Understanding the Role of Parallel Pathways via In‐Situ Switching of Quantum Interference in Molecular Tunneling Junctions

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window.     

Gradients of Rectification: Tuning Molecular Electronic Devices by the Controlled Use of Different‐Sized Diluents in Heterogeneous Self‐Assembled Monolayers

Molecular electronics has received significant attention in the last decades. To hone performance of devices, eliminating structural defects in molecular components inside devices is usually needed. We herein demonstrate this problem can be turned into a strength for modulating the performance of devices. We show the systematic dilution of a monolayer of an organic rectifier (2,2′‐bipyridine‐terminated n‐undecanethiolate) with electronically inactive diluents (n‐alkanethiolates of different lengths), gives remarkable gradients of rectification. Rectification is finely tunable in a range of approximately two orders of magnitude, retaining its polarity. Trends of rectification against the length of the diluent indicate the gradient of rectification is extremely sensitive to the molecular structure of the diluent. Further studies reveal that noncovalent intermolecular interactions within monolayers likely leads to gradients of structural defect and rectification.     

Constructive Quantum Interference in Single-Molecule Benzodichalcogenophene Junctions

Heteroatom substitution into the cores of alternant, aromatic hydrocarbons containing only even-membered rings is attracting increasing interest as a method of tuning their electrical conductance. Here, the effect of heteroatom substitution into molecular cores of non-alternant hydrocarbons, containing odd-membered rings, is examined. Benzodichalcogenophene (BDC) compounds are rigid, planar π-conjugated structures, with molecular cores containing five-membered rings fused to a six-membered aryl ring. To probe the sensitivity or resilience of constructive quantum interference (CQI) in these non-bipartite molecular cores, two C₂-symmetric molecules (I and II) and one asymmetric molecule (III) were investigated. I (II) contains S (O) heteroatoms in each of the five-membered rings, while III contains an S in one five-membered ring and an O in the other. Differences in their conductances arise primarily from the longer S−C and shorter O−C bond lengths compared with the C−C bond and the associated changes in their resonance integrals. Although the conductance of III is significantly lower than the conductances of the others, CQI was found to be resilient and persist in all molecules.     

Quantum Interference Enhanced Chemical Responsivity in Single-Molecule Dithienoborepin Junctions

Providing a chemical control over charge transport through molecular junctions is vital to developing sensing applications at the single-molecule scale. Quantum-interference effects that affect the charge transport through molecules offer a unique chance to enhance the chemical control. Here, we investigate how interference effects can be harnessed to optimize the response of single molecule dithienoborepin (DTB) junctions to the specific coordination of a fluoride ion in solution. The single-molecule conductance of two DTB isomers is measured using scanning tunneling microscopy break-junction (STM-BJ) before and after fluoride ion exposure. We find a significant change of conductance before and after the capture of a fluoride ion, the magnitude of which depends on the position of the boron atom in the molecular structure. This single-molecule sensor exhibits switching ratios of up to four orders of magnitudes, suggesting that the boron–fluoride coordination can lead to quantum-interference effects. This is confirmed by a quantum chemical characterization, pointing toward a cross-conjugated path through the molecular structure as the origin of the effect.     

Replacing AgTSSCH2‐R with AgTSO2C‐R in EGaIn‐Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

This paper compares rates of charge transport by tunneling across junctions with the structures Ag^TSX(CH₂)_2nCH₃ //Ga₂O₃ /EGaIn (n=1–8 and X= ? SCH₂? and ? O₂C? ); here Ag^TS is template‐stripped silver, and EGaIn is the eutectic alloy of gallium and indium. Its objective was to compare the tunneling decay coefficient (β, Å^?1) and the injection current (J₀, A cm^?2) of the junctions comprising SAMs of n‐alkanethiolates and n‐alkanoates. Replacing Ag^TSSCH₂‐R with Ag^TSO₂C‐R (R=alkyl chains) had no significant influence on J₀ (ca. 3×10³ A cm^?2) or β (0.75–0.79 Å^?1)—an indication that such changes (both structural and electronic) in the Ag^TSXR interface do not influence the rate of charge transport. A comparison of junctions comprising oligo(phenylene)carboxylates and n‐alkanoates showed, as expected, that β for aliphatic (0.79 Å^?1) and aromatic (0.60 Å^?1) SAMs differed significantly.     

Gating of Quantum Interference in Molecular Junctions by Heteroatom Substitution

To guide the choice of future synthetic targets for single‐molecule electronics, qualitative design rules are needed, which describe the effect of modifying chemical structure. Here the effect of heteroatom substitution on destructive quantum interference (QI) in single‐molecule junctions is, for the first time experimentally addressed by investigating the conductance change when a “parent” meta‐phenylene ethylene‐type oligomer (m‐OPE) is modified to yield a “daughter” by inserting one nitrogen atom into the m‐OPE core. We find that if the substituted nitrogen is in a meta position relative to both acetylene linkers, the daughter conductance remains as low as the parent. However, if the substituted nitrogen is in an ortho position relative to one acetylene linker and a para position relative to the other, destructive QI is alleviated and the daughter conductance is high. This behavior contrasts with that of a para‐connected parent, whose conductance is unaffected by heteroatom substitution. These experimental findings are rationalized by transport calculations and also agree with recent “magic ratio rules”, which capture the role of connectivity in determining the electrical conductance of such parents and daughters.     

Full Quantum Theory of Molecular Hot-Electroluminescence in Scanning Tunneling Microscope Tunnel Junctions

The pursuit of nanoscale photonics and molecular optoelectronics has stimulated a lot of interests in scanning tunneling microscope (STM) induced molecular emission. In this work, we have introduced a full quantum mechanical approach instead of the previous semiclassical theory to consider the quantized surface plasmon modes in this system. By considering the mutual interactions between a single molecule and the quantized surface plasmon, we have studied the molecular electroluminescence from STM tunnel junctions. Due to the coupling to the surface plasmons, the spontaneous emission rate and the fluorescence intensity of themolecule are both enormously enhanced. In particular, we show that when the radiative decay rate becomes comparable to the vibrational damping rate, hot-electroluminescence can be observed. All these findings are believed to be instructive for further developments of both molecular electronics and photonics.     

Generation of Molecular Fields, Quantum Similarity Measures and Related Questions

A straightforward discussion on how to generate molecular fields is developed within the postulates of quantum mechanics. The theoretical formalism points towards the generalization and extension of the well-known molecular field forms, associated to density function and electrostatic molecular potential (EMP), including another category of fields associated to quantum molecular similarity measures. The results show that the new formalism can be easily applied to obtain an unlimited number of new information about molecular behavior.     

Dependency of the Tunneling Decay Coefficient in Molecular Tunneling Junctions on the Topography of the Bottom Electrodes

A controversy in molecular electronics is the unexplained large spread in values of the tunneling decay coefficient β in tunneling junctions with self‐assembled monolayers of n‐alkanethiolates (SC_n). We show control of the β value over the range 0.4–1.0 n_C⁻¹ in junctions by changing the topography of the bottom electrodes that support the SAMs. Very low β values (0.4–0.5 n_C⁻¹) are obtained for rough surfaces with large areas of exposed grain boundaries, while β=1.0 n_C⁻¹ for smooth surfaces with small areas of exposed grain boundaries.     

High-Temperature Quantum Tunneling and Hydrogen Bonding Rearrangements Characterize the Solid-Solid Phase Transitions in a Phosphonium-Based Protic Ionic Liquid

We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C₈H₁₇PD₃][NTf₂] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD₃ group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD₃ deuterons. A transition enthalpy of about 12 kJ mol⁻¹ from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded P−D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD₃ group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.     

离子液体的分子模拟与量化计算

分子模拟方法是研究离子液体结构与性质关系非常有效的方法,可以从分子间相互作用出发研究离子液体的微观结构、热力学和动力学性质;量子化学计算则在分子、电子水平上对离子液体的结构、性能及催化机理进行理论研究。本文综述了分子模拟方法应用于离子液体体系的研究进展,重点介绍了利用分子动力学模拟和量子化学计算方法对不同离子液体进行研究,获得离子液体的结构性质、光谱性质(红外光谱、拉曼光谱)及离子液体催化反应机理等,为探讨离子液体结构-性质的关系、离子对的作用方式、催化反应活性中心、反应途径、反应活化能、振动模式和频率以及设计功能性离子液体提供理论导向。     

Quantum Mechanics/Molecular Mechanics Studies of the Mechanism of Cysteine Proteases Inhibition by Dipeptidyl Nitroalkenes

In this work a computational study of the mechanism of inhibition of cruzain, rhodesain, and cathepsin L cysteine proteases by the dipeptidyl nitroalkene Cbz-Phe-Ala-CH=CH-NO₂ has been carried out by means of molecular dynamics simulations with hybrid QM/MM potentials. The free-energy surfaces confirmed that the inhibition takes place by the formation of a covalent bond between the protein and the β-carbon atom of the inhibitor. According to the results, the tested inhibitor should be a much more efficient inhibitor of cruzain than of rhodesain, and little activity would be expected against cathepsin L, in total correspondence with the available experimental data. The origin of these differences may lie in the different stabilizing electrostatic interactions established between the inhibitor and the residues of the active site and S2 pocket of these enzymes. These results may be useful for the rational design of new dipeptidyl nitroalkenes with higher and more selective inhibitory activity against cysteine proteases.