Experimental and Computational Studies of Steric Factors in the Isomerization of Internal Alkynes within the Coordination Environment of Half-Sandwich Metal Complexes.

The isomerization of internal alkynes ArC≡CAr within the coordination environment of low-valent half-sandwich [Ru(dppe)Cp] complexes via a 1,2-migration process affords vinylidene species [Ru{=C=C(Ar)Ar}(dppe)Cp]. The rearrangement reactions of symmetrically and asymmetrically substituted substrates featuring different electron-donating and -withdrawing groups and of varying steric bulk were modelled using density functional theory (DFT), and the conclusions supported by experimental observations. Examination of the reaction pathway and associated activation barriers reveal a high solvent dependency for the generation of the key intermediate species [Ru(dppe)Cp] from [RuCl(dppe)Cp] by halide dissociation in the presence of Na salts of weakly coordinating anions, with the lattice enthalpy of the NaCl by-product playing a critical role in the overall thermochemical balance of the reaction.

Photocatalytic overall water splitting endowed by modulation of internal and external energy fields.

The pursuit of sustainable and clean energy sources has driven extensive research into the generation and use of novel energy vectors. The photocatalytic overall water splitting (POWS) reaction has been identified as a promising approach for harnessing solar energy to produce hydrogen to be used as a clean energy carrier. Materials chemistry and associated photocatalyst design are key to the further improvement of the efficiency of the POWS reaction through the optimization of charge carrier separation, migration and interfacial reaction kinetics.

Synthesis, Structure, and Redox Properties of Nonsymmetric 6-Oxoverdazyls.

Nonsymmetric 6π-electron ("oxidized") 6-oxoverdazyls have been synthesized for the first time. After formal incorporation of a hydrogen atom, the corresponding 7π-electron neutral verdazyl radical is generated. The 7π radical can undergo a further electrochemically reversible reduction to an 8π anion.

Methods for the analysis, interpretation, and prediction of single-molecule junction conductance behaviour.

This article offers a broad overview of measurement methods in the field of molecular electronics, with a particular focus on the most common single-molecule junction fabrication techniques, the challenges in data analysis and interpretation of single-molecule junction current-distance traces, and a summary of simulations and predictive models aimed at establishing robust structure-property relationships of use in the further development of molecular electronics.

Large area arrays of discrete single-molecule junctions derived from host-guest complexes.

The desire to continually reduce the lower limits of semiconductor integrated circuit (IC) fabrication methods continues to inspire interest in unimolecular electronics as a platform technology for the realization of future (opto)electronic devices. However, despite successes in developing methods for the construction and measurement of single-molecule and large-area molecular junctions, exercising control over the precise junction geometry remains a significant challenge. Here, host-guest complexes of the wire-like viologen derivative 1,1'-bis(4-(methylthio)-phenyl)-[4,4'-bipyridine]-1,1'-diium chloride ([1][Cl]) and cucurbit[7]uril (CB[7]) have been self-assembled in a regular pattern over a gold substrate.

Destructive quantum interference in -oligo(phenyleneethynylene) molecular wires with gold-graphene heterojunctions.

Quantum interference (QI) is well recognised as a significant contributing factor to the magnitude of molecular conductance values in both single-molecule and large area junctions. Numerous structure-property relationship studies have shown that -connected oligo(phenyleneethynylene) (OPE) based molecular wires exemplify the impact of constructive quantum interference (CQI), whilst destructive quantum interference (DQI) effects are responsible for the orders of magnitude lower conductance of analogous -contacted OPE derivatives, despite the somewhat shorter effective tunnelling distance. Since molecular conductance is related to the value of the transmission function, evaluated at the electrode Fermi energy, (), which in turn is influenced by the presence and relative energy of (anti)resonances, it follows that the relative single-molecule conductance of - and -contacted OPE-type molecules is tuned both by the anchor group and the nature of the electrode materials used in the construction of molecular junctions (gold|molecule|gold gold|molecule|graphene).

Advances in single-molecule junctions as tools for chemical and biochemical analysis.

The development of miniaturized electronics has led to the design and construction of powerful experimental platforms capable of measuring electronic properties to the level of single molecules, along with new theoretical concepts to aid in the interpretation of the data. A new area of activity is now emerging concerned with repurposing the tools of molecular electronics for applications in chemical and biological analysis. Single-molecule junction techniques, such as the scanning tunnelling microscope break junction and related single-molecule circuit approaches have a remarkable capacity to transduce chemical information from individual molecules, sampled in real time, to electrical signals.

Potential-Modulated Ion Distributions in the Back-to-Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer.

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential difference provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region.

Exploring relationships between chemical structure and molecular conductance: from α,ω-functionalised oligoynes to molecular circuits.

The quantum circuit rule (QCR) allows estimation of the conductance of molecular junctions, electrode|X-bridge-Y|electrode, by considering the molecule as a series of independent scattering regions associated with the anchor groups (X, Y) and bridge, provided the numerical parameters that characterise the anchor groups (, ) and molecular backbones () are known. Single-molecule conductance measurements made with a series of α,ω-substituted oligoynes (X-{(CC)}-X, = 1, 2, 3, 4), functionalised by terminal groups, X (4-thioanisole (CHSMe), 5-(3,3-dimethyl-2,3-dihydrobenzo[]thiophene) (DMBT), 4-aniline (CHNH), 4-pyridine (Py), capable of serving as 'anchor groups' to contact the oligoyne fragment within a molecular junction, have shown the expected exponential dependence of molecular conductance, , with the number of alkyne repeating units. In turn, this allows estimation of the anchor () and backbone () parameters.

Excited State Mixed-Valence Complexes: From the Special Pair to the Creutz-Taube Ion and Beyond.

Compounds and complexes with mixed-valence electronic ground states, such as the Creutz-Taube ion, have proven to be excellent vehicles through which to study intramolecular electron-transfer processes. In a recent contribution by Cadranel and co-workers, time-resolved pump-probe spectroscopy reveals photo-induced metal-to-bridge charge transfer within the homovalent analogue of the Creutz-Taube ion, [{(NH ) Ru}(μ-pz){Ru(NH ) }] , giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (τ=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure.

Multifunctional switching properties of "wire-like" dinuclear ruthenium bis-alkynyl spiropyran complexes.

Multifunctional switches are crucial to the development of smart molecular materials and molecular-electronic applications. Here, we describe the synthesis, structure, and characterization of several spiropyrans functionalized with alkynyl-[Ru(dppe)] moieties. Through electrochemical and spectroelectrochemical studies, we demonstrate access to several stable redox states, in addition to states accessed acidochromism and photoisomerisation.

Modulation of Charge Distribution in Cobalt-α-Diimine Complexes toward Valence Tautomerism.

Valence tautomerism (VT) and spin crossover (SCO) are promising avenues for developing a range of molecular materials for sensing, memory, and optoelectronic applications. However, these phenomena arise only when specific metal-ligand combinations are employed. The underexplored combination of cobalt(II/III) paired with bis((aryl)imino)acenapthene (Ar-BIAN) ligands, which can exist as neutral Ar-BIAN (L), monoanionic radical Ar-BIAN (L), and dianionic Ar-BIAN (L) forms, has potential to afford both VT and SCO.

Quantum Interference in Mixed-Valence Complexes: Tuning Electronic Coupling Through Substituent Effects.

Whilst 2- or 5-OMe groups on the bridging phenylene ring in [{Cp*(dppe)RuC≡C} (μ-1,3-C H )] have little influence on the electronic structure of this weakly coupled mixed-valence complex, a 4-OMe substituent enhances ground state electron delocalization, and increases the intensity of the IVCT transition. Vibrational frequency and TDDFT calculations (LH20t-D3(BJ), def2-SVP, COSMO (CH Cl )) on ([{Cp*(dppe)RuC≡C} (μ-1,3-C H -n-OMe)] (n=2, 4, 5) models are in excellent agreement with the experimental results. The stronger ground state coupling is attributed to the change in composition of the β-HOSO brought about by the 4-OMe group, which is ortho or para to each of the metal fragments.

The Use of Bridging Ligand Substituents to Bias the Population of Localized and Delocalized Mixed-Valence Conformers in Solution.

The electronic structure and associated spectroscopic properties of ligand-bridged, bimetallic 'mixed-valence' complexes of the general form {M}(μ-B){M } are dictated by the electronic couplings, and hence orbital overlaps, between the metal centers mediated by the bridge. In the case of complexes such as [{Cp*(dppe)Ru}(μ-C≡CC H C≡C){Ru(dppe)Cp*}] , the low barrier to rotation of the half-sandwich metal fragments and the arylene bridge around the acetylene moieties results in population of many energy minima across the conformational energy landscape. Since orbital overlap is also sensitive to the particular mutual orientations of the metal fragment(s) and arylene bridge through a Karplus-like relationship, the different members of the population range exemplify electronic structures ranging from strongly localized (weakly coupled Robin-Day Class II) to completely delocalized (Robin-Day Class III).

Redox-Addressable Single-Molecule Junctions Incorporating a Persistent Organic Radical.

Integrating radical (open-shell) species into non-cryogenic nanodevices is key to unlocking the potential of molecular electronics. While many efforts have been devoted to this issue, in the absence of a chemical/electrochemical potential the open-shell character is generally lost in contact with the metallic electrodes. Herein, single-molecule devices incorporating a 6-oxo-verdazyl persistent radical have been fabricated using break-junction techniques.

From ferrocene to 1,2,3,4,5-pentafluoroferrocene: halogen effect on the properties of metallocene.

The sequentially fluorinated ferrocenes (1-, 1,2-di, 1,2,3-tri, 1,2,3,4-tetra and 1,2,3,4,5-pentafluoroferrocene) have been synthesized from ferrocene. Rather than a 'perfluoro' effect, experimental and computational analysis of the complete series robustly demonstrates a linear additive effect of fluorine on the electrochemical and spectroscopic properties of ferrocene.

Syntheses and Structures of trans-bis(Alkenylacetylide) Ruthenium Complexes.

A series of ruthenium alkenylacetylide complexes trans-[Ru{C≡CC(=CH )R}Cl(dppe) ] (R=Ph (1 a), C H S (1 b), 4-MeS-C H (1 c), 3,3-dimethyl-2,3-dihydrobenzo[b]thiophene (DMBT) (1 d)) or trans-[Ru{C≡C- C H }Cl(dppe) ] (1 e) were allowed to react with the corresponding propargylic alcohol HC≡CC(Me)R(OH) (R=Ph (A), C H S (B), 4-MeS-C H (C), DMBT (D) or HC≡C- C H (OH) (E) in the presence of TlBF and DBU to presumably give alkenylacetylide/allenylidene intermediates trans-[Ru{C≡CC(=CH )R}{C=C=C(Me)}(dppe) ]PF ([2]PF ). These complexes were not isolated but deprotonated to give the isolable bis(alkenylacetylide) complexes trans-[Ru{C≡CC(=CH )R} (dppe) ] (R=Ph (3 a), C H S (3 b), 4-MeS-C H (3 c), DMBT (3 d)) and trans-[Ru{C≡C- C H } (dppe) ] (3 e). Analogous reactions of trans-[Ru(CH ) (dmpe) ], featuring the more electron-donating 1,2-bis(dimethylphosphino)ethane (dmpe) ancillary ligands, with the propargylic alcohols A or C and NH PF in methanol allowed isolation of the intermediate mixed alkenylacetylide/allenylidene complexes trans-[Ru{C≡CC(=CH )R}{C=C=C(Me)}(dmpe) ]PF (R=Ph ([4 a]PF ), 4-MeS-C H ([4 c]PF ).

Fabrication of metallic and non-metallic top electrodes for large-area molecular junctions.

Molecular junctions have proven invaluable tools through which to explore the electronic properties of molecules and molecular monolayers. In seeking to develop a viable molecular electronics based technology it becomes essential to be able to reliably create larger area molecular junctions by contacting molecular monolayers to both bottom and top electrodes. The assembly of monolayers onto a conducting substrate by self-assembly, Langmuir-Blodgett and other methods is well established.

Synthesis of a diferrocenylvinylidene complex by migration of a ferrocenyl substituent.

An unusual 1,2-ferrocenyl migration has been observed following reactions of [Ru(dppe)Cp][BArF4] with diferrocenylacetylene, extending the scope of group rearrangments beyond methyl (Wagner-Meerwein) and phenyl entities. Ferrocene-containing bis(alkynes) RC[triple bond, length as m-dash]CArC[triple bond, length as m-dash]CR (R = Fc, Ar = 1,4-phenylene; R = Ph, Ar = 1,1'-ferrocenylene) gave bimetallic bis(vinylidene) complexes following two consecutive rearrangements.

Rip It off: Nitro to Nitroso Reduction by Iron Half-Sandwich Complexes.

Activation of [FeCl(dppe)Cp] () by chloride abstraction with Na[BAr] (X = F, [B(3,5-(CF)-CH)]; X = Cl, [B(3,5-Cl-CH)]) permits reactions with a range of nitro aromatics, RCHNO (R = halogen, Me, OMe, NO or NMe), to give the cationic iron nitroso complexes [Fe{N(O)-CHR}(dppe)Cp][BAr]) ([][BAr]). Similar reactions of and Na[BAr] with [Fe(NCCHNO)(dppe)Cp][BAr] gave bimetallic [{Fe(dppe)Cp}{μ-N≡CCHN(O)}][BAr]. However, reactions of and Na[BAr] with 4-nitrophenol gave the first example of the bench-stable iron half-sandwich phenolate complex [Fe(OCHNO)(dppe)Cp] rather than NO activation.

Molecular Structure-(Thermo)electric Property Relationships in Single-Molecule Junctions and Comparisons with Single- and Multiple-Parameter Models.

The most probable single-molecule conductance of each member of a series of 12 conjugated molecular wires, 6 of which contain either a ruthenium or platinum center centrally placed within the backbone, has been determined. The measurement of a small, positive Seebeck coefficient has established that transmission through these molecules takes place by tunneling through the tail of the HOMO resonance near the middle of the HOMO-LUMO gap in each case. Despite the general similarities in the molecular lengths and frontier-orbital compositions, experimental and computationally determined trends in molecular conductance values across this series cannot be satisfactorily explained in terms of commonly discussed "single-parameter" models of junction conductance.

Mix and (Mis)match: further studies of the electronic structure and mixed-valence characteristics of 1,4-diethynylbenzene-bridged bimetallic complexes.

The 1,4-diethynylbenzene motif is commonly employed as a bridging ligand in bimetallic molecular systems intended to show pronounced intramolecular electronic interactions, delocalized electronic structures and 'wire-like' properties between the metal fragments at the ligand termini. In contrast to these expectations, the donor-acceptor compounds [{Cp'(CO)xM'}(μ-C[triple bond, length as m-dash]CC6H4C[triple bond, length as m-dash]C){M(PP)Cp'}]n+ [n = 0, 1; M'(CO)xCp' = Fe(CO)2Cp, W(CO)3Cp*; M(PP)Cp' = Fe(dppe)Cp, Fe(dppe)Cp*, Ru(PPh3)2Cp, Ru(dppe)Cp, Ru(dppe)Cp*] display remarkably little bridge-mediated electronic interaction between the electron-rich {M(PP)Cp'} and electron-poor {M'(CO)xCp'} fragments in the ground state. However, a relatively high-energy (26 000-30 000 cm-1) M-to-M' charge transfer can be identified.

Biodegradable Materials and Green Processing for Green Electronics.

There is little question that the "electronic revolution" of the 20th century has impacted almost every aspect of human life. However, the emergence of solid-state electronics as a ubiquitous feature of an advanced modern society is posing new challenges such as the management of electronic waste (e-waste) that will remain through the 21st century. In addition to developing strategies to manage such e-waste, further challenges can be identified concerning the conservation and recycling of scarce elements, reducing the use of toxic materials and solvents in electronics processing, and lowering energy usage during fabrication methods.