Redox-Active Molecular Nanowire Flash Memory for High-Endurance and High-Density Nonvolatile Memory Applications

Organometallics in molecular junctions: conductance, functions, and reactions

Molecular junctions, which involve sandwiching molecular structures between electrodes, play a crucial role in molecular electronics. Recent advances in this field have revealed the vital role of organometallic chemistry in the investigation of molecular junctions, which has added to their well-known contributions to catalysis and materials chemistry. This review summarizes the recent examples of organometallic chemistry applications in molecular junctions, which can be categorized into three types, i.e., class I encompassing molecular junctions with bridging organometallic complexes, class II involving molecular junctions with covalent and noncovalent metal electrode-carbon bonds, and class III comprising organometallic reactions within molecular junctions.

Bisaryl and Bisalkynyl Diruthenium (III,III) Compounds Based on an Electron-Deficient Building Block

Reported herein is a new series of diruthenium(III,III) bisalkynyl and bisaryl diruthenium(III,III) compounds supported with 2-amino-3-(trifluoromethyl)pyridinate (amtfmp). Using Ru₂(amtfmp)₄Cl₂ from a modified preparation, cis 2:2 Ru₂(amtfmp)₄(C≡CPh)₂ (1), cis 2:2 Ru₂(amtfmp)₄(Ph)₂ (2), and 3:1 Ru₂(amtfmp)₄(Ph)₂ (3) were synthesized via a lithium-halogen exchange reaction using LiC₂Ph and LiPh, respectively. Compounds 1-3 are all Ru₂(III,III) species with a ground-state configuration of π⁴δ²(π*)⁴ (S = 0) and were characterized via mass spectrometry, electron absorption and ¹H/¹⁹F NMR spectroscopies, and voltammetry. The molecular structures of 1-3 were established using single-crystal X-ray diffraction analysis, and preliminary density functional theory analysis was performed to elaborate the electronic structures of 1 and 2. Comparisons of the electrochemical properties of 1-3 against the Ru₂(amtfmp)₄Cl₂ starting material reveal cathodic shifts of the Ru₂^7+/6+ oxidation and the Ru₂^6+/5+ and Ru₂^5+/4+ reduction potentials. In comparison to related Ru₂(III,III) bisalkynyl and bisaryl compounds, the electrode potentials for 1-3 are anodically shifted up to ca. 0.95 V, highlighting the strong electron-withdrawing nature of the amtfmp ligand.

Bis-Alkynyl Complexes of Fe(III) Tetraaza Macrocycles─A Tale of Two Rings

Reported herein are new Fe bis-alkynyl complexes [Fe^III(L)(C₂R)₂]BPh₄ based on tetraimine macrocycle (L = HMTI = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; 1a-1c; R = C₆H₅ (a), C₁₀H₉ (b), SiMe₃ (c)) and tetraamine macrocycle (L = HMC = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; 2a-2c). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of 1a and 2a allowed for investigation of the Fe^II oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the Fe^II to C₂Ph metal-to-ligand charge transfer (MLCT) and the ν(C≡C). The ν(C≡C) was further influenced by the oxidation state, though sensitivity to the formal metal oxidation state was much higher in the case of 2a than in 1a. These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals via density functional theory calculations.

Phenylene as an efficient mediator for intermetallic electronic coupling

The new compound [(NC)Ru₂(ap)₄]₂(μ-1,4-C₆H₄) (ap = 2-anilinopyridinate) was prepared to address the open question of whether a 1,4-phenylene bridge can mediate intermetallic electronic coupling. As a manifestation of strong coupling, hole delocalization between the Ru₂ centers on the IR time scale (10^-14 s) was established using spectroelectrochemistry. An orbital mechanism for coupling was elaborated with DFT analysis.

Diruthenium aryl compounds - tuning of electrochemical responses and solubility

Reported herein are the two new series of diruthenium aryl compounds: Ru₂(DiMeOap)₄(Ar) (1a-6a) (DiMeOap = 2-(3,5-dimethoxyanilino)pyridinate) and Ru₂(m-ⁱPrOap)₄(Ar) (1b-5b) (m-ⁱPrOap = 2-(3-iso-propoxyanilino)pyridinate), prepared through the lithium-halogen exchange reaction with a variety of aryl halides (Ar = C₆H₄-4-NMe₂ (1), C₆H₄-4-^tBu (2), C₆H₄-4-OMe (3), C₆H₃-3,5-(OMe)₂ (4), C₆H₄-4-CF₃ (5), C₆H₅ (6)). The molecular structures of these compounds were established with X-ray diffraction studies. Additionally, these compounds were characterized using electronic absorption and voltammetric techniques. Compounds 1a-6a and 1b-5b are all in the Ru₂⁵⁺ oxidation state, with a ground state configuration of σ²π⁴δ²(π*δ*)³ (S = 3/2). Use of the modified ap ligands (ap') resulted in moderate increases of product yield when compared to the unsubstituted Ru₂(ap)₄(Ar) (ap = 2-anilinopyridinate) series. Comparisons of the electrochemical properties of 1a-6a and 1b-5b against the Ru₂(ap')Cl starting material reveals the addition of the aryl ligand cathodically shifted the Ru₂^6+/5+ oxidation and Ru₂^5+/4+ reduction potentials. These oxidation and reductions potentials are also strongly dependent on the p-substituent of the axial aryl ligands.

Control of dominant conduction orbitals by peripheral substituents in paddle-wheel diruthenium alkynyl molecular junctions

Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C6H4-NCHN-C6H4-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF3) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10-5 to 10-4 G 0 region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO-LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO-LUMO gaps.

Oxidized divinyl oligoacene-bridged diruthenium complexes: bridged localized radical characters and reduced aromaticity in bridge cores

A series of bimetallic ruthenium vinyl complexes 1-5 bridged by oligoacenes were synthesized and characterized in this study. Comparative cyclic voltammetry results from 1-5 indicated that the first oxidation potential decreased gradually with the extension of conjugate ligands. Upon oxidation to singly oxidized species 1+-5+, rather small ν(CO) changes in the infrared (IR) spectra and the characteristic bands of metal-to-ligand charge transfer absorptions in the near IR (NIR) region predicted via time-dependent DFT calculations suggested that strong bridged ligands participate in redox processes. NIR absorptions were not observed in complexes 4+ and 5+ possibly because of instability in their twisted and noncoplanar geometry. Electron paramagnetic resonance results and spin density distribution demonstrated that the bridged localized degrees of 1+-5+ successively increased with the extension of oligoacene from benzene to tetracene. Further comparative analysis of neutral molecules and monocations to the aromaticity and π-electron density of bridge cores indicated a step-by-step transformation process from an aromatic to quinoidal radical upon oxidation.

Covalent Grafting of Polyoxometalate Hybrids onto Flat Silicon/Silicon Oxide: Insights from POMs Layers on Oxides

Immobilization of polyoxometalates (POMs) onto oxides is relevant to many applications in the fields of catalysis, energy conversion/storage, or molecular electronics. Optimization and understanding the molecule/oxide interface is crucial to rationally improve the performance of the final molecular materials. We herein describe the synthesis and covalent grafting of POM hybrids with remote carboxylic acid functions onto flat Si/SiO₂ substrates. Special attention has been paid to the characterization of the molecular layer and to the description of the POM anchoring mode at the oxide interface through the use of various characterization techniques, including ellipsometry, AFM, XPS, and FTIR. Finally, electron transport properties were probed in a vertical junction configuration and energy level diagrams have been drawn and discussed in relation with the POM molecular electronic features inferred from cyclic-voltammetry, UV-visible absorption spectra, and theoretical calculations. The electronic properties of these POM-based molecular junctions are driven by the POM LUMO (d-orbitals) whatever the nature of the tether or the anchoring group.

Nonvolatile molecular memory with the multilevel states based on MoS2 nanochannel field effect transistor through tuning gate voltage to control molecular configurations

A new flexible memory element is crucial for mobile and wearable electronics. A new concept for memory operation and innovative device structure with new materials is certainly required to address the bottleneck of memory applications now and in the future. We report a new nonvolatile molecular memory with a new operating mechanism based on two-dimensional (2D) material nanochannel field-effect transistors (FETs). The smallest channel length for our 2D material nanochannel FETs was approximately 30 nm. The modified molecular configuration for charge induced in the nanochannel of the MoS₂ FET can be tuned by applying an up-gate voltage pulse, which can vary the channel conductance to exhibit memory states. Through controlling the amounts of triggered molecules through either different gate voltage pulses or gate duration time, multilevel states were obtained in the molecular memory. These new molecular memory transistors exhibited an erase/program ratio of more than three orders of current magnitude and high sensitivity, of a few picoamperes, at the current level. Reproducible operation and four-level states with stable retention and endurance were achieved. We believe this prototype device has potential for use in future memory devices.

Selective synthesis of iridium(iii) end-capped polyynes by oxidative addition of 1-iodopolyynes to Vaska's complex

The reaction of bis(triphenylphosphine)iridium(i) carbonyl chloride (Vaska's complex) with a series of 1-iodopolyynes (1-C_nI and 2-C_nI) gave σ-polyynyl iridium(iii) complexes with general formula R(C[triple bond, length as m-dash]C)_nIr(PPh₃)₂(Cl)(I)(CO). The use of acetonitrile as a solvent appeared crucial and allowed selectively obtaining only one from a few possible isomers. The X-ray single crystal diffraction experiment for 2-C₄[Ir]I allowed the determination of the exact structure of this complex. Further spectroscopic measurements, especially ³¹P NMR, confirmed the formation of the same type of isomers with trans coordinated phosphines in each case. All complexes were fully characterized with the use of NMR (¹H, ¹³C and ³¹P), IR, UV/Vis, cyclic voltammetry and (ESI)HRMS techniques. Moreover, DFT calculations were performed for all the resulting species. The complexes with a linear carbon chain from butadiyne to decapentayne are the longest iridium end-capped polyynes known to date since only compounds with a (C[triple bond, length as m-dash]C)₂ structural motif have been reported so far. Moreover, we confirmed that the synthetic approach, first used for palladium(ii) end-capped polyynes, may be also applied for the synthesis of other structurally new organometallic polyynes.

Diruthenium(ii)-capped oligothienylethynyl bridged highly soluble organometallic wires exhibiting long-range electronic coupling

Organometallic molecular wires with π-conjugation along their molecular backbones are of considerable interest for application in molecular-scale electronics. In this regard, thienylethynyl-based π-conjugated oligomers of three, five and seven thienylethynyl units with -C[triple bond, length as m-dash]C-H termini have been successfully synthesized through stepwise Pd(0)/Cu(i)-catalyzed Sonogashira coupling. The corresponding highly soluble diruthenium(ii) diacetylide complexes (O1-Ru2, O3-Ru2, O5-Ru2 and O7-Ru2, respectively) have been prepared by the reaction of cis-Ru(dppe)2Cl2 and NaPF6 in DCM with the corresponding rigid rod-like thienylethynyl oligomers with one, three, five and seven thienylethynyl π-conjugated segments containing alkynyl termini (O1, O3, O5 and O7). These Ru(ii)-Cl capped diacetylide complexes have been further functionalized by incorporating a phenylacetynyl moiety to afford [Ru(ii)-C[triple bond, length as m-dash]C-Ph]-capped diacetylide organometallic wires (O1-Ru2-Ph, O3-Ru2-Ph, O5-Ru2-Ph and O7-Ru2-Ph). The photophysical properties of the highly soluble thienylethynyl-based oligomers and Ru(ii)-organometallic wires have been explored to understand their electronic properties. Electrochemical studies of the binuclear ruthenium(ii)-alkynyl complexes showed highly interesting results, revealing long-range electrochemical communication between the two remote Ru(ii) termini connected even with five and seven thienylethynyl units. DFT computational studies further support the long range electrochemical communication between the redox active metal termini through heavy participation of the thienylethynyl bridge in the corresponding mono-oxidized mixed valence species of the organometallic wire-like complexes.

Molecular signature of polyoxometalates in electron transport of silicon-based molecular junctions

Polyoxometalates (POMs) are unconventional electro-active molecules with a great potential for applications in molecular memories, providing efficient processing steps onto electrodes are available. The synthesis of the organic-inorganic polyoxometalate hybrids [PM11O39{Sn(C6H4)C[triple bond, length as m-dash]C(C6H4)N2}]3- (M = Mo, W) endowed with a remote diazonium function is reported together with their covalent immobilization onto hydrogenated n-Si(100) substrates. Electron transport measurements through the resulting densely-packed monolayers contacted with a mercury drop as a top electrode confirms their homogeneity. Adjustment of the current-voltage curves with the Simmon's equation gives a mean tunnel energy barrier ΦPOM of 1.8 eV and 1.6 eV, for the Silicon-Molecules-Metal (SMM) junctions based on the polyoxotungstates (M = W) and polyoxomolybdates (M = Mo), respectively. This follows the trend observed in the electrochemical properties of POMs in solution, the polyoxomolybdates being easier to reduce than the polyoxotungstates, in agreement with lowest unoccupied molecular orbitals (LUMOs) of lower energy. The molecular signature of the POMs is thus clearly identifiable in the solid-state electrical properties and the unmatched diversity of POM molecular and electronic structures should offer a great modularity.

Redox-driven porphyrin based systems for new luminescent molecular switches

This work explores the possibility of controlling the fluorescence of porphyrins via oxidation of a ruthenium acetylide unit. The modulation depends on the nature of the porphyrin unit(s).

DFT computational correlations on conformational barriers of Zn2+ and Ni2+ chiral meso-(α,β-unsaturated)- porphyrins

Correlations between DFT and experimental measurements on Zn²⁺ and Ni²⁺ chiral meso-(α,β-unsaturated)- porphyrins were performed using Kohn-Sham methodology. The exchange-correlation Becke88-Perdew86 functional was used in conjunction with double-zeta Slater basis sets. An accurate description of the electronic processes depending on the metal ion (Zn, Ni) or ligand (perilaldehyde and myrtenal) was made, confirming experimental results in terms of structural and electronic modifications. Moreover, this theoretical study provides a stronger knowledge and interpretation of the dynamical conformational features of the free base, Zn and Ni structures. Fundamental links between the central metallic atom and distortions of the porphyrinic core and ligands were demonstrated, in agreement with experimental data. We observed that the core in ZnPeriP and ZnMyrtP species is almost flat, in comparison with the Ni porphyrinic core, which appeared much more distorted. The type of distortion differs between PeriP and MyrtP ligands, with a combined saddled-ruffled characteristic with the former and a pronounced ruffled twisting for the latter. Finally, conformational energy barriers were extracted by spinning one of the arms in steps of 20° in a 360° dihedral angle. The resulted conformational barriers for NiPeriP or NiMyrtP are lower in energy than for ZnPeriP or ZnMyrtP, in agreement with experimental data.

Interface Engineering for Nanoelectronics

Innovation in the electronics industry is tied to interface engineering as devices increasingly incorporate new materials and shrink. Molecular layers offer a versatile means of tuning interfacial electronic, chemical, physical, and magnetic properties enabled by a wide variety of molecules available. This paper will describe three instances where we manipulate molecular interfaces with a specific focus on the nanometer scale characterization and the impact on the resulting performance. The three primary themes include, 1-designer interfaces, 2-electronic junction formation, and 3-advancing metrology for nanoelectronics. We show the ability to engineer interfaces through a variety of techniques and demonstrate the impact on technologies such as molecular memory and spin injection for organic electronics. Underpinning the successful modification of interfaces is the ability to accurately characterize the chemical and electronic properties and we will highlight some measurement advances key to our understanding of the interface engineering for nanoelectronics.

Sustainable metal alkynyl chemistry: 3d metals and polyaza macrocyclic ligands

This article describes the recent development of sustainable metal-alkynyl chemistry based on the combination of 3d metals and polyaza macrocycles.

Zero-dimensional to three-dimensional nanojoining: current status and potential applications

As devices have become smaller, nanomaterials have become the preferred manufacturing building blocks due to lower material and joining energy costs. This review surveys progress in nanojoining methods, as compared to conventional joining processes.