Synthesis and self-assembly of functionalized hexa-peri-hexabenzocoronenes

Monolayers of hexa-alkyl substituted derivatives of hexa-peri-hexabenzocoronene (HBC) 1b have previously been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). It is expected that different functional groups (electron donating or withdrawing) connected to the aromatic core will influence the packing pattern and possibly the current-voltage characteristics as well. In order to provide suitable model systems, a new synthetic approach to synthesize functionalized HBC derivatives has been developed. This was accomplished by [4 + 2]-cycloaddition of suitably bromo-substituted diphenylacetylenes and 2,3,4,5-tetraarylcyclopenta-2,4-dien-1-ones followed by an oxidative cyclodehydrogenation with iron(III) chloride/nitromethane. Using this strategy three different substitution patterns were synthesized: 2-bromo-5,8,11.14,17-pentadodecylhexa-pecri-hexabenzocoronene (2a), 2,5-dibromo-8,11,14,17-pentadodecylhexa-peri-hexabenzocoronene (2b), and 2,11-dibromo5,8,14,17-pentadodecylhexa-peri-hexa-benzocoronene (2c). These bromo-substituted HBC derivatives were subjected to palladium catalyzed coupling reactions to give donor (alkoxy, amino) as well as acceptor (ester, cyano) substituted derivatives. The self-assembly of these new HBC derivatives was studied in the bulk as well as at an interface. DSC, optical microscopy, and X-ray diffraction revealed the existence of columnar mesophases. The bulk structure in the mesophase is largely insensitive to changes of the substitution pattern; however, in situ scanning tunneling microscopy at the solid-fluid interface between an organic solution of the HBC derivative and highly oriented pyrolytic graphite reveals different packing patterns of the first adsorbed monolayer.     

Synthesis and structural characterization of 2,6-dimesitylphenyl complexes of scandium, ytterbium, and yttrium

The molecular structures of a number of 2,6-dimesitylphenyl-based (2,6-dimesitylphenyl = Dmp) complexes of the group 3 elements scandium and yttrium as well as of the lanthanide element ytterbium are reported. Reaction of 1 equiv of DmpLi with 1 equiv of MCl(3) (M = Sc, Yb, Y) in tetrahydrofuran at room temperature followed by crystallization from toluene/hexanes at -30 degrees C produces DmpMCl(2)(THF)(2) (M = Sc: 1; M = Yb: 2) and DmpMCl(2)(THF)(3) (M = Y: 3), respectively. The one-pot reaction of DmpLi with 1 equiv of YbCl(3) in tetrahydrofuran at room temperature followed by addition of 1 equiv of KO(t)Bu produces the heterobimetallic monoalkoxide complex DmpYb(THF)(O(t)Bu)(mu-Cl)(2)Li(THF)(2) (4), which was crystallized from toluene/tetrahydrofuran (20:1) at -30 degrees C. Crystal data for 1: monoclinic, P2(1)/n; T = 203 K; a = 10.178(3) A; b = 15.468(3) A; c = 20.132(5) A; beta = 101.85(3) degrees; V = 3102.0(17) A(3); Z' = 4; D(calcd) = 1.228 g cm(-3); R(1) = 5.89%. Crystal data for 2: monoclinic, P2(1)/n; T = 173 K; a = 10.2447(7) A; b = 15.5683(12) A; c = 20.0979(14) A; beta = 101.749(4) degrees; V = 3238.3(5) A(3); Z' = 4; D(calcd) = 1.485 g cm(-3); R(1) = 4.32%. Crystal data for 3: monoclinic, P2(1)/n; T = 203 K; a = 15.950(3) A; b = 11.865(2) A; c = 18.254(3) A; beta = 92.323(3) degrees; V = 3451.9(10) A(3); Z' = 4; D(calcd) = 1.327 g cm(-)(3); R(1) = 4.43%. Crystal data for 4: triclinic, P1; T = 193 K; a = 10.2252(2) A; b = 11.3497(2) A; c = 18.5814(2) A; alpha = 98.7353(6) degrees; beta = 102.8964(6) degrees; gamma = 94.8058(5) degrees; V = 2062.09(5) A(3); Z' = 2; D(calcd) = 1.375 g cm(-3); R(1) = 4.56%. The molecular structures of 1-3 feature monomeric complexes with distorted trigonal-bipyramidal (1 and 2) or octahedral (3) coordination geometry about the metal atom, with the two chlorine atoms occupying the axial positions. 4 represents the first example of an alkoxide derivative of a terphenyl lanthanide complex. The molecular structure of the ate complex 4 exhibits a heavily distorted trigonal-bipyramidal coordination polyhedron about the ytterbium atom, with one of the mu-chlorine atoms and the oxygen atom of the tetrahydrofuran ligand representing the axial positions of the trigonal-bipyramidal arrangement. A terminal alkoxide ligand is another main feature of the molecular structure of complex 4.     

Catalytic partial oxidation of methane to synthesis gas over a ruthenium catalyst: the role of the oxidation state

The catalytic partial oxidation of methane to synthesis gas over ruthenium catalysts was investigated by thermogravimetry coupled with infrared spectroscopy (TGA-FTIR) and in situ X-ray absorption spectroscopy (XAS). It was found that the oxidation state of the catalyst influences the product formation. On oxidized ruthenium sites, carbon dioxide was formed. The reduced catalyst yielded carbon monoxide as a product. The influence of the temperature was also investigated. At temperatures below the ignition point of the reaction, the catalyst was in an oxidized state. At temperatures above the ignition point, the catalyst was reduced. This was also confirmed by the in situ XAS spectroscopy. The results indicate that both a direct reaction mechanism as well as a combustion-reforming mechanism can occur. The importance of knowing the oxidation state of the surface is discussed and a method to determine it under reaction conditions is presented.     

Adsorption-induced intramolecular dipole: correlating molecular conformation and interface electronic structure

The interfaces formed between pentacene (PEN) and perfluoropentacene (PFP) molecules and Cu(111) were studied using photoelectron spectroscopy, X-ray standing wave (XSW), and scanning tunneling microscopy measurements, in conjunction with theoretical modeling. The average carbon bonding distances for PEN and PFP differ strongly, that is, 2.34 A for PEN versus 2.98 A for PFP. An adsorption-induced nonplanar conformation of PFP is suggested by XSW (F atoms 0.1 A above the carbon plane), which causes an intramolecular dipole of approximately 0.5 D. These observations explain why the hole injection barriers at both molecule/metal interfaces are comparable (1.10 eV for PEN and 1.35 eV for PFP) whereas the molecular ionization energies differ significantly (5.00 eV for PEN and 5.85 eV for PFP). Our results show that the hypothesis of charge injection barrier tuning at organic/metal interfaces by adjusting the ionization energy of molecules is not always readily applicable.     

Functionalized silicone nanofilaments: a novel material for selective protein enrichment

We present a simple and versatile technique of tailoring functionalized surface structures for protein enrichment and purification applications based on a superhydrophobic silicone nanofilament coating. Using amino and carboxyl group containing silanes, silicone nanofilament templates were chemically modified to mimic anionic and cationic exchange resins. Investigations on the selectivity of the functionalized surfaces toward adsorption of charged model proteins were carried out by means of fluorescence techniques. Due to a high contact area resulting from the nanoroughness of the coating, excellent protein retention characteristics under various conditions were found. The surfaces were shown to be highly stable and reusable over several retention-elution cycles. Especially the full optical transparency and the possibility to use glass substrates as support material open new opportunities for the development of optical biosensors, open geometry microfluidics, or lab-on-a-chip devices.     

Conformational Analysis,Thermal Rearrangement,and EI‐MS Fragmentation Mechanism of (1(10)E,4E,6S,7R)‐Germacradien‐6‐ol by 13C‐Labeling Experiments

An uncharacterized terpene cyclase from Streptomyces pratensis was identified as (+)‐(1(10)E,4E,6S,7R)‐germacradien‐6‐ol synthase. The enzyme product exists as two interconvertible conformers, resulting in complex NMR spectra. For the complete assignment of NMR data, all fifteen (¹³C₁)FPP isotopomers (FPP=farnesyl diphosphate) and (¹³C₁₅)FPP were synthesized and enzymatically converted. The products were analyzed using various NMR techniques, including ¹³C, ¹³C COSY experiments. The (¹³C)FPP isotopomers were also used to investigate the thermal rearrangement and EI fragmentation of the enzyme product.     

Fabrication of large-scale ultra-smooth metal surfaces by a replica technique

The IR double-resonance techniques IR/R2PI (infrared/resonant 2-photon ionization), IR/PIRI (infrared-photo-induced Rydberg ionization) and IR-photodissociation spectroscopy are valuable tools to investigate structure, vibrations, and dynamical processes of neutral and ionic hydrogen-bonded clusters containing aromatic molecules. In this paper we report on the application of the IR double-resonance techniques to determine the NH and OH stretching vibrations of 4-aminophenol and 4-aminophenol(H₂O)₁, both in the neutral (S₀) and ionic (D₀) ground state. All vibrational frequencies obtained for 4-aminophenol and the cluster are compared with the values obtained from ab initio and DFT calculations. In the S₀ state, a trans-linear arrangement of 4-aminophenol(H₂O)₁ is obtained containing an O-H·· O hydrogen bond. In the D₀ state an overlay of two spectra can be observed resulting from the trans-linear structure and a second structure which contains a N-H·· O hydrogen bond. The observation of these two structures within the ion is an interesting example of a rearrangement reaction in the ionic state. Received: 3 November 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Die Kristallstrukturen von PPh4[MCl5(NCMe)] · MeCN (M = Ti,Zr), zwei Modifikationen von PPh4[TiCl5(NCMe)] und von cis‐TiCl4(NCMe)2 · MeCN

The Crystal Structures of PPh₄[MCl₅(NCMe)] · MeCN (M = Ti, Zr), two Modifications of PPh₄[TiCl₅(NCMe)] and of cis ‐TiCl₄(NCMe)₂ · MeCN The title compounds were obtained by reactions of TiCl₄ or ZrCl₄, respectively, with PPh₄Cl and acetonitrile in the presence of S₂Cl₂. PPh₄[TiCl₅(NCMe)] · MeCN is unstable and emanates the incorporated acetonitrile. PPh₄[TiCl₅(NCMe)] forms the two modifications aP114 and mP228, the latter being more stable. The crystal structures were determined by X‐ray diffraction. Triclinic PPh₄[TiCl₅(NCMe)]‐(aP114) crystallizes in a distorted variety at the tetragonal AsPh₄[RuNCl₄] type, i. e. with PPh₄⁺ ions that are piled to columns in the c direction; the [TiCl₅(NCMe)]^– ions are tilted vs. this direction and thus cause the symmetry reduction from P4/n to P1. PPh₄[TiCl₅(NCMe)] · MeCN and PPh₄[ZrCl₅(NCMe)] · MeCN also have the same packing principle as in AsPh₄[RuNCl₄] with a symmetry reduction from P4/n to P112₁/n and a doubled c axis. Instead, PPh₄[TiCl₅(NCMe)]‐(mP228) has a packing with (PPh₄⁺)₂ pairs. Orthorhombic TiCl₄(NCMe)₂ · MeCN contains molecules having two acetonitrile ligands attached to the Ti atom in a cis configuration.