH3I and (CH3)2SO4 to afford the thioethers CH3SR and first the mixed complexes [(neo)Zn(SR)X] (X = I, CH3SO4) and then [(neo)ZnX2]. Similar alkylations occur with benzyl iodide, but not with trimethyl phosphate in nonpolar media. Under these conditions, thiolate exchange with [PPN]SR does not occur which indicates that the alkylations take place at the zinc-bound thiolates. In polar solvents (methanol, DMSO), thiolate exchange occurs readily, and at higher temperatures (CH3)3PO4 also acts as an alkylating agent which indicates that under these conditions free thiolate is available in solution. Qualitative kinetic data support the associative alkylation mechanism in nonpolar media and the change of mechanism in polar media. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Enol Forms of 1,3-Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion-Assisted InterconversionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2010Mark Sigalov Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecularO,H···N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O,H···N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O,···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto,enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods. [source] Switchable Fluorescent and Solvatochromic Molecular Probes Based on 4-Amino- N -methylphthalimide and a Photochromic DiaryletheneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 15 2008Sergey F. Yan Abstract New fluorescent photochromic compounds (1 -H and 1 -Boc) have been synthesized and characterized in different solvents. The fluorescence emission can be switched "on" and "off" with visible light and UV, respectively, by means of the photochromic reaction. The emission wavelength and efficiency strongly depend on the polarity of the solvent. The compounds show a positive solvatochromic effect in the emission maxima, and their fluorescence quantum yield decreases as the solvent's polarity increases (from cyclohexane to dioxane). In solvents more polar than dioxane the emission is too weak and therefore undetectable, and thus 1 -H and 1 -Boc behave as "normal" photochromic compounds. The photochromic reaction is also sensitive to the environment. A decrease of more than an order of magnitude was found for the quantum yield of the colouring reaction (,OF,CF) for 1 -H in ethanol compared with cyclohexane, and an about threefold decrease in ,OF,CF was observed for the compound 1 -Boc in polar solvents (compared with apolar solvents). For both compounds the ring-opening reaction was found not to dependent on the solvent. The novel fluorescent molecular switches 1 -H and 1 -Boc are able to probe the polarity of their microenvironment. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] A Novel Bis(zinc,porphyrin),Oxoporphyrinogen Donor,Acceptor Triad: Synthesis, Electrochemical, Computational and Photochemical StudiesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 3 2006Jonathan P. Hill Abstract The first example of a porphyrin-quinonoid donor,acceptor triad featuring (tetraphenylporphinato)zinc(II) moieties covalently attached to an oxoporphyrinogen through its macrocyclic nitrogen atoms is reported. This arrangement of chromophores results in an interesting interplay between the electron-donating zinc,porphyrin(s) and the electron/energy accepting oxoporphyrinogen. The optical absorption of the triad reveals features corresponding to both the donor and acceptor entities. The geometry and electronic structure of the triad deduced from B3LYP/3-21G(*) calculations reveal an absence of inter-chromophoric interactions and localization of the HOMO on one zinc,porphyrin group and the LUMO on the oxoporphyrinogen scaffold. The electrochemical redox states of the triad were established from a comparative electrochemistry of the triad and the reference compounds. Both steady-state and time-resolved emission studies revealed quenching of the singlet excited state of zinc,porphyrin in the triad, and the free-energy calculations performed using Weller's approach indicate the possibility of electron transfer from the singlet excited zinc,porphyrin group to the oxoporphyrinogen in polar solvents. Time-resolved fluorescence studies reveal excited state energy transfer from zinc,porphyrin to oxoporphyrinogen in nonpolar solvents, while nanosecond transient absorption studies combined with time-resolved fluorescence studies in polar solvents are indicative of the occurrence of photoinduced charge separation from the singlet excited zinc,porphyrin to the oxoporphyrinogen. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Efficient Photosensitized Splitting of Thymine Dimer by a Covalently Linked Tryptophan in Solvents of High PolarityEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2005Qin-Hua Song Abstract Tryptophan-thymine dimer model compounds used to mimic the repair reaction of DNA photolyase have been synthesized. The photosensitized cleavage of the dimer by the covalently linked tryptophan is strongly solvent-dependent with the reaction rates increasing in increasingly polar solvents, for example, the quantum yield , = 0.004 in THF/hexane (5:95) and 0.093 in water. The fluorescence of the tryptophan residue is quenched by the dimer moiety by electron transfer from the excited tryptophan to the dimer. Fluorescence-quenching studies indicated that the electron transfer was efficient in polar solvents. The splitting efficiency of the dimer radical anion within the tryptophan·+,dimer·, species is also remarkably solvent-dependent and increases with the polarity of the solvents. The back-electron-transfer reaction in the charge-separated species, which competes with cleavage, was suppressed in polar solvents. These results are in contrast to those of earlier solvent-dependent studies of indole-dimer systems, but they can be rationalized in terms of the differences in the distances between the chromophore unit and the attached dimer. The pH-dependent measurements of the splitting reaction and the deuterium isotope effect showed that the tryptophan radical cation within the charge-separated species does not deprotonate prior to the cleavage of the dimer radical anion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Essential oil composition and antimicrobial activity of tuberous roots of Pimpinella tirupatiensis Bal.FLAVOUR AND FRAGRANCE JOURNAL, Issue 6 2002& Subr., India, an endemic taxon from eastern ghats Abstract The tuberous roots of Pimpinella tirupatiensis (Apiaceae) were subjected to sequential extraction with different polar solvents and the extracts were tested against eight bacterial and three fungal pathogenic strains for antimicrobial activity. The minimum inhibitory concentration of active extracts against six bacterial and two fungal strains were determined. The hexane and ethyl acetate fractions exhibited a broad spectrum of antimicrobial activity and were analysed for different phytochemicals. The active extracts contained significant amounts of alkaloids, flavonols, flavones and volatile oils. The hexane extract yielded an essential oil when subjected to GC with FID. The compounds were identified based on their retention indices and yielded 24 known compounds and one unknown compound. The major compounds are ,-bisabolene (9.2%), ,-3-carene (8.9%), cis -carveol (6.7%), elemol (5.8%), ,-cadinol (4.4%), methyl geranate (4.3%) and ,-nonalactone (3.4%). Copyright © 2002 John Wiley & Sons, Ltd. [source] Electron-Rich Alcohol-Soluble Neutral Conjugated Polymers as Highly Efficient Electron-Injecting Materials for Polymer Light-Emitting DiodesADVANCED FUNCTIONAL MATERIALS, Issue 15 2009Fei Huang Abstract We report the design and synthesis of three alcohol-soluble neutral conjugated polymers, poly[9,9-bis(2-(2-(2-diethanolaminoethoxy) ethoxy)ethyl)fluorene] (PF-OH), poly[9,9-bis(2-(2-(2-diethanol-aminoethoxy)ethoxy)ethyl)fluorene- alt -4,4,-phenylether] (PFPE-OH) and poly[9,9-bis(2-(2-(2-diethanolaminoethoxy) ethoxy)ethyl)fluorene- alt -benzothiadizole] (PFBT-OH) with different conjugation length and electron affinity as highly efficient electron injecting and transporting materials for polymer light-emitting diodes (PLEDs). The unique solubility of these polymers in polar solvents renders them as good candidates for multilayer solution processed PLEDs. Both the fluorescent and phosphorescent PLEDs based on these polymers as electron injecting/transporting layer (ETL) were fabricated. It is interesting to find that electron-deficient polymer (PFBT-OH) shows very poor electron-injecting ability compared to polymers with electron-rich main chain (PF-OH and PFPE-OH). This phenomenon is quite different from that obtained from conventional electron-injecting materials. Moreover, when these polymers were used in the phosphorescent PLEDs, the performance of the devices is highly dependent on the processing conditions of these polymers. The devices with ETL processed from water/methanol mixed solvent showed much better device performance than the devices processed with methanol as solvent. It was found that the erosion of the phosphorescent emission layer could be greatly suppressed by using water/methanol mixed solvent for processing the polymer ETL. The electronic properties of the ETL could also be influenced by the processing conditions. This offers a new avenue to improve the performance of phosphorescent PLEDs through manipulating the processing conditions of these conjugated polymer ETLs. [source] Covalent Functionalization of Carbon Nanohorns with Porphyrins: Nanohybrid Formation and Photoinduced Electron and Energy Transfer,ADVANCED FUNCTIONAL MATERIALS, Issue 10 2007G. Pagona Abstract The covalent attachment of carbon nanohorns (CNHs) to ,-5-(2-aminophenyl)-,-15-(2-nitrophenyl)-10,20-bis(2,4,6-trimethyl-phenyl)-porphyrin (H2P) via an amide bond is accomplished. The resulting CNH,H2P nanohybrids form a stable inklike solution. High-resolution transmission electron microscopy (HRTEM) images demonstrate that the original dahlia-flowerlike superstructure of the CNHs is preserved in the CNH,H2P nanohybrids. Steady-state and time-resolved fluorescence studies show efficient quenching of the excited singlet state of H2P, suggesting that both electron and energy transfer occur from the singlet excited state of H2P to CNHs, depending on the polarity of the solvent. In the case of electron transfer, photoexcitation of H2P results in the reduction of the nanohorns and the simultaneous oxidation of the porphyrin unit. The formation of a charge-separated state, CNH,,,H2P,+, has been corroborated with the help of an electron mediator, hexyl-viologen dication (HV2+), in polar solvents. Moreover, the charge-separated CNH,,,H2P,+ states have been identified by transient absorption spectroscopy. [source] Structures and properties of two diastereomeric cyclic sulfites derived from cis -3,4-di- tert -butylthiolane-3,4-diol and thionyl chlorideHETEROATOM CHEMISTRY, Issue 7 2003Sanae Tanaka cis-3,4-Di-tert-butylthiolane-3,4-diol (1) was treated with an equimolar amount of thionyl chloride in the presence of triethylamine or pyridine in several solvents of different polarity to furnish two diastereomeric sulfites 2a and 2b generally in excellent combined yields. Although 2a was consistently formed as the major diastereomer when pyridine was used as the base, 2a and 2b were formed in approximately equal amounts when triethylamine was used as the base in polar solvents. X-ray crystallographic analyses revealed that the SO group of 2a is anti to the thiolane ring and that of 2b syn to the thiolane ring. Density functional theory calculations (B3LYP/6-31G* level) revealed that 2a is less stable than 2b by 1.28 kcal mol,1, although 2a was formed generally as the predominant diastereomer. Spectroscopic data of 2a and 2b are discussed with emphasis on comparison with those obtained by calculations. Treatment of 2a and 2b with m-chloroperbenzoic acid resulted in the oxidation of the divalent sulfur atom of the thiolane ring and not the sulfite sulfur atom. The above oxidations took place exclusively at the syn-side with respect to the tert-butyl groups.© 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:587,595, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10192 [source] A generalized exo -anomeric effect.ISRAEL JOURNAL OF CHEMISTRY, Issue 3-4 2000Substituent, solvent effects on the conformational equilibria of 2-(arylseleno)cyclohexanones The effects of substitution and solvent on the conformational equilibria of 2-[(4-R-substituted-phenyl)seleno]cyclohexanones are described. The conformational equilibria were determined by comparison of the linewidths of the H-2 resonances in the 1H NMR spectra of the conformationally averaged systems with those of the anancomeric (highly biased) 4-isopropyl-2-substituted cyclohexanones. The substituent (R = NMe2, OMe, Me, H, F, Cl, CF3, NO2) and solvent ((CD3)2CO, CD3CN, CD2Cl2, CDCl3) effects are discussed in terms of electrostatic effects and the possible stabilizing orbital interactions. The values of Keq (axial-equatorial) increase as the substituent becomes more electron withdrawing, in agreement with the dominance of nSe , ,*C=O or ,C-Se , ,*C=O orbital interactions in the axial conformers. The increase in the proportion of the equatorial isomers in more polar solvents for a given substituent suggests a damping of the dipolar interactions in the equatorial isomers. However, the proportion of the equatorial isomers in a given solvent increases as the substituent becomes more electron withdrawing, indicating that electrostatic interactions do not dominate in controlling the conformational equilibria. Analysis of the equilibrium data by means of a dual substituent parameter approach indicates the best correlation with ,I and ,+R substituent constants in CD2Cl2 and with ,I and ,°R substituent constants in CD3CN, with similar sensitivities to the resonance and polar effects. The correlations are interpreted in terms of accommodation of effective positive charge on the selenium atom in the axial isomers in CD2Cl2, and a lesser sensitivity to the buildup of positive charge in the more polar solvent CD3CN. Comparison of the IR ,CO -stretching frequencies for the axial and equatorial ArSe-substituted anancomeric systems (R = NO2, NMe2) indicates a higher stretching frequency for the NO2 -substituted isomers. In the case of the NMe2 -substituted compounds, ,CO appears at a higher frequency in the equatorial isomer, whereas in the case of the NO2 -substituted compounds, ,CO is less sensitive to the axial or equatorial orientation of the substituent. The results are consistent with the operation of nse , ,*c=0 or ,C-Se , ,*C=O orbital interactions in the axial isomers. The JC2-H2 values in the axially-substituted anancomeric isomers are of greater magnitude than those in the equatorially-substituted isomers, which is also consistent with the operation of the orbital interactions described above. There is, however, no marked substituent effect on the JC2,H2 values within the series of axial or equatorial isomers. We argue that this does not support the dominance of ,C-Se , ,*C=O orbital interactions. Examination of crystal structures reported in the literature for related compounds indicates a particular gauche orientation about the C2,Se bond, which lends further support to the operation of an nSe , ,*C=O orbital interaction. We suggest that the latter interaction is a manifestation of a generalized exo -anomeric effect. [source] A Density Functional Theory Study of the Stille Cross-Coupling via Associative Transmetalation.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2007Coordinating Solvents, The Role of Ligands Abstract An associative mechanism has been computationally characterized for the Stille cross-coupling of vinyl bromide and trimethylvinylstannane catalyzed by PdL2 (L=PMe3, AsMe3) with or without dimethylformamide as coordinating ligand. All the species along the catalytic cycles that start from both the cis - and the trans -PdL(Y)(vinyl)Br complexes (Y=L or S; L=PMe3, AsMe3 or PH3; S=DMF) have been located in the gas phase and in the presence of polar solvents. Computations support the central role of species trans -PdL(DMF)(vinyl)Br which react by ligand dissociation and stannane coordination in the rate-limiting transmetalation step via a puckered four-coordinate (at palladium) transition state comprised of Pd, Br, Sn and sp2 C atoms. A donating solvent may enter the catalytic cycle assisting isomerization of cis -PdL2(vinyl)Br to trans -PdL(DMF)(vinyl)Br complexes via a pentacoordinate square pyramidal Pd intermediate. In keeping with experimental observations, the activation energies of the catalytic cycles with arsines as Pd ligands are lower than those with phosphines. Polytopal rearrangements from the three-coordinate T-shaped Pd complexes resulting from transmetalation account for the isomerization and the CC bond formation on the reductive elimination step. [source] | |||||||||||||||||||||||||||||||