Backbone Substituents Research Articles

How Chromophore Shape Determines the Spectroscopy of Phenylene−Vinylenes: Origin of Spectral Broadening in the Absence of Aggregation

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymer's optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.

Novel polyfluorinated polyimides derived from α,α-bis(4-amino-3,5-difluorophenyl)phenylmethane and aromatic dianhydrides: Synthesis and characterization

A novel aromatic diamine containing bulky lateral phenyl unit and fluorine groups ortho-substituted to the amino groups in the structure, α,α-bis(4-amino-3,5-difluorophenyl)phenylmethane (4FMA) was synthesized and characterized. 4FMA was polymerized with four aromatic dianhydrides to afford a series of polyimides (PIs) via a one-step high-temperature polycondensation procedure. All the PIs were amorphous and showed good solubility not only in polar aprotic solvents, but in many common solvents, such as cyclopentanone and tetrahydrofuran. The soluble polymers formed flexible, tough and transparent films. The films had a tensile strength, elongation at break, and tensile modulus in the ranges 85–107MPa, 4–7%, 1.7–2.8GPa, respectively. The polyimides also exhibited high-Tg (292–338°C), good thermal stability, good dielectric strength, low dielectric constants, and low water uptakes due to the introduction of the polyfluorinated substituents in the polymer backbone. As expected, the PI films showed good transparency in the UV–Vis light region with cutoff wavelength as low as 318nm and transmittance higher than 60% at 450nm. PI-4 derived from 4FMA and fluorinated dianhydride 6FDA exhibited low light-absorption in the near-infrared region, especially at the optocommunication wavelength of 1310nm and 1550nm. The remarkable combined properties indicate their potential applications in microelectronic and optoelectronic fabrications.

Synthesis and characterization of color-stable electroluminescent polymers: Poly(dinaphtho[1,2-a:1′,2′-g]-s-indacene)s

Two new stepladder conjugated polymers, that is, poly(7,7,15,15-tetraoctyldinaphtho[1,2-a:1′,2′-g]-s-indacene) (PONSI) and poly(7,7,15,15-tetra(4-octylphenyl)dinaphtho[1,2-a:1′,2′-g]-s-indacene) (PANSI) with alkyl and aryl substituents, respectively, have been synthesized and characterized. In comparison with poly(indenofluorene)s, both polymers have extended conjugation at the direction perpendicular to the polymer backbone because of the introduction of naphthalene moieties. The emission color of the polymers in film state is strongly dependent on the substituents. While PONSI emits at a maximum of 463 nm, PANSI with the same backbone but aryl substituents displays dramatically redshifted emission with a maximum at 494 nm. Both polymers show stable photoluminescence spectra while annealing at 200 °C in inert atmosphere. The PONSI-based devices with the configuration of ITO/PEDOT:PSS/polymer/Ca/Al turn on at 3.7 V, and emit at a maximum of 461 nm with the CIE coordinates of (0.19, 0.26), a maximum luminance efficiency of 1.40 cd/A, and a maximum brightness of 2036 cd/m2 at 13 V. Meanwhile, the emission color of the devices is independent of driving voltage and keeps unchanged during the continuous operation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4866–4878, 2008

Flexible coordination of bulky amidinates and guanidinates towards rhodium(i): conversion of kinetic to thermodymanic isomers

Reactions of the bulky amidinate and guanidinate salts K[(ArN)(2)CR] (R = Bu(t), NPr(i)(2) or N(C(6)H(11))(2); Ar = 2,6-diisopropylphenyl) with [{RhCl(eta(4)-COD)}(2)] (COD = 1,5-cyclooctadiene) lead to KCl elimination and the formation of the complexes, [Rh{(eta(5)-ArN)(ArN)CR}(COD)], in which the anionic ligand coordinates the rhodium centre in an unprecedented eta(5)-cyclohexadienyl mode. The thermal conversions of these complexes to their N,N'-chelated isomers, [Rh{kappa(2)-N,N'-(ArN)(2)CR}(COD)], were carried out and the kinetics of these processes have been shown to be first order. The rates of the isomerisations are inversely proportional to the size of the amidinate or guanidinate backbone substituent. Analogies between the ligating properties of the bulky amidinates and guanidinates used in the study, and those of beta-diketiminates are discussed.

Tandem Mass Spectrometry Characteristics of Silver-Cationized Polystyrenes: Internal Energy, Size, and Chain End versus Backbone Substituent Effects

The Ag(+) adducts of polystyrene (PS) oligomers with different sizes (6-19 repeat units) and initiating (alpha) or terminating (omega) end groups mainly decompose via free radical chemistry pathways upon collisionally activated dissociation. This reactivity is observed for ions formed by matrix-assisted laser desorption/ionization as well as electrospray ionization. With end groups lacking weak bonds (robust end groups), dissociation starts with random homolytic C-C bond cleavages along the PS chain, which lead to primary and benzylic radical ions containing either of the chain ends. The primary radical ions mainly depolymerize by successive beta C-C bond scissions. For the benzylic radical ions, two major pathways are in competition, namely, depolymerization by successive beta C-C bond scissions and backbiting via 1,5-H rearrangement followed by beta C-C bond scissions. The extent of backbiting decreases with internal energy. With short PS chains, the primary radical ions also undergo backbiting involving 1,4- and 1,6-H rearrangements; however, this process becomes negligible with longer chains. If the polystyrene contains a labile substituent at a chain end, this substituent is eliminated easily and, thus, not contained in the majority of observed fragments. Changes in the PS backbone structure can have a dramatic effect on the resulting dissociation chemistry. This is demonstrated for poly(alpha-methylstyrene), in which backbiting is obstructed due to the lack of benzylic H atoms; instead, this backbone connectivity promotes 1,2-phenyl shifts in the primary radical ions formed after initial C-C bond homolyses as well as H atom transfers between the incipient primary and benzylic radicals emerging from these homolyses.

Substituent Effects on Two-Dimensional Assembling and Chain Folding of Rigid-Rod Polymer Poly(p-phenyleneethynylene) Derivatives on the Solid/Liquid Interface

The two-dimensional assembling behavior of a series of poly(p-phenyleneethynylene) (PPE) derivatives having identical conjugated backbone but different substituents and degrees of polymerization has been studied using scanning tunneling microscopy (STM). Submolecularly resolved STM images reveal diverse forms of chain folding in the assembled monolayer, allowing the analysis of the effects of polarity and concentration of substituents on the assembling behavior and chain folding of PPE. Theoretical simulations have been performed to understand the structural effects on the chain folding characteristics on the surface. The submolecularly resolved STM images also enable the quasi-quantitative determination of molecular weight and number-averaged degree of polymerization. The histograms of the contour length measured from the STM images agree well with the theoretical Schulz−Zimm distribution, indicating the potential capacity of the STM method for the estimation of molecular weight distribution of rigid-rod...

Stabilizing Factors for Vanadium(IV) in Amavadin

AbstractSix secondary N‐hydroxy amino acids (amavadin‐based ligands) have been prepared through a strategy with nitrone reduction as its key step. Two of these amino acids are the new amavadin ligand analogues 4a and 4b containing either a phenyl or a benzyl group in combination with a small backbone substituent. In addition, the monoester 5 of the amavadin ligand as well as three N‐alkylated N‐hydroxy aminoacids 6 were prepared. Complexation studies with the new tri‐ and bidentate ligands using HRMS revealed that only amavadin and its two analogues with ligands 4a and 4b are stable in aqueous and aerobic environments and that the complexation of vanadium(IV) with ligands 5 and 6 does not lead to air‐stable vanadium(IV) non‐oxo compounds. An interesting spin‐off of the synthetic work on nitrones is an effective and easily applicable method for the synthesis of racemic N‐hydroxy amino acid esters. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The correlation of serum protein association and cellular uptake with lipophilicity and polarity of the backbone substituents of the technetium‐99m‐amine‐oxime chelates

AbstractDetection of tumor hypoxia is of importance because hypoxic cancer cells can resist radiation therapy and also induce molecules to help them surviving and metastasizing. Our previous study has reported the development of technetium‐99m labeled amine‐oxime compounds containing 2‐nitroimidazoles to target hypoxic cells. In order to develop new generation of 99mTc, 64Cu, and 67Cu based amine‐oxime chelates with improved selectivity on tumor hypoxia in the future, it is important to understand the correlation of some structural parameters such as lipophilicity and polarity of the backbone ‐constituents of a radiochelate with its serum protein binding potential and cellular‐uptake‐level. The correlation is investigated in this work by using five 99mTc‐amine‐oxime chelates containing 2‐NI group and another five containing aniline group (as control). The results indicate that the level of protein‐and cellular‐binding of a radiochelate increased with their lipophilicity which generally correlates with the lengthening of the alkyl chains on the backbone of the radiochelates. The greater lipophilicity of a radiochelate, the higher percentage it bound to serum proteins and cellular membrane. Our study also indicates that, in addition to lipophilicity, polarity of the constituents is also an important factor of determining the levels of serum protein binding and cellular accumulation of a radiochelate. Copyright © 2007 John Wiley & Sons, Ltd.

Mono(amidinate) Yttrium Alkyl Complexes: The Effect of Ligand Variation on Ethene Polymerization Catalysis

AbstractYttrium dialkyl complexes were prepared for three different 2,6‐diisopropylphenyl‐substituted benzamidinate ligands. The electron‐donating ability of the parent ligand (L1) was decreased by a pentafluorophenyl backbone substituent (L2), and enhanced by addition of a pendant Lewis base (L3). The corresponding cationic monoalkyl species were generated and compared in catalytic ethene polymerization.

Bisoxazoline ligands with an axial-unfixed biaryl backbone: the effects of the biaryl backbone and the substituent at oxazoline ring on Cu-catalyzed asymmetric cyclopropanation

The synthesis of novel C 2-symmetric bisoxazoline ligands with an axial-unfixed biaryl backbone and different substituents at the oxazoline ring is reported. The diastereomer equilibrium of these ligands in solution and their complexation behavior with the copper(I) ion were studied. Their application in Cu-catalyzed cyclopropanation of styrene, with diazoacetate, was carried out and the effects of the biaryl backbone and the substituent on the oxazoline ring in the catalysis were examined. The best result was obtained with the ligand having a 1,1′-binaphthyl backbone and a t-butyl group on the oxazoline ring, which afforded 89% de and 96% de for the trans- and cis-products of the above asymmetric cyclopropanation reaction, respectively.

Structure activity studies of the serine-AIB dipeptide domain in 2,3-dihydroisothiazole based growth hormone secretagogues

A series of growth hormone secretagogues (GHSs) based on 2,3-dihydroisothiazole has been synthesized in the search for a potential treatment of growth hormone deficiency or frailty in the elderly. This paper describes the evaluation of the SAR of the benzyl- d-Ser-aminoisobutyric acid dipeptide fragment. Introduction of substituents in the peptide backbone and in the phenyl ring has been investigated, as well as replacements for the benzyl group and for the AIB residue. A number of modifications resulted in enhanced potency over the parent benzyl- d-Ser-AIB derivative.

The gem-Dialkyl Effect as a Test for Preliminary Diphosphine Chelate Opening in a Reductive Elimination Reaction

A significant kinetic effect of the placement of gem-dialkyl substituents in the carbon backbone of a bidentate phosphine ligand on a reductive elimination reaction involving chelate opening as a preliminary step has been documented. This observation confirms the viability of the gem-dialkyl effect as a mechanistic probe for preliminary phosphine dissociation in reactions of complexes containing chelating diphosphine ligands.

Ligand-Induced Haptotropic Rearrangements in Bis(indenyl)zirconium Sandwich Complexes

Addition of principally sigma-donating ligands such as THF, chelating diethers, or 1,2-bis(dimethyl)phosphinoethane to eta(9),eta(5)-bis(indenyl)zirconium sandwich complexes, (eta(9)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))Zr (R = alkyl or silyl), induces haptotropic rearrangement to afford (eta(6)-C(9)H(5)-1,3-R(2))(eta(5)-C(9)H(5)-1,3-R(2))ZrL adducts. Examples where L = THF and DME have been characterized by X-ray diffraction and revealed significant buckling of the eta(6) benzo ring, consistent with reduction of the arene, and highlight the importance of the zirconium(IV) canonical form. For the THF-induced haptotropic rearrangements, the thermodynamic driving force for ring migration has been measured as a function of indenyl substituent and demonstrates silylated sandwiches favor THF coordination and the eta(6),eta(5) bonding motif over their alkylated counterparts. In the case of chelating diethers, measurement of the corresponding equilibrium constants establish more stable eta(6),eta(5) adducts with five- over four-membered chelates and with smaller oxygen and carbon backbone substituents. Kinetic studies on both THF and DME addition to (eta(9)-C(9)H(5)-1,3-(SiMe(3))(2))(eta(5)-C(9)H(5)-1,3-(SiMe(3))(2))Zr established a first-order dependence on the incoming ligand, consistent with a mechanism involving direct attack of the incoming nucleophile on the eta(9),eta(5) sandwich. These results further highlight the ability of the indenyl ligand to smoothly adjust hapticity to meet the electronic requirements of the metal center.

Influence of the Conformation of Salen Complexes on the Stereochemistry of the Asymmetric Epoxidation of Olefins

AbstractCompared with Katsuki’s or Jacobsen’s catalysts (R,R,R,R)‐1 or (R,R)‐2, the phenomenon of reversed asymmetric induction in the course of the epoxidation of unfunctionalised olefins in the presence of (salen)manganese complex (S,S)‐3 is indirectly explained by means of quantumchemical calculations, as well as by 1H NMR spectra of salen ligand (S,S)‐10 and diamagnetic low‐spin nickel complex (S,S)‐7 and by its X‐ray structure analysis. Whereas in (R,R,R,R)‐1 and (R,R)‐2 the phenyl and cyclohexyl substituents occupy equatorial positions, the origin for the reversed enantioselection caused by manganese catalyst (S,S)‐3, is based on an axial position of the acyclic heteroalkyl substituents in the diimine backbone of (S,S)‐3 as exemplarily confirmed by model nickel complex (S,S)‐7. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Synthesis and photochemical studies of substituted adjacent binaphthalophthalocyanines

Adjacent phthalocyanines with a binaphthalo backbone and phenoxy substituents were synthesized and their photochemical properties were investigated. The adjacent phthalocyanines are the binaphthalophthalocyanines, with the phenoxy, 4-tert-butylphenoxy and the sulfophenoxy substituents, respectively and bis-binaphthalophthalocyanine which has an extended π conjugation system and larger singlet oxygen quantum yield compared to the other compounds. The presence of the phenoxy substituents as well as the binaphthalo bridge does not cause a marked difference on the fluorescing properties of these complexes when compared to zinc phthalocyanine. The binaphthalo backbone allowed the molecules to photoswitch during photolysis affording them very high photostability.

Dramatic effect of heteroatom backbone substituents on the ethylene polymerization behavior of bis(imino)pyridine iron catalysts.

Bis(imino)pyridine iron complexes bearing ether and thioether backbone substituents have been synthesized and evaluated for the polymerization of ethylene. The methoxy derivative is inactive whereas bulky phenoxides or thioether derivatives afford activities as high as the most active systems reported to date.

Solution structure of domain 5 of a group II intron ribozyme reveals a new RNA motif.

Domain 5 (D5) is the central core of group II intron ribozymes. Many base and backbone substituents of this highly conserved hairpin participate in catalysis and are crucial for binding to other intron domains. We report the solution structures of the 34-nucleotide D5 hairpin from the group II intron ai5 gamma in the absence and presence of divalent metal ions. The bulge region of D5 adopts a novel fold, where G26 adopts a syn conformation and flips down into the major groove of helix 1, close to the major groove face of the catalytic AGC triad. The backbone near G26 is kinked, exposing the base plane of the adjacent A-U pair to the solvent and causing bases of the bulge to stack intercalatively. Metal ion titrations reveal strong Mg(2+) binding to a minor groove shelf in the D5 bulge. Another distinct metal ion-binding site is observed along the minor groove side of the catalytic triad, in a manner consistent with metal ion binding in the ribozyme active site.

Tandem Radical Reactions and Ring‐Closing Metathesis. Application in the Synthesis of Cyclooctenes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structure and reactivity of mono- and dinuclear diiminate zinc alkyl complexes

The synthesis, structure and reactivity of several diiminate ligands are presented. The syntheses of five representative β-diiminate (BDI) zinc alkyl complexes and one β-oxo-δ-diiminate (BODDI) zinc alkyl are described. BDI ligands with varying backbone and N-aryl substituents display different solid state structures. [(BDI)ZnR] are synthesized by the reaction of (BDI)H with ZnR 2 in quantitative yield. Previously reported (BDI- 1)ZnEt is a three-coordinate monomer in the solid state whereas [(BDI- 3)ZnEt] ∞ [(BDI- 3)=2-((2,6-diisopropylphenyl)amido)-3-cyano-4-((2,6-diisopropylphenyl)imino-2-pentene] and [(BDI- 4)ZnEt] ∞ [(BDI- 4)=2-((2,6-diethylphenyl)amido)-3-cyano-4-((2,6-diethylphenyl)imino-2-pentene] form one dimensional coordination polymers. The bimetallic complex [(BODDI- 1)(ZnEt) 2] [(BODDI- 1)=2,6-bis((2,6-diisopropylphenyl)amido)-2,5-heptadien-4-one] is prepared through the reaction of (BODDI- 1)H 2 with two equivalents ZnEt 2. Both [(BDI)ZnEt] and [(BODDI)ZnEt] complexes react with acetic acid to give the acetate complexes in moderate to high yields, offering a superior synthetic route to these complexes. [(BDI)ZnR] [BDI=(BDI- 3) or 1,1,1-trifluoro-2-((2,6-diisopropylphenyl)amido)-4-((2,6-diethylphenyl)imino-2-pentene), (BDI- 5)] complexes react with MeOH to produce [{(BDI)Zn(μ-OMe)} 2Zn(μ-OMe) 2] in moderate yields. The molecular structures of [(BDI- 3)ZnEt], [(BDI- 4)ZnEt], [(BODDI- 1)(ZnEt) 2], [(BODDI- 1)Zn 2(μ-OAc) 2], [{(BDI- 3)Zn(μ-OMe)} 2Zn(μ-OMe) 2] and [{(BDI- 5)Zn(μ-OMe)} 2Zn(μ-OMe) 2] have been determined by X-ray diffraction.

Tandem radical reactions and ring-closing metathesis. Application in the synthesis of cyclooctenes.

[reaction: see text] Fumarate- and acrylate-substituted oxazolidinones undergo tandem radical reaction to form dienes in moderate to good yields. The resulting dienes provide cyclooctenes in moderate to good yields after ring-closing metathesis (RCM). The role of the carbon backbone substituents and other variables in the efficiency of the eight-membered ring formation is discussed.