Control of single-molecule junction conductance of porphyrins via a transition-metal center

Shallow conductance decay along the heme array of a single tetraheme protein wire

Multiheme cytochromes (MHCs) are the building blocks of highly conductive micrometre-long supramolecular wires found in so-called electrical bacteria. Recent studies have revealed that these proteins possess a long supramolecular array of closely packed heme cofactors along the main molecular axis alternating between perpendicular and stacking configurations (TST = T-shaped, stacked, T-shaped). While TST arrays have been identified as the likely electron conduit, the mechanisms of outstanding long-range charge transport observed in these structures remain unknown. Here we study charge transport on individual small tetraheme cytochromes (STCs) containing a single TST heme array. Individual STCs are trapped in a controllable nanoscale tunnelling gap. By modulating the tunnelling gap separation, we are able to selectively probe four different electron pathways involving 1, 2, 3 and 4 heme cofactors, respectively, leading to the determination of the electron tunnelling decay constant along the TST heme motif. Conductance calculations of selected single-STC junctions are in excellent agreement with experiments and suggest a mechanism of electron tunnelling with shallow length decay constant through an individual STC. These results demonstrate that an individual TST motif supporting electron tunnelling might contribute to a tunnelling-assisted charge transport diffusion mechanism in larger TST associations.

Implementation of excellent spin-filtering effect in half-metallic electrode-based single-molecule optoelectronic devices

The application of half-metallic materials in single-molecule optoelectronic devices opens a promising way in advancing device performance and functionality, thus addressing a research question of significance. Here we propose a series of single-molecule devices with half-metallic FeN4-doped armchair graphene nanoribbon as electrodes and metalloporphyrin (MPr) molecules as photoresponsive materials for photon harvesting, which are driven by photogalvanic effects (PGEs). Through the quantum transport simulations, we systematically investigated the spin-polarized photocurrents under the linearly polarized light illumination in these devices. Since the exclusive opening only exists in the spin-up channel of the half-metallic nanoribbons, these devices can generate a large photocurrent in the spin-up direction whereas suppressing the spin-down photocurrent. Consequently, they exhibit an effective spin-filtering effect at numerous photon energies. Our study unveils the excellent spin-filtering effect achieved in single-molecule optoelectronic devices with half-metallic electrodes, showing instructive significance for the future design of new optoelectronic devices.

Robust binding between secondary amines and Au electrodes

While primary amines are one of the most widely used linker groups for forming single-molecule junctions, it remains elusive whether and how the substitution of one hydrogen in a primary amine with a methyl group (secondary amine) can alter its functional properties as a linker group. Here we show that a robust binding between a secondary amine and an Au electrode is absent with the use of a non-coated Au tip and is achieved when in contact with a wax-coated Au tip, which we propose is catalyzed by the more frequent formation of Au adatoms in measurements with a wax-coated tip.

Effectively Enhancing the Conductance of Asymmetric Molecular Wires by Aligning the Energy Level and Symmetrizing the Coupling

An asymmetric structure is an important strategy for designing highly conductive molecular wires for a gap-fixed molecular circuit. As the conductance enhancement in the current strategy is still limited to about 2 times, we inserted a methylene group as a spacer in a conjugated structure to modulate the structural symmetry. We found that the conductance drastically enhanced in the asymmetric molecular wire to 1.5 orders of magnitude as high as that in the symmetric molecular wire. First-principles quantum transport studies attributed the effective enhancement to the synchronization of improved energy alignment and nearly symmetric coupling between the frontier orbitals and the electrodes.

Influence of Peripheral Alkyl Groups on Junction Configurations in Single-Molecule Electronics

The addition of a lateral alkyl chain is a well-known strategy to reduce π-stacked ensembles of molecules in solution, with the intention to minimize the interactions between the molecules' backbones. In this paper, we study whether this concept generalizes to single-molecule junctions by using a combination of mechanically controllable break junction (MCBJ) measurements and clustering-based data analysis with two small series of model compounds decorated with various bulky groups. The systematic study suggests that introducing alkyl side chains also favors the formation of electrode-molecule configurations that are not observed in their absence, thereby inducing broadening of the conductance peak in the one-dimensional histograms. Thus, the introduction of alkyl chains in aromatic compounds for molecular electronics must be carefully designed and optimized for the specific purpose, balancing between increased solubility and the possibility of additional junction configurations.

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

STM studies on porphyrins and phthalocyanines at the liquid/solid interface for molecular-scale electronics

Porphyrins and phthalocyanines are promising candidates for single-molecule electronics. Among the many characterization tools, scanning tunneling microscopy (STM) represents a very powerful one to gain insight into the electronic properties at the molecular level, by correlating the charge transport behaviours of π-conjugated molecules with ultrahigh resolution imaging. In view of the sophistication of molecular self-assembly in the presence of a solution phase, in this frontier, we focus on STM studies on porphyrins and phthalocyanines at the liquid/solid interface, placing emphasis on the electronic and magnetic properties, as well as the switching behaviour of surface-confined or surface-anchored molecules. Furthermore, we have also addressed the topics of potential that can be exploited in this area.

Molecular Electronics: Creating and Bridging Molecular Junctions and Promoting Its Commercialization

Molecular electronics is driven by the dream of expanding Moore's law to the molecular level for next-generation electronics through incorporating individual or ensemble molecules into electronic circuits. For nearly 50 years, numerous efforts have been made to explore the intrinsic properties of molecules and develop diverse fascinating molecular electronic devices with the desired functionalities. The flourishing of molecular electronics is inseparable from the development of various elegant methodologies for creating nanogap electrodes and bridging the nanogap with molecules. This review first focuses on the techniques for making lateral and vertical nanogap electrodes by breaking, narrowing, and fixed modes, and highlights their capabilities, applications, merits, and shortcomings. After summarizing the approaches of growing single molecules or molecular layers on the electrodes, the methods of constructing a complete molecular circuit are comprehensively grouped into three categories: 1) directly bridging one-molecule-electrode component with another electrode, 2) physically bridging two-molecule-electrode components, and 3) chemically bridging two-molecule-electrode components. Finally, the current state of molecular circuit integration and commercialization is discussed and perspectives are provided, hoping to encourage the community to accelerate the realization of fully scalable molecular electronics for a new era of integrated microsystems and applications.

Searching for correlations between geometric and spectroscopic parameters of intramolecular hydrogen bonds in porphyrin-like macrocycles

Chemical bond lengths and angles are characteristic structural parameters of a molecule. Similarly, the frequencies of the vibrational modes and the NMR chemical shifts are unique "chemical fingerprints" specific to a compound. These are the basic parameters describing newly obtained compounds and enabling their identification. Intramolecular hydrogen bonding significantly influences the physicochemical properties of macrocyclic compounds with a porphyrin-like structure. This work presents the verification for correlations between geometric and spectroscopic parameters related to hydrogen bonds in this type of macrocyclic compounds. In particular, such relationships were investigated for a large group of porphyrin, porphycene, and dibenzotetraaza[14]annulene derivatives and a group of other macrocycles with similar structure. A very strong linear correlation was found only between the vibrational frequencies of the NH groups involved in a hydrogen bond and the length of this bond, which applied to all macrocyclic compounds of this type. Several other relationships were found between spectroscopic (IR, Raman, NMR) and geometric (X-ray) parameters, highlighting differences and similarities between different families of macrocycles. Apart from providing a better understanding of the nature of hydrogen bonds and their characteristics in porphyrin-like macrocyclic compounds, these relationships will facilitate the identification of new macrocycles and the extrapolation of their spectroscopic properties.

Tetrathiafulvalenes as anchors for building highly conductive and mechanically tunable molecular junctions

The interface between molecules and electrodes has great impact on charge transport of molecular devices. Precisely manipulating the structure and electronic coupling of electrode-molecule interface at a molecular level is very challenging. Here, we develop new molecular junctions based on tetrathiafulvalene (TTF)-fused naphthalene diimide (NDI) molecules which are anchored to gold electrodes through direct TTF-Au contacts formed via Au-S bonding. These contacts enable highly efficient orbital hybridization of gold electrodes and the conducting π-channels, yielding strong electrode-molecule coupling and remarkably high conductivity in the junctions. By further introducing additional thiohexyl (SHe) anchors to the TTF units, we develop molecular wires with multiple binding sites and demonstrate reversibly switchable electrode-molecule contacts and junction conductance through mechanical control. These findings show a superb electrode-molecule interface and provide a new strategy for precisely tunning the conductance of molecular devices towards new functions.

Formation and Evolution of Metallocene Single-Molecule Circuits with Direct Gold-π Links

Single-molecule circuits with group 8 metallocenes are formed without additional linker groups in scanning tunneling microscope-based break junction (STMBJ) measurements at cryogenic and room-temperature conditions with gold (Au) electrodes. We investigate the nature of this direct gold-π binding motif and its effect on molecular conductance and persistence characteristics during junction evolution. The measurement technique under cryogenic conditions tracks molecular plateaus through the full cycle of extension and compression. Analysis reveals that junction persistence when the metal electrodes are pushed together correlates with whether electrodes are locally sharp or blunt, suggesting distinct scenarios for metallocene junction formation and evolution. The top and bottom surfaces of the "barrel"-shaped metallocenes present the electron-rich π system of cyclopentadienyl rings, which interacts with the gold electrodes in two distinct ways. An undercoordinated gold atom on a sharp tip forms a donor-acceptor bond to a specific carbon atom in the ring. However, a small, flat patch on a dull tip can bind more strongly to the ring as a whole through van der Waals interactions. Density functional theory (DFT)-based calculations of model electrode structures provide an atomic-scale picture of these scenarios, demonstrating the role of these bonding motifs during junction evolution and showing that the conductance is relatively independent of tip atomic-scale structure. The nonspecific interaction of the cyclopentadienyl rings with the electrodes enables extended conductance plateaus, a mechanism distinct from that identified for the more commonly studied, rod-shaped organic molecular wires.

Fullerene binding effects in Al(III)/Zn(II) Porphyrin/Phthalocyanine photophysical properties and charge transport

We evaluate the fullerene C₆₀ binding effect; through the metal (Al) and through the ligand (Pc,TPP), on the photophysical and charge transport properties of M-porphyrin(TPP)/phthalocyanine(Pc) (M = Al(III), Zn(II)). We perform density functional theory (DFT) and time-dependent DFT calculations for the macrocycle-C₆₀ dyads, showing that all systems studied are thermodynamically favorable. The C₆₀ binding effect on the absorption spectrum is a red-shift of the Q and Soret (B) bands of TPPs and Pcs. The Pc-dyads show longer λ for Q bands (673 nm) than those with TPP (568 nm). AlTPP-C₆₀ and ZnTPP-C₆₀ show a more favorable electron injection to TiO₂ than the analogs Pcs, and the regeneration of the dye is preferred in AlTPP-C₆₀ and AlPc-C₆₀. Zero-bias conductance is computed (10^-4-10^-7 G₀) for the dyads using molecular junctions with Au(111)-based electrodes. When a bias voltage of around 0.6 V up to 1 V is applied, an increase in current is obtained for AlTPP-C₆₀ (10^-7 A), ZnTPP-C₆₀ (10^-7 A), and AlPc-C₆₀ (10^-8 A). Although there is not a unique trend in the behavior of the dyads, Pcs have better photophysical properties than TPPs and the latter are better in the charge transport. We conclude that AlTPP(ZnTPP)-C₆₀ dyads are an excellent alternative for designing new materials for dye-sensitized solar cells or optoelectronic devices.

Quasiparticle electronic structure of phthalocyanine:TMD interfaces from first-principles GW

Interfaces formed between monolayer transition metal dichalcogenides and (metallo)phthalocyanine molecules are promising in energy applications and provide a platform for studying mixed-dimensional molecule-semiconductor heterostructures in general. An accurate characterization of the frontier energy level alignment at these interfaces is key in the fundamental understanding of the charge transfer dynamics between the two photon absorbers. Here, we employ the first-principles substrate screening GW approach to quantitatively characterize the quasiparticle electronic structure of a series of interfaces: metal-free phthalocyanine (H2Pc) adsorbed on monolayer MX2 (M = Mo, W; X = S, Se) and zinc phthalocyanine (ZnPc) adsorbed on MoX2 (X = S, Se). Furthermore, we reveal the dielectric screening effect of the commonly used α-quartz (SiO2) substrate on the H2Pc:MoS2 interface using the dielectric embedding GW approach. Our calculations furnish a systematic set of GW results for these interfaces, providing the structure-property relationship across a series of similar systems and benchmarks for future experimental and theoretical studies.

Porphyrins as building blocks for single-molecule devices

Direct measurement of single-molecule electrical transparency by break junction experiments has become a major field of research over the two last decades. This review specifically and comprehensively highlights the use of porphyrins as molecular components and discusses their potential use for the construction of future devices. Throughout the review, the features provided by porphyrins, such as low level misalignments and very low attenuation factors, are shown with numerous examples, illustrating the potential and limitations of these molecular junctions, as well as differences emerging from applied integration/investigation techniques.

Single-Molecule Junctions with Highly Improved Stability

Single-molecule junctions (SMJs) have been fabricated using layers generated by diazonium electroreduction. This process creates a C-Au covalent bond between the molecule and the electrode. Rigid oligomers of variable length, based on porphyrin derivatives in their free base or cobalt complex forms, have been grafted on the surface. The conductance of the oligomers has been studied by a scanning tunneling microscopy break junction (STM-bj) technique and G(t) measurements, and the lifetime of the SMJs has been investigated. The conductance histograms indicate that charge transport in the porphyrins is relatively efficient and influenced by the presence of the cobalt center. With both systems, random telegraph G(t) signals are easily recorded, showing SMJ on/off states. The SMJs then stabilize and exhibit a surprisingly long lifetime around 10 s, and attenuation plots, obtained by both G(t) and STM-bj measurements, give identical values. This work shows that highly stable SMJs can be prepared using a diazonium grafting approach.

Single-atom control of electrical conductance and thermopower through single-cluster junctions

The control of single atoms offers fundamental insight into understanding the charge transport through single clusters, and the atomic precision of the clusters provides the opportunity to manipulate the charge transport even at the single-atom level. Herein, we designed and investigated the electrical conductance and thermopower of Anderson-type polyoxometalate (POM) clusters with single-atom variation using the scanning tunneling microscopy break-junction (STM-BJ) technique. Our results show the electrical conductance of single clusters can be changed by an order of magnitude by substituting different center-metal atoms, and the electrical conductance of clusters shows different bias-dependence. Furthermore, the Seebeck coefficients of the POM clusters also can be significantly changed by the center-metal atoms. The non-equilibrium quantum transport calculations reveal that the electrostatic potential profile is non-uniformly dependent on the center-metal atoms. This leads to gating of electrical conductance by bias voltage. This supports the tuning of thermopower and bias dependent transmission spectra. This work provides the fundamental understanding of single-atom control of charge transport in POM single-cluster junctions.

Origin of the Electron Transport Properties of Aromatic and Antiaromatic Single Molecule Circuits

Antiaromatic molecules have been predicted to exhibit increased electron transport properties when placed between two nanoelectrodes compared to their aromatic analogues. While some studies have demonstrated this relationship, others have found no substantial increase. We use atomistic simulations to establish a general relationship between the electronic spectra of aromatic, antiaromatic, and quinoidal molecules and illustrate its implications for electron transport. We compare the electronic properties of a series of aromatic-antiaromatic counterparts and show that antiaromaticity effectively p-dopes the aromatic electronic spectra. As a consequence, the conducting properties of aromatic-antiaromatic analogues are closely related. For similar attachment points to the electrodes, an interference feature is expected in the HOMO-LUMO gap of one whenever it is absent in the other one. We demonstrate how the relative conductance of aromatic-antiaromatic pairs can be tuned and even reversed through the choice of chemical linker groups. Our work provides a general picture relating connectivity, (anti)aromaticity, and quantum interference and establishes new design rules for single molecule circuits.

The Role of Metal Ions in the Electron Transport through Azurin-Based Junctions

We studied the coherent electron transport through metal–protein–metal junctions based on a blue copper azurin, in which the copper ion was replaced by three different metal ions (Co, Ni and Zn). Our results show that neither the protein structure nor the transmission at the Fermi level change significantly upon metal replacement. The discrepancy with previous experimental observations suggests that the transport mechanism taking place in these types of junctions is probably not fully coherent.

Role of the Binding Motifs in the Energy Level Alignment and Conductance of Amine-Gold Linked Molecular Junctions within DFT and DFT + Σ

We investigate, using density functional theory (DFT), the electronic and conducting properties of benzenediamine connected to gold electrodes via different tip structures. We examine a series of binding motifs to the electrodes and calculate the junction spectral properties. We consider corrections to the position of molecular resonances at the junction and discuss different approaches to the calculation of these shifts. We relate the magnitude of these corrections to resonance energies to the atomistic structure of the tip. Benzenediamine DFT-based transmission spectra can be well approximated by a Lorentzian model involving only the highest occupied molecular orbital (HOMO). We show how benzenediamine calculated conductance values in quantitative agreement with previous experiments can be achieved from the combination of DFT-based spectra and corrections to the DFT-based HOMO energy and an accessible Lorentzian model.

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.

The bimetallic and the anchoring group effects on both optical and charge transport properties of hexaphyrin amethyrin

Bimetallic Cu(ii)-hexaphyrin amethyrin proposed as a molecular switch operated by the application of an external magnetic field.

Tuning Single-Molecule Conductance in Metalloporphyrin-Based Wires via Supramolecular Interactions

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group, as compared to a more rigid 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms.

Designing bifuncitonal molecular devices with a metalloporphyrin dimer

Many organic molecules have unique magnetic properties and can potentially serve as excellent molecular spin devices, which is worth exploring deeply. Here, the spin transport properties of Mn, Fe, Co and Cu porphyrin dimer devices are investigated based on the first principles method. The spin filtering efficiencies of these molecular devices are maintained at 100% within certain applied voltage ranges and magnetoresistance ratios are higher than 10⁸% which increase as the voltage increases. To explain the excellent spin-filtering and giant magnetoresistance effects, analysis of spin electron densities and transmission spectra indicates that magnetic properties are mainly contributed by the metal atoms and their neighbouring N atoms. From the transmission pathway studies, spin electrons come mainly through the π-conjugated structure of the metal porphyrin ring. Interestingly, in the Cu porphyrin dimer device, magnetic moments of the Cu-N structure in the Cu porphyrin dimer device show spin behaviors different from those of Mn, Fe and Co porphyrin dimer devices.

Effect of the linkage modes of thiolated ethynyl groups on the spin-dependent electronic transport properties in transition metal porphyrin molecular junctions

Using density functional theory and nonequilibrium Green's function method, the spin-dependent electronic transport properties of six transition metal porphyrin molecules (VP, CrP, MnP, FeP, CoP, and NiP), which are linked to gold electrodes through the thiolated ethynyl groups, are investigated. Two different linkage modes (beta linkage and meso linkage) of the substituted ethynyl groups on the porphyrin macrocycle are considered. The results show that the linkage mode of ethynyl groups plays an important role on the spin transport properties of the molecular junctions and the beta linkage is more favorable for the spin filtering efficiency of current than the meso linkages. The spin-up and spin-down energy levels show the different evolutions which is responsible for the difference of spin filtering efficiency between the two linkage modes. The computational results of total current show that the meso-linked molecular junctions have the better conductive performances than the beta-linked ones which may be caused by the different electronic transport paths.

Structure, Properties, and Reactivity of Porphyrins on Surfaces and Nanostructures with Periodic DFT Calculations

Porphyrins are fascinating molecules with applications spanning various scientific fields. In this review we present the use of periodic density functional theory (PDFT) calculations to study the structure, electronic properties, and reactivity of porphyrins on ordered two dimensional surfaces and in the formation of nanostructures. The focus of the review is to describe the application of PDFT calculations for bridging the gaps in experimental studies on porphyrin nanostructures and self-assembly on 2D surfaces. A survey of different DFT functionals used to study the porphyrin-based system as well as their advantages and disadvantages in studying these systems is presented.

Enhanced Energy Transfer in a Donor-Acceptor Photosensitizer Triggers Efficient Photodynamic Therapy

Photosensitizers (PSs) play a vital role in photodynamic therapy (PDT) for combating bacterial resistance and treating tumor. In this study, we report new donor-acceptor porphyrin PSs with a cationic conjugated oligomer (OPV) as a donor unit and porphyrin (TPP) as an acceptor unit by covalent linkage and achieved a fluorescence resonance energy transfer efficiency of 99% owing to their strong spectral overlap and short distance. The ¹O₂ yield of porphyrin derivatives is 121% (rose bengal as the standard reference) by virtue of OPVs' excellent light-harvesting ability and high fluorescence resonance energy transfer efficiency, greatly exceeding those of oligomer and porphyrin derivatives reported in the literature. Additionally, the cationic donors significantly improved the water solubility, decreased the aggregation of porphyrin, and promoted the adherence of the PSs to cell membranes through electrostatic interactions. As a result, the D-A porphyrin PSs exhibit dramatic PDT treatment efficiency. The half-inhibitory concentration is as low as 33 and 88 nM for methicillin-resistant Staphylococcus aureus and Escherichia coli, respectively. Therefore, this study provides a new strategy to construct PSs with high ¹O₂ yield and an excellent treatment effect at a low dose of PSs, which is promising for application in PDT used to treat cancer and microbial infections.

Unravelling the conductance path through single-porphyrin junctions

By studying transport through seven structurally related porphyrin derivatives with a machine learning algorithm we could identify and distinguish three different electronic paths.

Porphyrin derivatives are key components in natural machinery enabling us to store sunlight as chemical energy. In spite of their prominent role in cascades separating electrical charges and their potential as sensitizers in molecular devices, reports concerning their electronic transport characteristics are inconsistent. Here we report a systematic investigation of electronic transport paths through single porphyrin junctions. The transport through seven structurally related porphyrin derivatives was repeatedly measured in an automatized mechanically controlled break-junction set-up and the recorded data were analyzed by an unsupervised clustering algorithm. The correlation between the appearances of similar clusters in particular sub-sets of the porphyrins with a common structural motif allowed us to assign the corresponding current path. The small series of model porphyrins allowed us to identify and distinguish three different electronic paths covering more than four orders of magnitude in conductance.

Electric-field induced bistability in single-molecule conductance measurements for boron coordinated curcuminoid compounds

We have studied the single-molecule conductance of a family of curcuminoid molecules (CCMs) using the mechanically controlled break junction (MCBJ) technique. The CCMs under study contain methylthio (MeS-) as anchoring groups: MeS-CCM (1), the free-ligand organic molecule, and two coordination compounds, MeS-CCM-BF2 (2) and MeS-CCM-Cu (3), where ligand 1 coordinates to a boron center (BF2 group) and to a CuII moiety, respectively. We found that the three molecules present stable molecular junctions allowing detailed statistical analysis of their electronic properties. Compound 3 shows a slight increase in the conductance with respect to free ligand 1, whereas incorporation of BF2 (compound 2) promotes the presence of two conductance states in the measurements. Additional experiments with control molecules point out that this bistability is related to the combination of MeS- anchoring groups and the BF2 moiety within the structure of the molecules. Theoretical calculations show that this can be explained by the presence of two conformers once compound 2 is anchored between the gold electrodes. An energy minimum is found for a flat structure but there is a dramatic change in the magnitude and orientation of dipole moment (favouring a non-flat conformer in the presence of an external electric field) due to a conformational change of one of the terminal MeS- groups. The results thus point to an intricate interplay between the applied bias voltage and the molecule dipole moment which could be the basis for designing new molecules aiming at controlling their conformation in devices.

Functionalized Zinc Porphyrins with Various Peripheral Groups for Interfacial Electron Injection Barrier Control in Organic Light Emitting Diodes

Here, we use a simple and effective method to accomplish energy level alignment and thus electron injection barrier control in organic light emitting diodes (OLEDs) with a conventional architecture based on a green emissive copolymer. In particular, a series of functionalized zinc porphyrin compounds bearing π-delocalized triazine electron withdrawing spacers for efficient intramolecular electron transfer and different terminal groups such as glycine moieties in their peripheral substitutes are employed as thin interlayers at the emissive layer/Al (cathode) interface to realize efficient electron injection/transport. The effects of spatial (i.e., assembly) configuration, molecular dipole moment and type of peripheral group termination on the optical properties and energy level tuning are investigated by steady-state and time-resolved photoluminescence spectroscopy in F8BT/porphyrin films, by photovoltage measurements in OLED devices and by surface work function measurements in Al electrodes modified with the functionalized zinc porphyrins. The performance of OLEDs is significantly improved upon using the functionalized porphyrin interlayers with the recorded luminance of the devices to reach values 1 order of magnitude higher than that of the reference diode without any electron injection/transport interlayer.

Electronically Mediated Magnetic Anisotropy in Vibrating Magnetic Molecules

We address the electronically induced anisotropy field acting on a spin moment in a vibrating magnetic molecule located in the junction between ferromagnetic metals. Under weak coupling between the electrons and molecular vibrations, the nature of the anisotropy can be changed from favoring a high spin (easy-axis) magnetic moment to a low spin (easy plane) by applying a temperature difference or a voltage bias across the junction. For unequal spin polarizations in ferromagnetic metals, it is shown that the character of the anisotropy is essentially determined by the properties of the weaker ferromagnet. By increasing the temperature in this metal or introducing a voltage bias, its influence can be suppressed such that the dominant contribution to the anisotropy is interchanged to the stronger ferromagnet. With increasing coupling strength between the molecular vibrations and the electrons, the nature of the anisotropy is locked into favoring easy-plane magnetism.

Probing Charge Transport through Peptide Bonds

We measure the conductance of unmodified peptides at the single-molecule level using the scanning tunneling microscope-based break-junction method, utilizing the N-terminal amine group and the C-terminal carboxyl group as gold metal-binding linkers. Our conductance measurements of oligoglycine and oligoalanine backbones do not rely on peptide side-chain linkers. We compare our results with alkanes terminated asymmetrically with an amine group on one end and a carboxyl group on the other to show that peptide bonds decrease the conductance of an otherwise saturated carbon chain. Using a newly developed first-principles approach, we attribute the decrease in conductance to charge localization at the peptide bond, which reduces the energy of the frontier orbitals relative to the Fermi energy and the electronic coupling to the leads, lowering the tunneling probability. Crucially, this manifests as an increase in conductance decay of peptide backbones with increasing length when compared with alkanes.

High cross-plane thermoelectric performance of metallo-porphyrin molecular junctions

We investigated the thermoelectric properties of flat-stacked 5,15-diphenylporphyrins containing divalent metal ions Ni, Co, Cu or Zn, which are strongly coordinated with the nitrogens of pyridyl coated gold electrodes. Changing metal atom has little effect on the thermal conductance due to the phonons. The room-temperature Seebeck coefficients of these junctions are rather high, ranging from 90 μV K^-1 for Cu, Ni and Zn-porphyrins to -16 μV K^-1 for Co-porphyrin. These values could be further increased by lowering molecular energy levels relative to the DFT-predicted Fermi energy. In contrast, the phonon contribution to the thermal conductance of these junctions is rather insensitive to the choice of metal atom. The thermopower, thermal conductance and electrical conductance combined to yield the room-temperature values for the thermoelectric figure of merit ZT ranging from 1.6 for Cu porphyrin to ∼0.02 for Ni-porphyrin.

Robust Molecular Anchoring to Graphene Electrodes

Recent advances in the engineering of picoscale gaps between electroburnt graphene electrodes provide new opportunities for studying electron transport through electrostatically gated single molecules. But first we need to understand and develop strategies for anchoring single molecules to such electrodes. Here, for the first time we present a systematic theoretical study of transport properties using four different modes of anchoring zinc-porphyrin monomer, dimer, and trimer molecular wires to graphene electrodes. These involve either amine anchor groups, covalent C-C bonds to the edges of the graphene, or coupling via π-π stacking of planar polyaromatic hydrocarbons formed from pyrene or tetrabenzofluorene (TBF). π-π stacked pyrene anchors are particularly stable, which may be advantageous for forming robust single-molecule transistors. Despite their planar, multiatom coupling to the electrodes, pyrene anchors can exhibit both destructive interference and different degrees of constructive interference, depending on their connectivity to the porphyrin wire, which makes them attractive also for thermoelectricity. TBF anchors are more weakly coupled to both the graphene and the porphyrin wires and induce negative differential conductance at finite source-drain voltages. Furthermore, although direct C-C covalent bonding to the edges of graphene electrodes yields the highest electrical conductance, electron transport is significantly affected by the shape and size of the graphene electrodes because the local density of states at the carbon atoms connecting the electrode edges to the molecule is sensitive to the electrode surface shape. This sensitivity suggests that direct C-C bonding may be the most desirable for sensing applications. The ordering of the low-bias electrical conductances with different anchors is as follows: direct C-C coupling > π-π stacking with the pyrene anchors > direct coupling via amine anchors > π-π stacking with TBF anchors. Despite this dependency of conductances on the mode of anchoring, the decay of conductance with the length of the zinc-porphyrin wires is relatively insensitive with the associated attenuation factor β lying between 0.9 and 0.11 Å^-1.

Highly-conducting molecular circuits based on antiaromaticity

Aromaticity is a fundamental concept in chemistry. It is described by Hückel’s rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current–voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.

Antiaromatic molecules are predicted to have unusual charge transport properties, but are notoriously unstable and reactive. Here, the authors successfully fabricate an antiaromatic molecular circuit, based on a macrocyclic complex, displaying much higher conductance than its aromatic counterpart.

The enhanced spin-polarized transport behaviors through cobalt benzene-porphyrin-benzene molecular junctions: the effect of functional groups

The modification effects of the groups amino (NH₂) and nitro (NO₂) on the spin polarized transport properties of the cobalt benzene-porphyrin-benzene (Co-BPB) molecule coupled to gold (Au) nanowire electrodes are investigated by the nonequilibrium Green's function method combined with the density functional theory. The calculation results show that functional groups can lead to the significant spin-filter effect, enhanced low-bias negative differential resistance (NDR) behavior and novel reverse rectifying effect in Co-BPB molecular junction. The locations and types of functional groups have distinct influences on spin-polarized transport performances. The configuration with NH₂ group substituting H atom in central porphyrin ring has larger spin-down current compared to that with NO₂ substitution. And Co-BPB molecule junction with NH₂ group substituting H atom in side benzene ring shows reverse rectifying effect. Detailed analyses confirm that NH₂ and NO₂ group substitution change the spin-polarized transferred charge, which makes the highest occupied molecular orbitals (HOMO) of spin-down channel of Co-BPB closer to the Fermi level. And the shift of HOMO strengthens the spin-polarized coupling between the molecular orbitals and the electrodes, leading to the enhanced spin-polarized behavior. Our findings might be useful in the design of multi-functional molecular devices in the future.

Tuning the electrical conductance of metalloporphyrin supramolecular wires

In contrast with conventional single-molecule junctions, in which the current flows parallel to the long axis or plane of a molecule, we investigate the transport properties of M(II)-5,15-diphenylporphyrin (M-DPP) single-molecule junctions (M=Co, Ni, Cu, or Zn divalent metal ions), in which the current flows perpendicular to the plane of the porphyrin. Novel STM-based conductance measurements combined with quantum transport calculations demonstrate that current-perpendicular-to-the-plane (CPP) junctions have three-orders-of-magnitude higher electrical conductances than their current-in-plane (CIP) counterparts, ranging from 2.10⁻² G₀ for Ni-DPP up to 8.10⁻² G₀ for Zn-DPP. The metal ion in the center of the DPP skeletons is strongly coordinated with the nitrogens of the pyridyl coated electrodes, with a binding energy that is sensitive to the choice of metal ion. We find that the binding energies of Zn-DPP and Co-DPP are significantly higher than those of Ni-DPP and Cu-DPP. Therefore when combined with its higher conductance, we identify Zn-DPP as the favoured candidate for high-conductance CPP single-molecule devices.

Achieving Predictive Description of Molecular Conductance by Using a Range-Separated Hybrid Functional

The conductance of molecular bridges tends to be overestimated by computational studies in comparison to measured values. While this well-established trend may be related to difficulties for achieving robust bridges, the employed computational scheme can also contribute to this tendency. In particular, caveats of the traditional functionals employed in first-principles-based calculations can lead to discrepancies reflected in exaggerated conductance. Here, we show that by employing a range-separated hybrid functional the calculated values are within the same order as the measured conductance for all four considered cases. On the other hand, with B3LYP, which is a widely used functional, the calculated values greatly overestimate the conductance (by about 1-2 orders of magnitude). The improved description of the conductance with a RSH functional builds on achieving a physically meaningful treatment of the quasi particles associated with the frontier orbitals.