Experimental and Computational Studies of the Single-Molecule Conductance of Ru(II) and Pt(II) trans-Bis(acetylide) Complexes

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.

Thermoelectric Properties of Porphyrin Nano Rings: A Theoretical and Modelling Investigation

Propagation of De Broglie waves through nanomolecular junctions is greatly affected by molecular topology changes, which in turn plays a key role in determining the electronic and thermoelectric properties of source|molecule|drain junctions. The probing and realization of the constructive quantum interference (CQI) and a destructive quantum interference (DQI) are well established in this work. The critical role of quantum interference (QI) in governing and enhancing the transmission coefficient T(E), thermopower (S), power factor (P) and electronic figure of merit (ZelT) of porphyrin nanorings has been investigated using a combination of density functional theory (DFT) methods, a tight binding (Hückel) modelling (TBHM) and quantum transport theory (QTT). Remarkably, DQI not only dominates the asymmetric molecular pathways and lowering T(E), but also improves the thermoelectric properties.

Carbon Nanohoops: Multiple Molecular Templates for Exploring Spectroscopic, Electronic, and Thermoelectric Properties

A combination of density functional theory (DFT) methods and quantum transport theory (QTT) has been used to investigate the spectroscopic, electronic, and thermoelectric properties of carbon nanohoop molecules with different molecular templates. The connectivity type, along with inherent strain, impacts the transport behavior and creates a destructive quantum interference (DQI), which proves itself to be a powerful strategy to enhance the thermoelectric properties of these molecules, making them promising candidates for thermoelectric applications.

A stronger acceptor decreases the rates of charge transfer: ultrafast dynamics and on/off switching of charge separation in organometallic donor-bridge-acceptor systems

To unravel the role of driving force and structural changes in directing the photoinduced pathways in donor-bridge-acceptor (DBA) systems, we compared the ultrafast dynamics in novel DBAs which share a phenothiazine (PTZ) electron donor and a Pt(ii) trans-acetylide bridge (-C[triple bond, length as m-dash]C-Pt-C[triple bond, length as m-dash]C-), but bear different acceptors conjugated into the bridge (naphthalene-diimide, NDI; or naphthalene-monoimide, NAP). The excited state dynamics were elucidated by transient absorption, time-resolved infrared (TRIR, directly following electron density changes on the bridge/acceptor), and broadband fluorescence-upconversion (FLUP, directly following sub-picosecond intersystem crossing) spectroscopies, supported by TDDFT calculations. Direct conjugation of a strong acceptor into the bridge leads to switching of the lowest excited state from the intraligand 3IL state to the desired charge-separated 3CSS state. We observe two surprising effects of an increased strength of the acceptor in NDI vs. NAP: a ca. 70-fold slow-down of the 3CSS formation-(971 ps)-1vs. (14 ps)-1, and a longer lifetime of the 3CSS (5.9 vs. 1 ns); these are attributed to differences in the driving force ΔGet, and to distance dependence. The 100-fold increase in the rate of intersystem crossing-to sub-500 fs-by the stronger acceptor highlights the role of delocalisation across the heavy-atom containing bridge in this process. The close proximity of several excited states allows one to control the yield of 3CSS from ∼100% to 0% by solvent polarity. The new DBAs offer a versatile platform for investigating the role of bridge vibrations as a tool to control excited state dynamics.

Not So Innocent After All: Interfacial Chemistry Determines Charge-Transport Efficiency in Single-Molecule Junctions

Most studies in molecular electronics focus on altering the molecular wire backbone to tune the electrical properties of the whole junction. However, it is often overlooked that the chemical structure of the groups anchoring the molecule to the metallic electrodes influences the electronic structure of the whole system and, therefore, its conductance. We synthesised electron-accepting dithienophosphole oxide derivatives and fabricated their single-molecule junctions. We found that the anchor group has a dramatic effect on charge-transport efficiency: in our case, electron-deficient 4-pyridyl contacts suppress conductance, while electron-rich 4-thioanisole termini promote efficient transport. Our calculations show that this is due to minute changes in charge distribution, probed at the electrode interface. Our findings provide a framework for efficient molecular junction design, especially valuable for compounds with strong electron withdrawing/donating backbones.

Acene Size-Dependent Transition of The Radical Centers From the Metal to The Acene Parts In Monocationic Dinuclear (Diethynylacene)diyl Complexes

Controlling radical localization/delocalization is important for functional materials. The present paper describes synthesis and results of electrochemical, spectroscopic, and theoretical studies of diruthenium (p-diethynylacene)diyl complexes, Me₃ Si-(C≡C)₂ -Ru(dppe)₂ -C≡C-Ar-C≡C-Ru(dppe)₂ -(C≡C)₂ -SiMe₃ (1-6) (dppe: 1,2-bis(diphenylphosphino)ethane), and their monocationic radical species ([1]⁺ -[6]⁺ ). The HOMO-LUMO energy gaps can be finely tuned by the acene rings in the bridging ligands installed, as indicated by the absorption maxima of the electronic spectra of 1-6 ranging from the UV region even to the NIR region. The cationic species [1]⁺ -[6]⁺ show two characteristic NIR bands, which are ascribed to the charge resonance (CR) and π-π* transition bands, as revealed by spectroelectrochemistry. Expansion of the acene rings in [1]⁺ -[6]⁺ causes (1) blue shifts of the CR bands and red shifts of the π-π* transition bands and (2) charge localization on the acene parts as evidenced by the ESR, DFT and TD-DFT analyses. Notably, the monocationic complexes of the larger acene derivatives are characterized as the non-classical acene-localized radicals.

Electrostatic Fermi level tuning in large-scale self-assembled monolayers of oligo(phenylene-ethynylene) derivatives

Understanding and controlling the orbital alignment of molecules placed between electrodes is essential in the design of practically-applicable molecular and nanoscale electronic devices. The orbital alignment is highly determined by the molecule-electrode interface. Dependence of orbital alignment on the molecular anchor group for single molecular junctions has been intensively studied; however, when scaling-up single molecules to large parallel molecular arrays (like self-assembled monolayers (SAMs)), two challenges need to be addressed: 1. Most desired anchor groups do not form high quality SAMs. 2. It is much harder to tune the frontier molecular orbitals via a gate voltage in SAM junctions than in single molecular junctions. In this work, we studied the effect of the molecule-electrode interface in SAMs with a micro-pore device, using a recently developed tetrapodal anchor to overcome challenge 1, and the combination of a single layered graphene top electrode with an ionic liquid gate to solve challenge 2. The zero-bias orbital alignment of different molecules was signalled by a shift in conductance minimum vs. gate voltage for molecules with different anchoring groups. Molecules with the same backbone, but a different molecule-electrode interface, were shown experimentally to have conductances that differ by a factor of 5 near zero bias. Theoretical calculations using density functional theory support the trends observed in the experimental data. This work sheds light on how to control electron transport within the HOMO-LUMO energy gap in molecular junctions and will be applicable in scaling up molecular electronic systems for future device applications.

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

A series of ruthenium alkenylacetylide complexes trans-[Ru{C≡CC(=CH₂ )R}Cl(dppe)₂ ] (R=Ph (1 a), ^c C₄ H₃ S (1 b), 4-MeS-C₆ H₄ (1 c), 3,3-dimethyl-2,3-dihydrobenzo[b]thiophene (DMBT) (1 d)) or trans-[Ru{C≡C-^c C₆ H₉ }Cl(dppe)₂ ] (1 e) were allowed to react with the corresponding propargylic alcohol HC≡CC(Me)R(OH) (R=Ph (A), ^c C₄ H₃ S (B), 4-MeS-C₆ H₄ (C), DMBT (D) or HC≡C-^c C₆ H₁₀ (OH) (E) in the presence of TlBF₄ and DBU to presumably give alkenylacetylide/allenylidene intermediates trans-[Ru{C≡CC(=CH₂ )R}{C=C=C(Me)}(dppe)₂ ]PF₆ ([2]PF₆ ). These complexes were not isolated but deprotonated to give the isolable bis(alkenylacetylide) complexes trans-[Ru{C≡CC(=CH₂ )R}₂ (dppe)₂ ] (R=Ph (3 a), ^c C₄ H₃ S (3 b), 4-MeS-C₆ H₄ (3 c), DMBT (3 d)) and trans-[Ru{C≡C-^c C₆ H₉ }₂ (dppe)₂ ] (3 e). Analogous reactions of trans-[Ru(CH₃ )₂ (dmpe)₂ ], featuring the more electron-donating 1,2-bis(dimethylphosphino)ethane (dmpe) ancillary ligands, with the propargylic alcohols A or C and NH₄ PF₆ in methanol allowed isolation of the intermediate mixed alkenylacetylide/allenylidene complexes trans-[Ru{C≡CC(=CH₂ )R}{C=C=C(Me)}(dmpe)₂ ]PF₆ (R=Ph ([4 a]PF₆ ), 4-MeS-C₆ H₄ ([4 c]PF₆ ). Deprotonation of [4 a]PF₆ or [4 c]PF₆ gave the symmetric bis(alkenylacetylide) complexes trans-[Ru{C≡CC(=CH₂ )R}₂ (dmpe)₂ ] (R=Ph (5 a), 4-MeS-C₆ H₄ (5 c)), the first of their kind containing the dmpe ancillary ligand sphere. Attempts to isolate bis(allenylidene) complexes [Ru{C=C=C(Me)R}₂ (PP)₂ ]²⁺ (PP=dppe, dmpe) from treatment of the bis(alkenylacetylide) species 3 or 5 with HBF₄  ⋅ Et₂ O were ultimately unsuccessful.

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. Rather, the trends in molecular conductance are better rationalized from consideration of the complete molecular junction, with conductance values well described by transport calculations carried out at the DFT level of theory, on the basis of the Landauer-Büttiker model.

Single-Molecule Junction of a Cationic Rh(III) Polyyne Molecular Wire

Single-molecule conductance studies on metal-containing inorganic and organometallic molecular wires are relatively less explored compared to those on organic molecular wires. Furthermore, conductance and transmission profiles of the metal-containing wires insensitive to the metal centers often hinder rational design for high performance wires. Here, synthesis and single-molecule conductance measurements of the bis(butadiynyl)rhodium wires with tetracarbene ligands 1^H and 1^Au are reported as rare examples for Rh(III) diacetylide molecular wires. The rhodium wires derived from the terminal acetylene and gold-functionalized precursors show comparable, high single-molecule conductance ((6-7) × 10^-3 G₀) as determined by the STM break-junction measurements, suggesting formation of virtually the same covalently linked metal electrode-molecule-metal electrode junctions. The values for the metallapolyynes are larger than those of the organic polyyne wires having the similar molecular lengths. The hybrid DFT-NEGF calculations of the model systems suggest that profiles of transmission spectra are highly sensitive to the presence and species of the metal fragments doped into the polyyne molecular wire because the conductance orbitals of the metallapolyynes molecular junctions carry significant metal fragment characters. Thus, the metallapolyyne junctions turn out to be suitable platforms for rationally designed molecular wires.

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.

Dinuclear ruthenium acetylide complexes with diethynylated anthrahydroquinone and anthraquinone frameworks: a multi-stimuli-responsive organometallic switch

The anthrahydroquinone (AHQ)/anthraquinone (AQ) system is of particular interest as a molecular switching system for molecule-based devices because AHQ and AQ can be interconverted by redox stimuli and their π-conjugated systems are distinct from each other. To date, however, a way to modify and/or functionalize the switching behavior based on the AHQ/AQ system is still limited. In the present contribution, the synthesis and properties of multi-responsive dinuclear molecular switches having Ru(dppe)₂ fragments bridged by the diethynylated diacetoxyanthracene (AcAHQ) and AQ units (μ-AcAHQ/AQ){C[triple bond, length as m-dash]C-Ru(R)(dppe)₂}₂ (R = Cl, C₄TMS) are presented. The terminal Ru(dppe)₂ fragments are redox-active and, therefore, the intermetallic interaction can be estimated by electrochemical as well as IVCT band analysis. As a result, the organometallic AcAHQ/AQ-Ru system turns out to be an effective bimodal molecular switch, which is triggered not only by redox stimuli but also by pH stimuli.

Metal bis(acetylide) complex molecular wires: concepts and design strategies

The past decade has seen a remarkable surge in studies of thin-film and single-molecule electronics, due in no small part to the development and advancement of experimental methods for the construction and measurement of metal|molecule|metal junctions. Within the plethora of molecular structures that have been investigated, metal complexes of general form trans-M(C[triple bond, length as m-dash]CR)2(Ln) have attracted attention from the inorganic and organometallic chemistry community in the search for efficient molecular wires due to the potential π-d-π orbital mixing along the molecular backbone. In this article progress towards this goal will be summarised, and design strategies for future molecular components discussed.

Push-pull D-π-Ru-π-A chromophores: synthesis and electrochemical, photophysical and second-order nonlinear optical properties

The present work describes the one-pot synthesis and electrochemical, photophysical and second-order nonlinear optical (NLO) properties of a series of dipolar π-delocalized Ru(ii) dialkynyl complexes. The eight new asymmetrical D-π-Ru-π-A push-pull chromophores incorporate pyranylidene ligands as pro-aromatic donor groups (D) and formaldehyde, indane-1,3-dione, pyrimidine or pyrimidinium as electron-attracting groups (A) separated by ruthenium bis-acetylide fragments and π-conjugated linkers. The second-order nonlinear optical (NLO) properties of all eight complexes were determined by the Electric-Field-Induced Second Harmonic generation (EFISH) technique (operating at 1907 nm), and were compared to those of their purely organic analogs. All investigated compounds (organic and organometallic) exhibited positive μβ values, which dramatically increased for the complexes due to the presence of ruthenium in the π-conjugated core. The second-order NLO response could also be easily modulated by changing the nature of alkynyl substituents. The most promising ruthenium complexes 7 and 8 of the series with the pyrimidinium fragment displayed μβ values of 14 000 × 10^-48 esu. The effect of structural modifications on the redox and spectroscopic properties of the complexes was also studied. The intramolecular charge transfer (ICT) occurring through the ruthenium center of the push-pull σ-dialkynyl complexes was investigated by combining experimental and theoretical data.

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).

Molecular rectifiers based on five-coordinate iron(iii)-containing surfactants

The state-of-the-art of metallorganic-based molecular rectification is reviewed with an emphasis on asymmetric five-coordinate Fe^III-containing surfactants in electrode|LB film|electrode assemblies.

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d⁸ square-planar platinum(ii) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (C[triple bond, length as m-dash]CSiMe₃) contacting groups, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₄C[triple bond, length as m-dash]CSiMe₃}₂(PR₃)₂ (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3-5 × 10^-5G₀) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh₃ supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal C[triple bond, length as m-dash]CSiMe₃ group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₂(Ohex)₂C[triple bond, length as m-dash]CSiMe₃}₂(PPh₃)₂ (Ohex = OC₆H₁₃), hinders contact of the STM tip to the PPh₃ groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt₃), rather than triarylphosphine (PPh₃), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions.

Ferrocene- and Biferrocene-Containing Macrocycles towards Single-Molecule Electronics

Cyclic multiredox centered systems are currently of great interest, with new compounds being reported and developments made in understanding their behavior. Efficient, elegant, and high‐yielding (for macrocyclic species) synthetic routes to two novel alkynyl‐conjugated multiple ferrocene‐ and biferrocene‐containing cyclic compounds are presented. The electronic interactions between the individual ferrocene units have been investigated through electrochemistry, spectroelectrochemistry, density functional theory (DFT), and crystallography to understand the effect of cyclization on the electronic properties and structure.