Long-lived charged states of single porphyrin-tape junctions under ambient conditions

The ability to control the charge state of individual molecules wired in two-terminal single-molecule junctions is a key challenge in molecular electronics, particularly in relation to the development of molecular memory and other computational componentry. Here we demonstrate that single porphyrin molecular junctions can be reversibly charged and discharged at elevated biases under ambient conditions due to the presence of a localised molecular eigenstate close to the Fermi edge of the electrodes. In particular, we can observe long-lived charge-states with lifetimes upwards of 1–10 seconds after returning to low bias and large changes in conductance, in excess of 100-fold at low bias. Our theoretical analysis finds charge-state lifetimes within the same time range as the experiments. The ambient operation demonstrates that special conditions such as low temperatures or ultra-high vacuum are not essential to observe hysteresis and stable charged molecular junctions

Earthquake surface ruptures on the altiplano and geomorphological evidence of normal faulting in the December 2016 (Mw 6.1) Parina earthquake, Peru

The 2016 Mw 6.1 Parina earthquake ruptured a shallow-crustal normal fault within the high Andes of south Peru. We use high-resolution DEMs and field mapping of the surface ruptures generated by the earthquake, in combination with co-seismic and post-seismic InSAR measurements, to investigate how different features of the geomorphology at Parina are generated by the earthquake cycle on the Parina Fault. We systematically mapped 12 km of NW-SE trending surface ruptures with up to ~27 cm vertical displacement and ~25 cm tensional opening along strike, separated by a gap with no observable surface ruptures. Co- and post-seismic InSAR measurements require slip below this gap in surface ruptures, implying that surface offsets observed in paleoseismic trenches may not necessarily be representative of slip at seismogenic depths, and will typically yield an underestimate of paleo-earthquake magnitudes. The surface ruptures developed along 10–20 m high cumulative scarps cutting through late Quaternary fluvio-glacial deposits and bedrock. The 2016 Parina earthquake did not rupture the full length of the late Quaternary scarps, implying that the Parina Fault does not slip in characteristic, repeat earthquakes. At Parina, and across most of the Peruvian Altiplano, normal faults are most-easily identified from recent scarps cutting late Quaternary moraine crests. In regions where there are no recently-deposited moraines, faults are difficult to identify and lack time constraints to quantify rates of fault slip. For this reason, current fault maps may underestimate the seismic hazard in the Altiplano

Biodiversity of Fusarium species in Mexico associated with ear rot in maize, and their identification using a phylogenetic approach

Fusariumproliferatum, F. subglutinans, and F. verticillioides are known causes of ear and kernel rot in maize worldwide. In Mexico, only F. verticillioides and F. subglutinans, have been reported previously as causal agents of this disease. However, Fusarium isolates with different morphological characteristics to the species that are known to cause this disease were obtained in the Highland-Valley region of this country from symptomatic and symptomless ears of native and commercial maize genotypes. Moreover, while the morphological studies were not sufficient to identify the correct taxonomic position at the species level, analyses based in the Internal Transcribed Spacer region and the Nuclear Large Subunit Ribosomal partial sequences allowed for the identification of F. subglutinans, F. solani, and F. verticillioides, as well as four species (F. chlamydosporum, F. napiforme, F. poae, and F. pseudonygamai) that had not previously been reported to be associated with ear rot. In addition, F. napiforme and F. solani were absent from symptomless kernels. Phylogenetic analysis showed genetic changes in F. napiforme, and F. pseudonygamai isolates because they were not true clones, and probably constitute separate sibling species. The results of this study suggest that the biodiversity of Fusarium species involved in ear rot in Mexico is greater than that reported previously in other places in the world. This new knowledge will permit a better understanding of the relationship between all the species involved in ear rot disease and their relationship with maize

Bias-driven conductance increase with length in porphyrin tapes

A key goal in molecular electronics has been to find molecules that facilitate efficient charge transport over long distances. Normally molecular wires become less conductive with increasing length. Here we report a series of fused porphyrin oligomers for which the conductance increases substantially with length by > 10-fold at a bias of 0.7 V. This exceptional behavior can be attributed to the rapid decrease of the HOMO-LUMO gap with the length of fused porphyrins. In contrast, for butadiyne-linked porphyrin oligomers with moderate inter-ring coupling, a normal conductance decrease with length is found for all bias voltages explored (± 1 V), although the attenuation factor (β) decreases from ca. 2 nm-1 at low bias to < 1 nm-1 at 0.9 V, highlighting that β is not an intrinsic molecular property. Further theoretical analysis using density functional theory underlines the role of inter-site coupling and indicates that this large increase in conductance with length at increasing voltages can be generalized to other molecular oligomers

Detecting mechanochemical atropisomerization within an STM break junction

We have employed the scanning tunneling microscope break-junction technique to investigate the single-molecule conductance of a family of 5,15-diaryl porphyrins bearing thioacetyl (SAc) or methylsulfide (SMe) binding groups at the ortho position of the phenyl rings (S2 compounds). These ortho substituents lead to two atropisomers, cis and trans, for each compound, which do not interconvert in solution under ambient conditions; even at high temperatures, isomerization takes several hours (half-life 15 h at 140 °C for SAc in C2Cl4D2). All the S2 compounds exhibit two conductance groups, and comparison with a monothiolated (S1) compound shows the higher group arises from a direct Au−porphyrin interaction. The lower conductance group is associated with the S-to-S pathway. When the binding group is SMe, the difference in junction length distribution reflects the difference in S−S distance (0.3 nm) between the two isomers. In the case of SAc, there are no significant differences between the plateau length distributions of the two isomers, and both show maximal stretching distances well exceeding their calculated junction lengths. Contact deformation accounts for part of the extra length, but the results indicate that cis-to-trans conversion takes place in the junction for the cis isomer. The barrier to atropisomerization is lower than the strength of the thiolate Au−S and Au−Au bonds, but higher than that of the Au−SMe bond, which explains why the strain in the junction only induces isomerization in the SAc compound

Chilling-Dependent Release of Seed and Bud Dormancy in Peach Associates to Common Changes in Gene Expression

Reproductive meristems and embryos display dormancy mechanisms in specialized structures named respectively buds and seeds that arrest the growth of perennial plants until environmental conditions are optimal for survival. Dormancy shows common physiological features in buds and seeds. A genotype-specific period of chilling is usually required to release dormancy by molecular mechanisms that are still poorly understood. In order to find common transcriptional pathways associated to dormancy release, we analyzed the chilling-dependent expression in embryos of certain genes that were previously found related to dormancy in flower buds of peach. We propose the presence of short and long-term dormancy events affecting respectively the germination rate and seedling development by independent mechanisms. Short periods of chilling seem to improve germination in an abscisic acid-dependent manner, whereas the positive effect of longer cold treatments on physiological dwarfing coincides with the accumulation of phenylpropanoids in the seed

Corrigendum to:Unusual Length Dependence of the Conductance in Cumulene Molecular Wires (Angewandte Chemie International Edition, (2019), 58, 25, (8378-8382), 10.1002/anie.201901228)

The authors of this Communication have noticed a mistake in Table 1. In the second column of this table, the single-molecule conductances for compounds 3 and 5 were incorrect (but the values plotted in Figure 3 g were correct). The correct log(G/G0) values for compounds 3 and 5 are −3.70 and −3.77, respectively. No changes in analysis or conclusions result from this correction. The authors apologize for this transcription error

The Electrical Properties of Porphyrin Single Molecule Wires

Measurement of the electrical properties of porphyrin single molecular wires sandwiched between metal contacts is now an experimental reality and such methods have contributed to understanding charge flow through and the current-voltage response of porphyrin wires. We have exploited scanning-tunneling-microscopy-based methods such as the STM break junction method and the non-contact I(s) technique (I=current and s=distance between STM tip and substrate) for achieving this feat. In both these techniques single molecule junctions are formed by bringing a gold STM tip into contact or very close to the gold substrate surface. As the tip is rapidly retracted the conductance of the porphyrin molecule junction or the complete current-voltage response can be recorded. When combined with statistical analysis this provides a strong platform for investigating the electrical properties of porphyrin junctions. This presentation will review some of our findings on the electrical properties of porphyrin single molecule wires as well as presenting our latest data recorded using the STM break junction method, with a focus on the mechanochemical properties of 5,15-diaryl porphyrins with thiol end groups. Topics to be discussed include: (1) The mechanochemical properties of 5,15-diaryl porphyrins with thiol end groups. (2) Single molecule junction formation mechanisms and evolution during junction stretching. (3) The influence of contacting groups on porphyrin electrical junctions. (4) Long range electron transport in porphyrin oligomers, their current-voltage response and unusual voltage dependent length decay of conductance. (5) Mechanisms of charge transport in porphyrin single molecule wires. REFERENCES to our work on porphyrin single molecule electronics (1) G. Sedghi, K. Sawada, L. J. Esdaile, M. Hoffmann, H. L. Anderson, D. Bethell, W. Haiss, S. J. Higgins, and R. J. Nichols, Single Molecule Conductance of Porphyrin Wires with Ultralow Attenuation, Journal of the American Chemical Society 130, 8582 (2008). (2) G. Sedghi, V.M. Garcia-Suarez, L.J. Esdaile, H.L. Anderson, C.J. Lambert, S. Martin, D. Bethell, S.J. Higgins, M. Elliott, N. Bennett, J.E. Macdonald and R.J. Nichols, Long-range electron tunnelling in oligo-porphyrin molecular wires, Nature Nanotechnology, 6, 517-523, (2011). (3) G. Sedghi, L.J. Esdaile, H.L. Anderson, S. Martin, D. Bethell, S.J. Higgins and R.J. Nichols, Comparison of the Conductance of Three Types of Porphyrin-Based Molecular Wires: beta,meso,beta-Fused Tapes, meso-Butadiyne-Linked and Twisted meso-meso Linked Oligomers, Advanced Materials, 24, 653-, (2012). (4) Edmund Leary, Cécile Roche, Hua-Wei Jiang, Iain Grace, M. Teresa González, Gabino Rubio-Bollinger, Yaoyao Xiong, Qusiy Al-Galiby, Maria A. Lebedeva, Kyriakos Porfyrakis, Nicolás Agrait, Andrew Hodgson, Simon J. Higgins, Colin J. Lambert, Harry L. Anderson, and Richard J. Nichols, JACS submitted 2017. Figure 1 <jats:p /