Molecules for charge-based information storage

Origins of non-ideal behaviour in voltammetric analysis of redox-active monolayers

Disorder in redox-active monolayers convolutes electrochemical characterization. This disorder can come from pinhole defects, loose packing, heterogeneous distribution of redox-active headgroups, and lateral interactions between immobilized redox-active molecules. Identifying the source of non-ideal behaviour in cyclic voltammograms can be challenging as different types of disorder often cause similar non-ideal cyclic voltammetry behaviour such as peak broadening, large peak-to-peak separation, peak asymmetry and multiple peaks for single redox processes. This Review provides an overview of ideal voltammetric behaviour for redox-active monolayers, common manifestations of disorder on voltammetric responses, common experimental parameters that can be varied to interrogate sources of disorder, and finally, examples of different types of disorder and how they impact electrochemical responses.

Efficient Spin-Adapted Implementation of Multireference Algebraic Diagrammatic Construction Theory. I. Core-Ionized States and X-ray Photoelectron Spectra

We present an efficient implementation of multireference algebraic diagrammatic construction theory (MR-ADC) for simulating core-ionized states and X-ray photoelectron spectra (XPS). Taking advantage of spin adaptation, automatic code generation, and density fitting, our implementation can perform calculations for molecules with more than 1500 molecular orbitals, incorporating static and dynamic correlation in the ground and excited electronic states. We demonstrate the capabilities of MR-ADC methods by simulating the XPS spectra of substituted ferrocene complexes and azobenzene isomers. For the ground electronic states of these molecules, the XPS spectra computed using the extended second-order MR-ADC method (MR-ADC(2)-X) are in a very good agreement with available experimental results. We further show that MR-ADC can be used as a tool for interpreting or predicting the results of time-resolved XPS measurements by simulating the core ionization spectra of azobenzene along its photoisomerization, including the XPS signatures of excited states and the minimum energy conical intersection. This work is the first in a series of publications reporting the efficient implementations of MR-ADC methods.

An enzymolysis-induced energy transfer co-assembled system for spontaneously recoverable supramolecular dynamic memory

The continuing growth of the digital world requires new ways of constructing memory devices to process and store dynamic data, because the current ones suffer from inefficiency, limited reads, and difficulty to manufacture. Here we propose a supramolecular dynamic memory (SDM) strategy based on an enzymolysis-induced energy transfer co-assembly derived from a naphthalene-based cationic monomer and organic dye sulforhodamine 101, enabling the construction of spontaneously recoverable dynamic memory devices. Benefitting from the large exciton migration rate (4.48 × 1015 L mol-1 s-1) between the monomer and sulforhodamine 101, the energy transfer process between the two is effectively achieved. Since alkaline phosphatase can selectively hydrolyze adenosine triphosphate, leading to the disruption of the co-assemblies, an enzyme-mediated time-dependent fluorochromic system is realized. On this basis, a SDM system featuring spontaneous recovery and enabling the memory of dynamic information in optical and electrical modes is successfully constructed. The current study represents a promising step in the nascent development of supramolecular materials for computational systems.

Unusual Stabilisation of Remarkably Bent Tetra-Cationic Tetra-radical Intermolecular Fe(III) μ-Oxo Tetranuclear Complexes

A hitherto unknown series of air stable, π-conjugated, remarkably bent tetra-cation tetra-radical intermolecular Fe(III) μ-oxo tetranuclear complex, isolated from the dication diradical diiron(III) porphyrin dimers, has been synthesised and spectroscopically characterised along with single crystal X-ray structure determination of two such molecules. These species facilitate long-range charge/radical delocalisation through the bridge across the entire tetranuclear unit manifesting an unusually intense NIR band. Assorted spin states of Fe(III) centres are stabilised within these unique tetranuclear frameworks: terminal six-coordinate iron centres stabilise the admixed intermediate spin states while the central five-coordinate iron centres stabilise the high-spin states. Variable temperature magnetic susceptibility measurements indicated strong antiferromagnetic coupling for the Fe(III)-O-Fe(III) unit while the exchange interactions between the Fe centres and the porphyrin π-cation radicals are weaker as supported both by magnetic data and DFT calculations. The nature of orbital overlap between the SOMOs of Fe(III) and π* orbital of the porphyrin was found to rationalise the observed exchange coupling, establishing such a complex magnetic exchange in this tetranuclear model with a significant bioinorganic relevance.

Reversible open-closed conformational switching of nano-size metalloporphyrin dimers triggered by light and temperature

The current work demonstrates the reversible control of substantial molecular motion in 'nano-sized' molecules, where two structural isomers can 'open' and 'close' their cavities in response to light or heat. The isomers differ widely in their photophysical properties, including colour, polarity, two-photon absorption and π-conjugation, and can easily be separated through column chromatography and thus have wide applicability.

Stable Dication Diradicals of Triply Fused Metallo Chlorin-Porphyrin Heterodimers: Impact of the Bridge on the Control of Spin Coupling to Reactivity

We report an unexpected rearrangement, controlled by the nature of the bridge, leading to the formation of novel, remarkably stable triply fused dinickel(II)/dicopper(II) chlorin-porphyrin dication diradical heterodimers in excellent yields. Here, a dipyrromethene bridge gets completely fused between two porphyrin macrocycles with two new C-C and one C-N bonds. The two macrocycles exhibit extensive π-conjugation through the bridge, which results in an antiferromagnetic coupling between the two π-cation radicals. In addition, the macrocyclic distortion also favours a rare intramolecular ferromagnetic interaction between the CuII and π-cation radical spins to form a triplet state. The structural and electronic perturbation in the unconjugated dication diradical possibly enables the bridging pyrrolic nitrogen to undergo a nucleophilic attack at the nearby β-carbon of the porphyrin π-cation radical with a computed free energy barrier of >20 kcal mol-1 which was supplied in the form of reflux condition to initiate such a rearrangement process. UV-vis, EPR and ESI-MS spectroscopies were used to monitor the rearrangement process in situ in order to identify the key reactive intermediates leading to such an unusual transformation.

Analytical nonadiabatic coupling and state-specific energy gradient for the crystal field Hamiltonian describing lanthanide single-ion magnets

Paramagnetic molecules with a metal ion as an electron spin center are promising building blocks for molecular qubits and high-density memory arrays. However, fast spin relaxation and decoherence in these molecules lead to a rapid loss of magnetization and quantum information. Nonadiabatic coupling (NAC), closely related to spin-vibrational coupling, is the main source of spin relaxation and decoherence in paramagnetic molecules at higher temperatures. Predicting these couplings using numerical differentiation requires a large number of computationally intensive ab initio or crystal field electronic structure calculations. To reduce computational cost and improve accuracy, we derive and implement analytical NAC and state-specific energy gradient for the ab initio parametrized crystal field Hamiltonian describing single-ion molecular magnets. Our implementation requires only a single crystal field calculation. In addition, the accurate NACs and state-specific energy gradients can be used to model spin relaxation using sophisticated nonadiabatic molecular dynamics, which avoids the harmonic approximation for molecular vibrations. To test our implementation, we calculate the NAC values for three lanthanide complexes. The predicted values support the relaxation mechanisms reported in previous studies.

Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Herein, we introduce a comprehensive study of the photophysical behaviors and CO2 reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H2, Co(ii), Ni(ii), Cu(ii), Zn(ii)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H2, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO2 reduction reaction than the other metallic CPOC-M (M = Ni(ii), Cu(ii), Zn(ii)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm-2 at -0.90 V with 90% faradaic efficiency of CO.

Remarkable Enhancement of Catalytic Activity of Cu-Complexes in the Electrochemical Hydrogen Evolution Reaction by Using Triply Fused Porphyrin

A bimetallic triply fused copper(II) porphyrin complex (1) was prepared, comprising two monomeric porphyrin units linked through β-β, meso-meso, β'-β' triple covalent linkages and exhibiting remarkable catalytic activity for the electrochemical hydrogen evolution reaction in comparison to the analogous monomeric copper(II) porphyrin complex (2). Electrochemical investigations in the presence of a proton source (trifluoroacetic acid) confirmed that the catalytic activity of the fused metalloporphyrin occurred at a significantly lower overpotential (≈320 mV) compared to the non-fused monomer. Controlled potential electrolysis combined with kinetic analysis of catalysts 1 and 2 confirmed production of hydrogen, with 96 and 71 % faradaic efficiencies and turnover numbers of 102 and 18, respectively, with an observed rate constant of around 10⁷  s^-1 for the dicopper complex. The results thus firmly establish triply fused porphyrin ligands as outstanding candidates for generating highly stable and efficient molecular electrocatalysts in combination with earth-abundant 3d transition metals.

Chemical Conformation Induced Transport Carrier Switching in Molecular Junction based on Carboxylic-Terminated Thiol Molecules

The paper demonstrates the effect of the chemical conformation of the -COOH group on the transport characteristic including conductance, rectification, and length effect in molecular junctions (MJs) formed by self-assembled monolayers of carboxylic-terminated thiol molecules. For an alkyl chain shorter than C₁₁, the transport mechanism was attributed to a direct off-resonant tunneling of a hole carrier, located at the Au-S interface, whereas a hopping mechanism was assigned to the alkyl chain longer than the C₁₁ chain located at the -COOH group. The hopping mechanism may be operated by electron transport associated with the breaking of the -OH bonding likely driven by a voltage. Importantly, at the C₁₁ alkyl chain, we observed that the transport carrier operating in MJs could change from a hole carrier into an electron carrier. The result strongly proves that the chemical conformation should be considered in analyzing molecular electronics and provides a basis for the rational design of molecular electronic devices.

Helical Molecule as an Efficient Rectifier: Effects of Molecular Conformation and Transverse Electric Field

The phenomenon of charge current rectification is critically investigated using a single stranded helical molecule in presence of transverse electric field. Two different helical molecules, DNA and protein, are taken into account to explore the specific roles of molecular conformation on rectification, which have not been addressed so far to the best of our concern. Sandwiching the molecular system within source and drain electrodes, we compute charge currents for two bias polarities and the degree of current rectification based on non-equilibrium Green's function formalism within a tight-binding framework. At non-zero electric field, site energies of the molecule are modulated in a cosine form, similar to the well known Aubry-André-Harper relation, resulting an atypical and fragmented energy band spectrum. The appearance of non-uniform site energies plays the central role for generating different currents in two bias polarities, and thus, the current rectification. We find that a high degree of current rectification can be established using the helical system and it becomes more effective for the protein molecule than the DNA one. At the end, the rectification operation considering a more general helical structure is discussed to make the present communication a self-contained one. Our proposition may provide a new route of getting controlled current rectification using similar kind of biological molecules and other tailor made helical geometries.

Structure Determination of Binuclear Triple-Decker Phthalocyaninato Complexes by NMR via Paramagnetic Shifts Analysis Using Symmetry Peculiarities

The detailed knowledge about the structure of multinuclear paramagnetic lanthanide complexes for the targeted design of these compounds with special magnetic, sensory, optical and electronic properties is a very important task. At the same time, establishing the structure of such multinuclear paramagnetic lanthanide complexes in solution, using NMR is a difficult task, since several paramagnetic centers act simultaneously on the resulting chemical shift of a particular nucleus. In this paper, we have demonstrated the possibility of molecular structure determination in solution on the example of binuclear triple-decker lanthanide(III) complexes with tetra-15-crown-5-phthalocyanine Ln₂[(15C5)₄Pc]₃ {where Ln = Tb (1) and Dy (2)} by quantitative analysis of the pseudo-contact lanthanide-induced shifts (LIS). The symmetry of complexes was used for the simplification of the calculation of pseudo-contact shifts on the base of the expression for the magnetic susceptibility tensor in the arbitrary oriented magnetic axis system. Good agreement between the calculated and experimental shifts in the ¹H NMR spectra indicates the similarity of the structure for the complexes 1 and 2 in solution of CDCl₃ and the structure in the crystalline phase, found from the data of the X-ray structural study of the similar complex Lu₂[(15C5)₄Pc]₃. The described approach can be useful for LIS analysis of other polynuclear symmetric lanthanide complexes.

Highly Oxidized Cobalt Porphyrin Dimer: Control of Spin Coupling via a Bridge

A cobalt porphyrin dimer is constructed in which two Co(II)porphyrins are connected covalently through a redox-active diethylpyrrole moiety via a flexible but "nonconjugated" methylene bridge. Upon oxidation with even a mild oxidant such as iodine, each cobalt(II) center and porphyrin ring undergo 1e^- oxidation, leading to the formation of a 4e^--oxidized cobalt(III)porphyrin dication diradical complex. Other oxidants such as Cl₂ and Br₂ also produce similar results. To stabilize such highly oxidized dication diradicals, the "nonconjugated" methylene spacer undergoes a facile and spontaneous oxidation to form a methine group with a drastic structural change, thereby making the bridge fully π-conjugated and enabling through-bond communication. This results in a strong spin coupling between two π-cation radicals which stabilizes the singlet state. The experimental observations are also strongly supported by extensive density functional theory calculations. The present study highlights the crucial role played by the nature of the bridge in the long-range electronic communication.

Photophysical and Computational Insights into Ag(I) Complexation of Porphyrinic Covalent Cages Equipped with Triazoles-Incorporating Linkers

The photophysical characterization of four supramolecular complexes based on covalent cages 2H-S-2H, 2H-L-2H, Zn-S-2H, and Zn-L-2H, consisting in either two free-base porphyrins or one Zn(II) porphyrin and one free-base porphyrin connected by four flexible linkers of different lengths incorporating triazole binding sites, and their Ag(I) complexation are reported. The complexation processes have been followed by means of absorption and emission spectroscopies, and a comprehensive computational study explains the behavior of the free-base porphyrin-containing cages. Absorption and emission features have been interpreted on the bases of conformational changes, metalation processes, and modification of energy transfer efficiencies occurring in the different cases. In all cages, except 2H-L-2H, the coordination of four Ag(I) ions to the lateral triazole groups of the linkers leads to the enlargement of their cavity. Only for 2H-L-2H is a different behavior observed, where the process of silver metalation of the porphyrins’ core prevails.

Long-Range Intramolecular Spin Coupling through a Redox-Active Bridge upon Stepwise Oxidations: Control and Effect of Metal Ions

Dinickel(II) and dicopper(II) porphyrin dimers have been constructed in which two metalloporphyrin units are widely separated by a long unconjugated dipyrrole bridge. Two macrocycles are aligned somewhat orthogonally to each other, while oxidation of the bridge generates a fully π-conjugated butterfly-like structure, which, in turn, upon stepwise oxidations by stronger oxidants result in the formation of the corresponding one- and two-electron-oxidized species exhibiting unusual long-range charge/radical delocalization to produce intense absorptions in the near-infrared (NIR) region and electron paramagnetic resonance (EPR) signals of a triplet state due to interaction between the unpaired spins on the Cu(II) ions. Although the two metal centers have a large physical separation through the bridge (more than 16 Å), they share electrons efficiently between them, behaving as a single unit rather than two independent centers. Detailed UV-vis-NIR, electrospray ionization mass spectrometry, IR, variable-temperature magnetic study, and EPR spectroscopic investigations along with X-ray structure determination of unconjugated, conjugated, and one electron-oxidized complexes have been exploited to demonstrate the long-range electronic communication through the bridge. The experimental observations are also supported by density functional theory (DFT) and time-dependent DFT calculations. The present study highlights the crucial roles played by a redox-active bridge and metal in controlling the long-range electronic communication.

Meso bromination and derivatization of synthetic bacteriochlorins

Twelve bacteriochlorin building blocks featuring meso-substitution have been prepared including a set with finely tuned long-wavelength absorption (725–757 nm) for studies in photonics.

Octopus-Type Crown-Bisphthalocyaninate Anchor for Bottom-Up Assembly of Supramolecular Bilayers with Expanded Redox-Switching Capability

Achievement of information storage at molecular level remains a pressing task in miniaturization of computing technology. One of the promising approaches for its practical realization is development of nanoscale molecular switching materials including redox-active systems. The present work demonstrates a concept of expansion of a number of available redox-states of self-assembled monolayers through supramolecular approach. For this, the authors synthesized an octopus-like heteroleptic terbium(III) bisphthalocyaninate bearing one ligand with eight thioacetate-terminated "tentacles" (octopus-Pc) and a ligand with four crown-ether moieties (H₂ [(15C5)₄ Pc]). It is shown that octopus-Pc forms stable monolayers on gold, where its face-on orientation allows for subsequent binding of crown-phthalocyanine molecules via potassium ion bridges. This chemistry is utilized to form a heterogeneous bilayer, in which a single molecule thick adlayer brings an additional redox-state to the system, thus expanding the multistability of the system as a whole. All four redox states available to this system exhibit characteristic absorbance in visible range, allowing for the switching to be easily read out using optical density measurements. The proposed approach can be used in wide range of switchable materials-single-molecule magnets, conductive, and optical devices, etc.

A Modular Strategy for Expanding Electron-Sink Capacity in Noncanonical Cluster Assemblies

A modular synthetic strategy is described whereby organometallic complexes exhibiting considerable electron-sink capacity may be assembled by using only a few simple molecular components. The Fe₂(PPh₂)₂(CO)₅ fragment was selected as a common electroactive component and was assembled around aromatic cores bearing one, two, or three isocyanide functional groups, with the resultant complexes possessing electron-sink capacities of two, four, and six electrons, respectively. The latter complex is noteworthy in that its electron-sink capacity was found to rival that of large multinuclear clusters (e.g., [Ni₃₂C₆(CO)₃₆]^6– and [Ni₃₈Pt₆(CO)₄₈]^6–), which are often considered as benchmarks of electron-sink behavior. Moreover, the modular assembly bearing three Fe₂(PPh₂)₂(CO)₅ fragments was observed to undergo reduction to a hexaanionic state over a potential window of about −1.4 to −2.1 V (vs Fc/Fc⁺), the relatively compressed range being attributed to potential inversions operative during the addition of the second, fourth, and sixth electrons. Such complexes may be designated noncanonical clusters because they exhibit redox properties similar to those of large multinuclear clusters yet lack the extensive network of metal–metal bonds and the condensed metallic cores that typify the latter.

By use of a modular synthetic strategy and relatively few molecular components, organometallic complexes exhibiting considerable electron-sink capacity have been characterized. Complexes bearing one, two, or three Fe₂(PPh₂)₂(CO)₅ fragments bound to aromatic isocyanide cores were found to possess electron-sink capacities of two, four, and six electrons, respectively, the latter rivaling the electron-sink capacity of large polynuclear cluster benchmarks.

Codes in Code: AIE Supramolecular Adhesive Hydrogels Store Huge Amounts of Information

With the continuous advancement of information technology, the requirements for the information storage capacity of materials are getting higher and higher. However, information code materials usually only store a single piece of information. In order to improve their storage capacity, aggregation-induced emission (AIE) supramolecular adhesive hydrogels with different fluorescent colors are prepared, and a "Codes in Code" method is used to demonstrate the storage capacity for large amounts of information. Four kinds of poly(vinyl alcohol) (PVA) supramolecular hydrogels with different fluorescent colors are prepared; based on the hydrogen bonds on the hydrogel surface, these hydrogels can be assembled into a hydrogel, G5, which shows multiple fluorescent colors under the irradiation of UV light. When many 1D barcode patterns or/and 2D code patterns are incorporated into G5, not only a kind of 3D information but also plenty of 1D or/and 2D information can be stored. Therefore, the information codes prepared by the "Codes in Code" method can store a large amount of information.

Electrochemical Potential-Driven High-Throughput Molecular Electronic and Spintronic Devices: From Molecules to Applications

Molecules are fascinating candidates for constructing tunable and electrically conducting devices by the assembly of either a single molecule or an ensemble of molecules between two electrical contacts followed by current-voltage (I-V) analysis, which is often termed "molecular electronics". Recently, there has been also an upsurge of interest in spin-based electronics or spintronics across the molecules, which offer additional scope to create ultrafast responsive devices with less power consumption and lower heat generation using the intrinsic spin property rather than electronic charge. Researchers have been exploring this idea of utilizing organic molecules, organometallics, coordination complexes, polymers, and biomolecules (proteins, enzymes, oligopeptides, DNA) in integrating molecular electronics and spintronics devices. Although several methods exist to prepare molecular thin-films on suitable electrodes, the electrochemical potential-driven technique has emerged as highly efficient. In this Review we describe recent advances in the electrochemical potential driven growth of nanometric various molecular films on technologically relevant substrates, including non-magnetic and magnetic electrodes to investigate the stimuli-responsive charge and spin transport phenomena.

Multi-Redox Active Carbons and Hydrocarbons: Control of their Redox Properties and Potential Applications

Precise control over redox properties is essential for high-performance organic electronic devices such as organic batteries, electrochromic devices, and information storage devices. In this context, multi-redox active carbons and hydrocarbons, represented as C_x H_y molecules (x≥1, y≥0), are highly sought after, because they can switch between multiple redox states. Herein, we outline the redox properties of C_x H_y molecules as solutes and adsorbed species. Furthermore, the limitations of evaluating their redox properties and the possible solutions are summarized. Additionally, the theoretical capacity (mAh/g) and gravimetric energy density (Wh/kg) of secondary batteries were estimated based on the redox properties of 185 C_x H_y molecules, which have primarily been reported in the last decade. Among them, seven C_x H_y molecules were found to have the potential to surpass the energy density of LiNi_0.6 Mn_0.2 Co_0.2 O₂ /graphite batteries. The use of C_x H_y molecules in multielectrochromic devices and multi-bit memory is also explained. We believe that this review will encourage further utilization of C_x H_y molecules thereby promoting its applications in organic electronic devices.

Covalently Conjugated Gold-Porphyrin Nanostructures

Gold nanoparticles show important electronic and optical properties, owing to their size, shape, and electronic structures. Indeed, gold nanoparticles containing no more than 30–40 atoms are only luminescent, while nanometer-sized gold nanoparticles only show surface plasmon resonance. Therefore, it appears that gold nanoparticles can alternatively be luminescent or plasmonic and this represents a severe restriction for their use as optical material. The aim of our study was the fabrication of nanoscale assembly of Au nanoparticles with bi-functional porphyrin molecules that work as bridges between different gold nanoparticles. This functional architecture not only exhibits a strong surface plasmon, due to the Au nanoparticles, but also a strong luminescence signal due to porphyrin molecules, thus, behaving as an artificial organized plasmonic and fluorescent network. Mutual Au nanoparticles–porphyrin interactions tune the Au network size whose dimension can easily be read out, being the position of the surface plasmon resonance strongly indicative of this size. The present system can be used for all the applications requiring plasmonic and luminescent emitters.

Electric-field-driven dual-functional molecular switches in tunnel junctions

To avoid crosstalk and suppress leakage currents in resistive random access memories (RRAMs), a resistive switch and a current rectifier (diode) are usually combined in series in a one diode-one resistor (1D-1R) RRAM. However, this complicates the design of next-generation RRAM, increases the footprint of devices and increases the operating voltage as the potential drops over two consecutive junctions¹. Here, we report a molecular tunnel junction based on molecules that provide an unprecedented dual functionality of diode and variable resistor, resulting in a molecular-scale 1D-1R RRAM with a current rectification ratio of 2.5 × 10⁴ and resistive on/off ratio of 6.7 × 10³, and a low drive voltage of 0.89 V. The switching relies on dimerization of redox units, resulting in hybridization of molecular orbitals accompanied by directional ion migration. This electric-field-driven molecular switch operating in the tunnelling regime enables a class of molecular devices where multiple electronic functions are preprogrammed inside a single molecular layer with a thickness of only 2 nm.

Electrospray ionization: an efficient approach to deposit polymetallic molecular switches onto gold surfaces

Electrospray ionization (EI) deposition is proven efficient in obtaining monolayers of a polymetallic charge transfer complex on gold surfaces. The molecule's integrity is monitored by using PM-IRRAS and XPS. This approach broadens the perspective of molecular magnetic switch deposition, which is currently dominated by the thermal evaporation of monometallic spin crossover (SCO) complexes.

N-Heterocyclic Carbenes for the Self-Assembly of Thin and Highly Insulating Monolayers with High Quality and Stability

As an organic nanostructure, self-assembled monolayers (SAMs) play a central role in many aspects of nanotechnology, including molecular electronics. In this work, we show that SAMs based on N-heterocyclic carbenes on a Au(111) substrate offer a high level of crystallinity and also exhibit the highest possible packing density. As a result of this structural optimization, defect concentrations were reduced by 2-3 orders of magnitude and thermal stability was ∼100 K higher than those of any other SAMs on Au. The conductivity of these SAMs is ∼4 orders of magnitude lower than that of standard alkanethiols of comparable length, which together with very low defect concentration and high thermal stability makes them a highly interesting material for potential application in organic thin film transistors. The self-assembly of such dense, highly crystalline, and notably stable structures is associated with strong C-Au bonding and the rational design of assembled molecules, resulting in the high mobility of both adsorbate and substrate atoms, as confirmed by the size of the molecular domains and the adsorbate-driven modification of the Au(111) substrate, respectively.

Molecular and Supramolecular Multiredox Systems

The design and synthesis of molecular and supramolecular multiredox systems have been summarized. These systems are of great importance as they can be employed in the next generation of materials for energy storage, energy transport, and solar fuel production. Nature provides guiding pathways and insights to judiciously incorporate and tune the various molecular and supramolecular design aspects that result in the formation of complex and efficient systems. In this review, we have classified molecular multiredox systems into organic and organic-inorganic hybrid systems. The organic multiredox systems are further classified into multielectron acceptors, multielectron donors and ambipolar molecules. Synthetic chemists have integrated different electron donating and electron withdrawing groups to realize these complex molecular systems. Further, we have reviewed supramolecular multiredox systems, redox-active host-guest recognition, including mechanically interlocked systems. Finally, the review provides a discussion on the diverse applications, e. g. in artificial photosynthesis, water splitting, dynamic random access memory, etc. that can be realized from these artificial molecular or supramolecular multiredox systems.

Thermo- and electro-switchable Cs⊂{Fe4–Fe4} cubic cage: spin-transition and electrochromism

The Cs⊂{Fe₄–Fe₄} cube is a dual redox and magnetic switch showing remarkable electrochromic properties and spin-transition in its neutral state.

Elucidating π–π interaction-induced extension effect in sandwich phthalocyaninato compounds

π–π interaction-linked extension in the perpendicular direction to the monomers and corresponding effect on nonlinear optic properties have been clearly disclosed over the multiple-decker sandwich-type phthalocyaninato metal compounds.

Photophysical properties of porphyrinic covalent cages endowed with different flexible linkers

In-depth photophysical studies of four flexible covalent cages bearing either two free-base porphyrins or one free-base porphyrin and one Zn(II) porphyrin, connected by linkers of different lengths, are reported. In the case of the cages with two free-base porphyrins, exciton coupling between the porphyrins is evidenced by large and split Soret bands in the absorption spectra, but the different length of the linkers has only a slight effect on their emission properties. Strong electronic interactions between the porphyrins are also evidenced for the cages that incorporate a free-base porphyrin and a Zn(II) porphyrin, with a more pronounced splitting of the Soret band for the system with longer linkers. In these cages, following excitation of the Zn-porphyrin component, an almost quantitative energy transfer to the free-base unit occurs, with a rate 1.4 times faster in the cage with longer linkers (1.4 × 10[Formula: see text] s[Formula: see text] vs. 1.0 × 10[Formula: see text] s[Formula: see text]. This difference might reflect the more flattened conformation adopted by the cage equipped with longer and more flexible linkers, the latter allowing for a shorter interplanar distance between the porphyrins. The results are discussed in terms of classical and short-range energy transfer mechanisms.

Multidentate Anchors for Surface Functionalization

The bottom-up functionalization of solid surfaces shows increasing importance for a wide range of interdisciplinary applications. Multidentate anchors with more than two contact points can bind to solid surfaces with strong chemisorption, well-defined upright configuration, and tailored functionality. The surface functionalization using multidentate anchors with three (tripodal), four (quadripodal), or more binding points is summarized herein, with a focus on those beyond classical tripodal anchors. In particular, the molecular design on how to achieve multisite interaction between anchor and substrate and the introduction of functional groups to thin films are discussed.

In situ reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes

In this article we report the reversible redox switching of first hyperpolarizability of bimetallic ruthenium complexes bridged by bipyridyl tetrazine (bptz) ligands by second harmonic light scattering experiments (SHLS). We have synthesised [RuII(acac)₂(CH₃CN)₂] and [(acac)₂Ru-bptz-Ru(acac)₂] complexes and measured their first hyperpolarizabilities as a function of in situ electrochemical oxidation/reduction of the metal centres. As a result of the oxidation of ruthenium centre from Ru(ii) to Ru(iii), the molecular hyperpolarizability of the complexes went up. The mixed-valence intermediate bimetallic complex and not the fully oxidized complex exhibit the highest β value of 780 × 10^-30 esu. We also demonstrated that the hyperpolarizability is reversible with the change of the oxidation state of the metal and both the complexes are stable for several cycles of redox switching. The experimental results are also supported by calculations.

Charge transport pathways in metal porphyrin as interplay between long and short range scattering processes

We have investigated the ballistic transport for Mn, Fe-porphyrin molecules in contact with Au(111) electrodes by using density functional theory. We show that the information resulted from the projected density of electronic states does not provide a complete picture of the transport mechanism. Instead, we propose a methodology based on the concept of the orbital projected transmissions for selected groups of atoms. We have found that the transmission channels for occupied states can be assigned to short range scattering processes at gold-molecule interface, while for the states above Fermi level the transmission takes place via long range scattering processes. The interplay between these two conduction mechanisms is responsible for the transport path in metal-porphyrin; our model is in qualitative agreement with the existing experimental data on transport pathway on single porphyrin molecules.

Charge transport through redox active [H7P8W48O184]33- polyoxometalates self-assembled onto gold surfaces and gold nanodots

Polyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects.

Electron transport behavior of quinoidal heteroacene-based junctions: effective electron-transport pathways and quantum interference

The electron transport behavior through a series of molecular junctions composed of tetracene (TC) and S/O substituted-TC (S/O-TC) has been studied using density functional theory (DFT) combined with the non-equilibrium Green's function (NEGF) method. The unique transport behavior has been interpreted using correlated quantum interference and electron transport pathway models. In the TC system, two dominant electron transfer channels exist as demonstrated by a detailed transmission pathway analysis. In the substituted S/O-TC systems, the electron transport behavior is regulated through either constructive or destructive quantum interference due to the existence of additional p-electrons, leading to a significant diversity of current-voltage curves. Compared to the TC molecule in the bias region from 0 to 1.0 V, an α-connected molecular junction exhibits a greater current, whereas a β-connected molecular junction shows a smaller current. The substitution with O and S atoms shows a minor effect on the conductance of the molecular junctions. In order to clarify the role of heteroatoms, a series of artificial models designed by removing specific sulfur and carbon atoms in α-S-TC have been investigated in detail. The results have demonstrated that only the S heteroatom on one side of the molecule contributes to the junction conductivity through constructive quantum interference. It has also been observed that current exchange occurs between the two electron transfer channels.

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.