Antiaromatic non-alternant heterocyclic compounds as molecular wires

“The dataset that supports the findings of this study are archived in the Universidad Autónoma de Madrid data repository e‐cienciaDatos in https://doi.org/10.21950/BLB9NR”We have theoretically studied the electron-transport properties of a family of molecular junctions containing the non-alternant antiaromatic pentalene moiety stabilised with various 5-membered heterocycles. For this purpose, we used a combination of density functional theory and Green’s function techniques. We have focussed on dithieno derivatives to understand if the relative position of the heteroatom influences the transport properties as significantly as it does the degree of antiaromaticity. We found that the heteroatom position does significantly affect the shape of the transmission curves, but there is no correlation between the degree of antiaromaticity and the magnitude of the transmission at the Fermi level. Overall, we find that this behaviour is well-modelled by tight-binding calculations and the graphical prediction scheme. On the other hand, curly arrow rules fail for certain isomers, regardless of the degree of antiaromaticity. Reasons for this discrepancy are discusseWe thank the Spanish MICIN for the Marı´a de Maeztu Programme for Units of Excellence in R&D (grant No. CEX2018-000805-M). Financial support from MCIN/AEI/10.13039/501100011033 is acknowledged by L. A. Z. (grant PID2021-125604NB-I00), E. L. (PID2021-127964NB-C21). We also thank the Universidad Auto´noma de Madrid and the Comunidad de Madrid (grants No. SI3/PJI/ 2021-00191). E. L. thanks the Comunidad de Madrid Atraccio´n de Talento grant 2019-T1/IND-16384. IMDEA Nanociencia acknowledges support from the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (CEX2020-001039-S

A Molecular Platinum Cluster Junction: A Single-Molecule Switch

We present a theoretical study of the electronic transport through single-molecule junctions incorporating a Pt6 metal cluster bound within an organic framework. We show that the insertion of this molecule between a pair of electrodes leads to a fully atomically engineered nano-metallic device with high conductance at the Fermi level and two sequential high on/off switching states. The origin of this property can be traced back to the existence of a HOMO which consists of two degenerate and asymmetric orbitals, lying close in energy to the Fermi level of the metallic leads. Their degeneracy is broken when the molecule is contacted to the leads, giving rise to two resonances which become pinned close to the Fermi level and display destructive interference.Comment: 4 pages, 4 figures. Reprinted (adapted) with permission from J. Am. Chem. Soc., 2013, 135 (6), 2052. Copyright 2013 American Chemical Societ

Single-molecule conductance of a chemically modified, {\pi}-extended tetrathiafulvalene and its charge-transfer complex with F4TCNQ

We describe the synthesis and single molecule electrical transport properties of a molecular wire containing a ${\pi}$-extended tetrathiafulvalene (exTTF) group and its charge-transfer complex with F4TCNQ. We form single molecule junctions using the in-situ break junction technique using a home-built scanning tunneling microscope with a range of conductance between 10 G$_{0}$ down to 10$^{-7}$ G$_{0}$. Within this range we do not observe a clear conductance signature of the neutral parent molecule, suggesting either that its conductance is too low or that it does not form stable junctions. Conversely, we do find a clear conductance signature in the experiments carried out on the charge-transfer complex. Due to the fact we expected this species to have a higher conductance than the neutral molecule, we believe this supports the idea that the conductance of the neutral molecule is very low, below our measurement sensitivity. This is further supported by our theoretical calculations. To the best of our knowledge, these are the first reported single molecule conductance measurements on a molecular charge-transfer species

Single-Molecule Conductance of 1,4-Azaborine Derivatives as Models of BN-doped PAHs

The single–molecule conductance of a series of BN-acene-like derivatives has been measured by using scanning tunneling break-junction techniques. A strategic design of the target molecules has allowed us to include azaborine units in positions that unambiguously ensure electron transport through both heteroatoms, which is relevant for the development of customized BN-doped nanographenes. We show that the conductance of the anthracene azaborine derivative is comparable to that of the pristine all-carbon anthracene compound. Notably, this heteroatom substitution has also allowed us to perform similar measurements on the corresponding pentacene-like compound, which is found to have a similar conductance, thus evidencing that B–N doping could also be used to stabilize and characterize larger acenes for molecular electronics applications. Our conclusions are supported by state-of-the-art transport calculations.This work has been supported by the MICINN projects PID2019-105458RB-I00, PID2019-106732GB-I00, PGC2018-101873-A-I00, CTQ2017-85454-C2-1-P, FIS2016-77889-R and MAT2017-88693-R, the European Research Council (ERC) (677023), the FEDER/Junta de Andalucía project A-FQM-221-UGR18 and the Comunidad de Madrid project NanoMagCOST (CM S2018/NMT-4321). We also thank Severo Ochoa Programme for Center of Excellence in R&D (SEV-2016-0686) and María de Maeztu Programme for Units of Excellence in R&D (CEX2018-000805-M). We acknowledge the allocation of computer time by the Red Española de Supercomputación and the Centro de Computación Científica at the Universidad Autónoma de Madrid (CCC-UAM). J.G.F. thanks the PFI program of the MICINN co-financed by the European Social Fund. I.R.M. thanks MICINN for a Personal Técnico de Apoyo contract (PTA2017-13681-I). E.L. thanks the Comunidad de Madrid grant Atracción de Talento 2019-T1/IND-16384

Chiral Single-Molecule Potentiometers Based on Stapled ortho- Oligo(phenylene)ethynylenes

We report on the chemical design of chiral molecular junctions with stress-dependent conductance, whose helicity is maintained during the stretching of a single molecule junction due to the stapling of both ends of the inner helix. In the reported compounds, different conductive pathways are observed, with clearly different conductance values and plateau-length distributions, attributed to different conformations of the helical structures. The large chiro-optical responses and the potential use of these molecules as unimolecular spin filters have been theoretically proved using state-of-the-art Density Functional Theory (DFT) calculations, including a fully ab-initio estimation of the CISS-originating spin polarization which is done, for the first time, for a realistic molecular system

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