Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas-phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor-like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π-system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas-phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas-phase) ionization energy of an electron donor or the (gas-phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.
Size matters: A relationship between the gas-phase ionization energy and the experienced redox potential in solution is derived. The results highlight the importance of molecular volume in determining the reduction potential in solution (see figure).
Despite intensive research on photochemical activation of sol–gel metal oxide materials, the relatively long processing time and lack of deep understanding of the underlying chemical courses have limited their broader impact on diverse materials and applications such as thin-film electronics, photovoltaics, and catalysts. Here, in-depth studies on the rapid photochemical activation of diverse sol–gel oxide films using various spectroscopic and electrical investigations for the underlying physicochemical mechanism are reported. Based on the exhaustive chemical and physical analysis, it is noted that deep ultraviolet-promoted rapid film formation such as densification, polycondensation, and impurity decomposition is possible within 5 min via in situ radical-mediated reactions. Finally, the rapid fabrication of all-solution metal oxide thin-film-transistor circuitry, which exhibits stable and reliable electrical performance with a mobility of >12 cm2 V−1 s−1 and an oscillation frequency of >650 kHz in 7-stage ring oscillator even after bending at a radius of <1 mm is demonstrated.
The general physicochemical mechanisms underlying photoactivated sol–gel reactions are described, with comprehensive chemical and structural analysis inducing rapid (<5 min) fabrication of various metal oxide films at low temperatures (<150 °C), and all-solution processed high-performance electronic devices and circuitry on ultrathin polymeric substrates are demonstrated. This will open new possibilities to prepare future electronic materials in a fast, scalable, and economic manner.
The 2D semiconductor MoS2 in its mono- and few-layer form is expected to have a significant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration. Here, modulation spectroscopy in the sub-ps and ms time scales is used to study the photoexcitation dynamics in few-layer MoS2. The results suggest that the primary photoexcitations are excitons that efficiently dissociate into charges with a characteristic time of 700 fs. Based on these findings, simple suggestions for the design of efficient MoS2 photovoltaic and photodetector devices are made.
Few-layer MoS2 flakes are intermediates between conventional semiconductors and excitonic nanomaterials. By femtosecond optical pump–probe spectroscopy it is shown that photoexcitation creates excitons as the primary species. The excitons efficiently dissociate into charge carriers with a time constant of 700 fs, making few-layer MoS2 an excellent candidate for efficient photodetectors and photovoltaic devices.
The front cover demonstrates the fabrication of sol–gel derived metal oxide electronic devices and circuits by low-temperature photochemical activation via in-situ radical-mediated reactions. On page 2807, M.-H. Yoon, S. K. Park, and colleagues show that the rapid photoactivation process enables the conversion of the sol–gel precursors into metal oxide electronic materials directly on ultra-flexible plastic substrates, which will serve as a general methodology in a rapid, scalable, and economic manner.
Mammoth genomes provide recipe for creating Arctic elephants
Nature 521, 7550 (2015). http://www.nature.com/doifinder/10.1038/nature.2015.17462
Author: Ewen Callaway
Catalogue of genetic differences between woolly mammoths and elephants reveals how ice-age giants braved the cold.
Copper-exchanged SSZ-13 is a very efficient material in the selective catalytic reduction of NOx using ammonia (deNOx-SCR) and characterizing the underlying distribution of copper sites in the material is of prime importance to understand its activity. The IR spectrum of NO adsorbed to divalent copper sites are modeled using ab initio molecular dynamics simulations. For most sites, complex multi-peak spectra induced by the thermal motion of the cation as well as the adsorbate are found. A finite temperature spectrum for a specific catalyst was constructed, which shows excellent agreement with previously reported data. Additionally these findings allow active and inactive species in deNOx-SCR to be identified. To the best of our knowledge, this is the first time such complex spectra for single molecules adsorbed to single active centers have been reported in heterogeneous catalysis, and we expect similar effects to be important in a large number of systems with mobile active centers.
Theory and practice: The IR spectra of NO adsorbed on Cu centers in a copper-containing zeolite were modeled using molecular dynamics simulations. The spectra are complex, which is due to the thermal motions of the cations and the adsorbates, and are in excellent agreement with the experimental spectra.
Chemistry: Imaging of excited electron orbitals
Nature 519, 7544 (2015). doi:10.1038/519392d
A technique could pave the way for imaging electron behaviour as chemical reactions happen.Many reactions are governed by the behaviour of electrons in excited orbital states, but these states are difficult to capture because they last only a few picoseconds (10−12 seconds).
The future of the postdoc
Nature 520, 7546 (2015). http://www.nature.com/doifinder/10.1038/520144a
Author: Kendall Powell
There is a growing number of postdocs and few places in academia for them to go. But change could be on the way.
Poly(triazine imides) intercalated with Li+ and X− (PTI/X, X=Cl or Br), which are described widely as crystalline polymeric carbon nitrides, were synthesized in a facile manner by heating a mixture of melamine and LiX. This method has the advantages of low cost, scalable production, and high efficiency. Importantly, both PTI/Cl and PTI/Br exhibit an enhanced photocatalytic performance compared to conventional graphitic polymeric carbon nitride in the degradation of rhodamine B under visible-light irradiation because of their higher visible-light-harvesting ability and charge carrier separation efficiency.
Easy does it: Crystalline polymeric graphitic carbon nitrides with an enhanced visible-light-driven photocatalytic performance are prepared by heating a mixture of melamine and lithium halides.
The cover picture shows effective hydrogen production from an aqueous solution of ammonia-borane (NH3-BH3, AB) by bimetallic FeNi nanoparticles supported on CeO2. In their Full Paper, K. Mori, T. Taga, and H. Yamashita show highly dispersed and partially oxidized amorphous FeNi NPs stabilized by strong interaction with the CeO2 is active for the catalytic dehydrogenation of AB. The advantages of this catalytic system, such as the facile preparation method, free of noble metals, and high recyclability are particularly desirable for a hydrogen vector in terms of potential industrial application in fuel cells. The article highlighted by this cover can be found on p. 1285 ff. of Issue 8, 2015.
“… There is an unprecedented global consensus that an energetic transition, not to say a revolution, is urgently required. The transition can be defined as the process by which all fossil fuels (and nuclear energy) are replaced by renewable energies. How can we achieve this transition in the shortest time? …” Read more in the Editorial by Marc Fontecave.
Nature Chemistry 7, 328 (2015). doi:10.1038/nchem.2194
Authors: Hyun Gil Cha & Kyoung-Shin Choi
Photoelectrochemical water-splitting produces hydrogen at the cathode and oxygen at the anode. The anode reaction is, however, kinetically unfavourable. Now, reduction of water at the cathode has been combined with oxidation of 5-hydroxymethylfurfural at the anode resulting in a photoelectrochemical cell that produces fuel and a useful platform chemical.