Highlights 2016

Synthesis, Structure, and Chiroptical Properties of a Double [7]Heterohelicene
Xiao-Ye Wang, Xin-Chang Wang, Akimitsu Narita, Manfred Wagner, Xiao-Yu Cao, Xinliang Feng, and Klaus Müllen
Synthesis, Structure, and Chiroptical Properties of a Double [7]Heterohelicene
Double helicenes (structures with two helicenes fused together) have recently attracted great interest due to their inherent chiral properties and potential applications in asymmetric catalysis, molecular machines, and chiroptical devices. However, compared to the inspiring advances in the synthesis of higher monohelicenes, a double helicene longer than one circle of a helix has remained elusive. Herein, we report the first double [7]helicene with intramolecular π-layers, which exhibits the highest isomerization energy barrier, facilitating the separation of enantiomers by chiral HPLC and the investigation of chiroptical properties by CD spectroscopy. This work has established a new record on the way to higher double helicenes, and provided conformationally stable materials for further applications.
© ACS (2016)
Double [7]heterohelicene as a new record
High-precision estimate of the hydrodynamic radius for self-avoiding walks
Nathan Clisby and Burkhard Dünweg
High-precision estimate of the hydrodynamic radius for self-avoiding walks
The universal asymptotic amplitude ratio between the gyration radius and the hydrodynamic radius of self-avoiding walks is estimated by high-resolution Monte Carlo simulations. By studying chains of length of up to N=225≈34×106 monomers, we find that the ratio takes the value RG/RH=1.5803940(45), which is several orders of magnitude more accurate than the previous state of the art. This is facilitated by a sampling scheme which is quite general and which allows for the efficient estimation of averages of a large class of observables. The competing corrections to scaling for the hydrodynamic radius are clearly discernible. We also find improved estimates for other universal properties that measure the chain dimension. In particular, a method of analysis which eliminates the leading correction to scaling results in a highly accurate estimate for the Flory exponent of ν=0.58759700(40).
© Nathan Clisby (2016)
Typical conformation of a 5000-step self-avoiding walk on the 3D simple cubic lattice, generated by N. Clisby's highly efficient SAW-tree algorithm. The actual study used chains of up to 34 million steps.
Synthesis of graphene nanoribbons by ambient-pressure chemical vapor deposition and device integration
Zongping Chen, Wen Zhang, Carlos-Andres Palma, Alberto Lodi Rizzini, Bilu Liu, Ahmad N. Abbas, Nils Richter, Leonardo Martini, Xiao-Ye Wang, Nicola Cavani, Hao Lu, Neeraj Mishra, Camilla Coletti, Reinhard Berger, Florian Klappenberger, Mathias Kläui, Andrea Candini, Marco Affronte, Chongwu Zhou, Valentina De Renzi, Umberto del Pennino, Johannes V. Barth, Hans Joachim Räder, Akimitsu Narita, Xinliang Feng, and Klaus Müllen
Synthesis of graphene nanoribbons by ambient-pressure chemical vapor deposition and device integration
Graphene nanoribbons (GNRs), quasi-one-dimensional graphene strips, have shown great potential for nano-scale electronics, optoelectronics and photonics. Atomically precise GNRs can be “bottom-up” synthesized by surface-assisted assembly of molecular building blocks under ultrahigh vacuum conditions. However, a large-scale and efficient synthesis of such GNRs at low cost remains a significant challenge. Here we report an efficient “bottom-up” chemical vapor deposition (CVD) process for inexpensive and high-throughput growth of structurally defined GNRs with varying structures under ambient-pressure conditions. The high quality of our CVD-grown GNRs is validated by a combination of different spectroscopic and microscopic characterizations. Facile, large-area transfer of GNRs onto insulating substrates and subsequent device fabrication demonstrate their promising potential as semiconducting material, exhibiting high current on/off ratio up to 6,000 in field-effect transistor devices. This value is three orders of magnitude higher than values reported so far for other thin film transistors of structurally defined GNRs. Notably, on-surface mass-spectrometry analyses of polymer precursors provide unprecedented evidences for the chemical structures of the resulting GNRs, especially the heteroatom-doping and heterojunctions. These results pave the way toward the scalable and controllable growth of GNRs for future applications.
© ACS (2016)
High-throughput bottom-up synthesis of structurally defined graphene nanoribbons by ambient-pressure CVD process, and their unprecedentedly strong gate modulation in field-effect transistor devices.
Both Inter- and Intramolecular Coupling of O−H Groups Determine the Vibrational Response of the Water/Air Interface
Jan Schaefer, Ellen H. G. Backus, Yuki Nagata, and Mischa Bonn
Both Inter- and Intramolecular Coupling of O−H Groups Determine the Vibrational Response of the Water/Air Interface
For many geo-chemical and biological processes, interfacial water is of particular importance. However, developing an accurate understanding of its macroscopic properties requires a molecular level insight into its structure and dynamics which can be accessed by for example vibrational spectroscopy. As such, Sum Frequency Generation Spectroscopy can be used to probe the vibrational response of the first few molecular layers at interfaces. For addressing the local environment of aqueous interfacial molecules, the O−H stretch vibrational band of water is a widely employed fingerprint signal. However, it has been recognized that inter- and intramolecular coupling between O−H groups is not only critically affecting the band shape but also relevant for understanding dissipation of excess energy after chemical reactions. A unifying picture of contributing coupling effects is therefore necessary to connect the vibrational spectra with physical insight into the system. In this work, we disentangle inter- from intra-molecular coupling effects in bulk and surface water through comparison between the water O−H stretch band, for which both coupling effects are present and simple alcohols, for which intermolecular coupling dominates.
© MPI-P (2016)
Schematic picture of inter- and intramolecular coupling between O−H groups of surface water molecules probed with Vibrational Sum Frequency Generation Spectroscopy.
Spanning the Solar Spectrum: Azopolymer Solar Thermal Fuels for Simultaneous UV and Visible Light Storage
Andrew K. Saydjari, Philipp Weis, and Si Wu
Spanning the Solar Spectrum: Azopolymer Solar Thermal Fuels for Simultaneous UV and Visible Light Storage
A UV-sensitive azopolymer and a visible-light-sensitive azopolymer are combined with a fluorescent dye and a filter to create a four-layer solar thermal cell with record efficiency gravimetric energy density. While most azopolymers discharge under visible irradiation, this device can use a true solar spectrum because fluorescent down-conversion enables storage of light that would otherwise cause discharging.
© Wiley (2016)
A four-layer solar thermal cell consisting of a UV-sensitive azopolymer and a visible-light-sensitive azopolymer, combined with a fluorescent dye and a filter, is used to store solar energy.
Nonlinear control of high-frequency phonons in spider silk
Dirk Schneider, Nikolaos Gomopoulos, Cheong Y. Koh, Periklis Papadopoulos, Friedrich Kremer, Edwin L. Thomas & George Fytas
Nonlinear control of high-frequency phonons in spider silk
Spider dragline silk possesses superior mechanical properties compared with synthetic polymers with similar chemical structure due to its hierarchical structure comprised of partially crystalline oriented nanofibrils. To date, silk’s dynamic mechanical properties have been largely unexplored. Here we report an indirect hypersonic phononic bandgap and an anomalous dispersion of the acoustic-like branch from inelastic (Brillouin) light scattering experiments under varying applied elastic strains. We show the mechanical nonlinearity of the silk structure generates a unique region of negative group velocity, that together with the global (mechanical) anisotropy provides novel symmetry conditions for gap formation. The phononic bandgap and dispersion show strong nonlinear strain-dependent behaviour. Exploiting material nonlinearity along with tailored structural anisotropy could be a new design paradigm to access new types of dynamic behaviour.
© Nature Materials (2016)
Phonon band diagram for wave propagation parallel (modes 1 and 3) and normal (mode 2) to the spider silk fiber in the natural (0%) and strained (18%) state along with the theoretical prediction based on an anharmonic linear chain.
 
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