Publications

Straight facets and sharp edges are among the most distinctive indicators of well-defined crystals and often reflect the polyhedral geometry and symmetry of the underlying close-packed, molecular structure. Curved morphologies are sometimes observed in biogenic crystals where templating or nonclassical crystallization processes (e.g., crystallization via amorphous states) occur. Here we report the formation and growth of copper-based metal-organic frameworks (MOFs) that crystallize in the hexagonal space group P622. The individual MOFs are homochiral and are formed from achiral compounds. The crystals begin as hexagonal-like structures, developing over time into complex flower-like structures having two decks joined in the center. Each deck layer has six well-defined petals with curved lateral surfaces. The growth mechanism shows initial straight petals that become progressively more curved with increased faceting. Remarkably, despite its multidomain appearance, the entire entity is a single crystal. The curved morphology is correlated to the crystallographic structure and the arrangement of nanosized channels within this structure. Crystal habits are typically considered to be inconsistent with curved morphologies. This work suggests that crystallographic explanations can support the development of such surfaces for low-density structures.

In this study, the precise positioning and alignment of arrays of two different guest molecules in a crystalline host matrix has been engineered and resulted in new optically active materials. Sub-nm differences in the diameters of two types of 1D channels are sufficient for size-selective inclusion of dyes. Energy transport occurs between the arrays of different dyes that are included in parallel-positioned nanochannels by Förster resonance energy transfer (FRET). The color of individual micro-sized crystals are dependent on their relative position under polarized light. This angular-dependent behavior is a result of the geometrically constrained orientation of the dyes by the crystallographic packing of the host matrix and is concentration dependent.

We demonstrate the formation of highly interpenetrated frameworks. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal-organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63% of the unit cell volume. The frameworks are chiral but formed from achiral components. The chirality is manifested by the coordination chemistry frameworks around the metal center, the structure of the helicoidal channels and the motifs of the individual networks. Channels of both handedness are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated to the degree of interpenetration 10-, 16- and 32-fold. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å × 14.8 Å.

We demonstrate the formation of both metallo-organic crystals and nanoscale films that have entirely different compositions and structures despite using the same set of starting materials. This difference is the result of an unexpected cation exchange process. The reaction of an iron polypyridyl complex with a copper salt by diffusion of one solution into another resulted in iron-to-copper exchange, concurrent ligand rearrangement, and the formation of metal-organic frameworks (MOFs). This observation shows that polypyridyl complexes can be used as expendable precursors for the growth of MOFs. In contrast, alternative depositions of the iron polypyridyl complex with a copper salt by automated spin coating on conductive metal oxides resulted in the formation of electrochromic coatings, and the structure and redox properties of the iron complex were retained. The possibility to form such different networks from the same set of molecular building blocks by "in solution"versus "on surface"coordination chemistry broadens the synthetic space to design functional materials.

We demonstrate here a unique metalloorganic material where the appearance and the internal crystal structure are in contradiction. The eggshaped (ovoid) crystals have a brainlike texture. Although these microsized crystals are monodispersed; like fingerprints their grainy surfaces are never exactly alike. Remarkably, our Xray and electron diffraction studies unexpectedly revealed that these structures are singlecrystals comprising a continuous coordination network of two differently shaped homochiral channels. By using the same building blocks under different reaction conditions, a rare series of crystals have been obtained that are uniquely rounded in their shape. In stark contrast to the brainlike crystals, these isostructural and monodispersed crystals have a comparatively smooth appearance. The sizes of these crystals vary by several orders of magnitude.The eggshaped homochiral crystals: A brainlike texture combined with single crystallinity. A series of isomorphous crystals found in a rare space group have been formed with varied morphologies. These crystals contain chiral channels and are made from achiral components.

We demonstrate the formation of uniform and oriented metal-organic frameworks using a combination of anion-effects and surface-chemistry. Subtle but significant morphological changes result from the nature of the coordinative counter-anion of the following metal salts: NiX2 with (X = Br-, Cl-, NO3-, and OAc-). Crystals could be obtained in solution or by template surface growth. The latter resulting in truncated crystals that resemble a half-structure of the solution-grown ones. The oriented surface-bound metal-organic frameworks (sMOFs) are obtained via a one-step solvothermal approach, rather than in a layer-by-layer approach. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion, nor by solution versus the surface chemistry. A propeller-type arrangement of the non-chiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. To demonstrate the accessibility and porosity of the macroscopically-oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. These "colored" crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman-symmetry forbidden non-linear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single non-linear optical (NLO) devices.

Naturally occurring single crystals having a multidomain morphology are a counterintuitive phenonomon: the macroscopic appearance is expected to follow the symmetry of the unit cell. Growing such crystals in the lab is a great challenge, especially from organic molecules. We achieve here uniform metallo-organic crystals that exhibit single crystallinity with apparently distinct domains and chirality. The chirality is present at both the molecular and macroscopic levels, although only achiral elements are used. \u201cYo-yo\u201d-like structures having opposite helical handedness evolve from initially formed seemingly achiral cylinders. This non-polyhedral morphology coexists with a continuous coordination network forming homochiral channels. This work sheds light on the enigmatic aspects of fascinating crystallization processes occurring in biological mineralization. Our findings open up opportunities to generate new porous and hierarchical chiral materials.

Herein, we present an approach that integrates molecular logic functions using surface-confined metallo-organic assemblies. These assemblies are electrochromic and mimic the behaviour of logic elements. The logic elements are addressed individually by electrochemical methods, and their outputs are simultaneously read-out optically by UV/Vis absorption spectroscopy. The versatility of our setup is demonstrated by the integration of two multi-component assemblies; each acting as ternary logic elements. We used also a laminated cell configuration to demonstrate color-to-color and color-to-transparent transitions. This concept offers a route for the future development of devices with multiple logic states.

Electrochromic films undergo optical changes in response to a redox stimulus. This intriguing phenomenon can be used for a wide range of applications, including smart windows, sensors, color displays, and memory elements. Despite the rapid progress of late, designing suitable electrochromic materials that offer low-cost production, appealing colors, and pronounced optical contrast with high efficiency, as well as long-term stability remains an engineering challenge. Solid-state metal oxides, liquid crystals, and organic polymers have been for many years the leading candidates, successfully making their way into commercial products. An alternative class of materials relies on metal complexes that can be processed from solution, offer a variety of colors, and have metal-centered stable and reversible redox chemistry. These metallo-organic materials possess a full range of electrochromic properties, including ultrahigh coloration efficiencies, and cyclic stability. Here, some of the recent scientific developments in this field are highlighted.

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.

In this study we have used a tetrahedral oligopyridine and four different fluoroiodides to obtain three-dimensional (3D) nanoporous halogen-bonded cocrystals. Many of the halogen-bonded cocrystals reported to date are one-dimensional chains or two-dimensional sheet-like structures; these new cocrystals possess multiple channels of 300-800 Å
³ volume per unit cell. The extended 3D channels can be designed by varying the molecular structure of the halogen bond donor and were found to occupy 20-40% of the unit cell volume. The N···I distances in our cocrystals are ∼80% of the sum of the van der Waals radii of the nitrogen and iodine atoms, and the N···I-C angles are nearly linear. Noncovalent stacking (π-π) interactions as well as H-bonding to solvents were also observed in some of the cocrystals. The supramolecular structures obtained in this study are effectively derived out of different donor-acceptor XB interactions, solvent and other noncovalent interactions. The weak nature of halogen bonds as well as the existence of multiple interactions make these cocrystal structures and their supramolecular organization difficult to predict. Even though this work does not attempt to single out the individual contributions of different factors affecting the supramolecular assemblies, we show here how the structure and hence the potential porosity of the halogen-bonded organic frameworks can be varied via careful design and combination of structurally different donors and acceptors.

Redox reactions play key roles in fundamental biological processes. The related spatial organization of donors and acceptors is assumed to undergo evolutionary optimization facilitating charge mobilization within the relevant biological context. Experimental information from submolecular functional sites is needed to understand the organization strategies and driving forces involved in the self-development of structure-function relationships. Here we exploit chemically resolved electrical measurements (CREM) to probe the atom-specific electrostatic potentials (ESPs) in artificial arrays of bacteriochlorophyll (BChl) derivatives that provide model systems for photoexcited (hot) electron donation and withdrawal. On the basis of computations we show that native BChl's in the photosynthetic reaction center (RC) self-assemble at their ground-state as aligned gates for functional charge transfer. The combined computational and experimental results further reveal how site-specific polarizability perpendicular to the molecular plane enhances the hot-electron transport. Maximal transport efficiency is predicted for a specific, ∼5 Å, distance above the center of the metalized BChl, which is in remarkably close agreement with the distance and mutual orientation of corresponding native cofactors. These findings provide new metrics and guidelines for analysis of biological redox centers and for designing charge mobilizing machines such as artificial photosynthesis.

We report here how the crystallinity of AuNPs and the choice of binding sites of molecular cross-linkers control their aggregation. The combination of different binding moieties (N-oxides, Ar_F-I) and the reactivity of the particles' facets allow control over the organization and crystallinity of the AuNP assemblies.

We demonstrate a process that results in the formation of palladium nanoparticles during the assembly of molecular thin films. These nanoparticles are embedded in the films and are generated by a chemical reaction of the counter anions of the molecular components with the metal salt that is used for cross-linking these components.

We demonstrate how the distance over which electron transfer occurs through organic materials can be controlled and extended. Coating of conductive surfaces with nanoscale layers of redox-active metal complexes allows the electrochemical addressing of distant layers that are otherwise electrochemically silent. Our materials can pass electrons selectively in directions that are determined by positioning of layers of metal complexes and the distances between them. These electron-transfer processes can be made dominantly uni- or bidirectional. The design involves 1) a set of isostructural metal complexes with different electron affinities, 2) a scalable metal-organic spacer, and 3) a versatile assembly approach that allows systematic variation of composition, structure, and electron-transfer properties. We control the electrochemical communication between interfaces by the deposition sequence and the spacer length, therefore we are able to program the bulk properties of the assemblies.

We demonstrate high-performance electrochromic assemblies that exhibit a practical combination of low-voltage operation and efficient electrochromic switching as well as long-term thermal and redox stability (1.12 × 10⁵ cycles). Our molecular assemblies can be integrated into a solid-state configuration. Furthermore, we also show how the molecular structure of the chromophores correlates with the materials' growth and function. The coloration efficiencies of our assemblies are higher than those of inorganic materials and many conducting polymers, in addition to offering an alternative fabrication approach.

Enabling and understanding new methodologies to fabricate molecular assemblies driven by intermolecular interactions is fundamental in chemistry. Such forces can be used to control crystal growth and enable surface-confinement of these materials, which remains challenging. Here we demonstrate for the first time, a solvent-free on-surface crystal-to-co-crystal conversion process driven by halogen bonding (XB). By exposing a polycrystalline organic material, consisting of a XB-acceptor moiety, to the vapors of a complementary XB-donor compound, the corresponding halogen-bonded co-crystals were formed. Furthermore, we demonstrate that this approach can also be utilized for non-crystalline materials to afford surface-confined organic composites. Our stepwise vapor-based approach offers a new strategy for the formation of hybrid supramolecular materials.

The controlled deposition of metal complexes from solution on inorganic surfaces offers access to functional materials that otherwise would be elusive. For such surface-confined interfaces to form, specific assembly sequences are often used. We show here that varying the assembly sequence of two well-defined and iso-structural osmium and ruthenium polypyridyl complexes results in interfaces with strikingly different spectroelectrochemical properties. Successive deposition of redox-active layers of osmium and ruthenium polypyridyl complexes, leads to self-propagating molecular assemblies (SPMAs) with distinct internal interfaces and individually addressable components. In contrast, the clear separation of these interfaces upon sequential deposition of these two complexes, results in charge trapping or electrochemical communication between the metal centers, as a function of layer thickness and applied assembly sequence. The SPMAs were characterized using a variety of techniques, including: UV-vis spectroscopy, spectroscopic ellipsometry, electrochemistry, synchrotron X-ray reflectivity, angle-resolved X-ray photoelectron spectroscopy, and spectroelectrochemistry. The combined data demonstrate that the sequence-dependent assembly is a decisive factor that influences and provides the material properties that are difficult to obtain otherwise.

We present a mechanistic study demonstrating selective Ar_f-Cl bond activation preceded by η² coordination of Pd(PEt ₃)₂ to a C=C moiety of a partially fluorinated substrate. An intramolecular ring-walking process to activate the Ar_f-Cl bond is plausible, but an intermolecular reaction becomes dominant in the presence of PEt₃. The latter pathway is significantly enhanced, since PEt ₃ promotes dissociation of Pd(PEt₃)₃ from the C=C moiety followed by activation of the Ar_f-Cl bond. Our observations also show that PEt₃ can be used to control reaction selectivity. The experimental observations are supported by density functional theory (DFT) calculations (at the SMD(toluene)-DSD-PBEP86/cc-pV(D+d)Z-PP//DF- PBE+d_v2/SDD(d) level of theory).

Halogen bonding between complementary organic monolayers was directly observed in an organic environment using force spectroscopy. This non-covalent interaction is significantly affected by the nature of the organic media. We also demonstrated the effect of lateral packing interactions on the optical properties of the monolayers.x

Plug and play: The mimicking of integrated circuits by using two individual monolayers (molecular chips) is shown. These monolayers can be individually addressed using identical inputs. Upon combination of their optical outputs, the input/output characteristics of a molecular encoder is obtained. Since the encoder functionality is only displayed when both chips are active, the device behaves according to a plug-and-play principle.

We report here that the undesired hydrodehalogenation in cross-coupling reactions with fluorinated substrates involves water as a possible hydrogen source. Moreover, the product distribution (hydrodehalogenation vs carbon-carbon coupling) can be controlled by varying the phosphine substituents. Significant hydrodehalogenation occurs prior to the formation of Ar _F-Pd(II)-Br complexes. DFT calculations were used to evaluate a direct hydrodehalogenation route with a phosphine and water. These findings provide new mechanistic insight into aryl-Br bond activation with fluorinated substrates and selective arene functionalization.

Since it was first suggested that molecules could be used for information processing, there has been a significant effort to generate systems that behave according to various logic schemes. Here we will summarize and discuss various approaches that serve one common goal: constructing a (bio)-molecular flip-flop. This logic circuit is at the core of random access memory (RAM) and is one of the pillars of sequential logic. We will highlight the concept underlying various approaches towards flip-flops and we will discuss how these constitute and expand the field of molecular logic. This multi-disciplinary approach results in various schemes that range from all photonic systems to transition metal complexes and hybrid nanoparticle/protein-based systems on solid supports.

Stimuli responsive materials are capable of mimicking the operation characteristics of logic gates such as AND, OR, NOR, and even flip-flops. Since the development of molecular sensors and the introduction of the first AND gate in solution by de Silva in 1993, Molecular (Boolean) Logic and Computing (MBLC) has become increasingly popular. In this Account, we present recent research activities that focus on MBLC with electrochromic polymers and metal polypyridyl complexes on a solid support. Metal polypyridyl complexes act as useful sensors to a variety of analytes in solution (i.e., H(2)O, Fe(2+/3+), Cr(6+), NO(+)) and in the gas phase (NO in air). This information transfer, whether the analyte is present, is based on the reversible redox chemistry of the metal complexes, which are stable up to 200 degrees C in air. The concurrent changes in the optical properties are nondestructive and fast. In such a setup, the input is directly related to the output and, therefore, can be represented by one-input logic gates. These input-output relationships are extendable for mimicking the diverse functions of essential molecular logic gates and circuits within a set of Boolean algebraic operations. Such a molecular approach towards Boolean logic has yielded a series of proof-of-concept devices: logic gates, multiplexers, half-adders, and flip-flop logic circuits. MBLC is a versatile and, potentially, a parallel approach to silicon circuits: assemblies of these molecular gates can perform a wide variety of logic tasks through reconfiguration of their inputs. Although these developments do not require a semiconductor blueprint, similar guidelines such as signal propagation, gate-to-gate communication, propagation delay, and combinatorial and sequential logic will play a critical role in allowing this field to mature. For instance, gate-to-gate communication by chemical wiring of the gates with metal ions as electron carriers results in the integration of stand-alone systems: the output of one gate is used as the input for another gate. Using the same setup, we were able to display both combinatorial and sequential logic.We have demonstrated MBLC by coupling electrochemical inputs with optical readout, which resulted in various logic architectures built on a redox-active, functionalized surface. Electrochemically operated sequential logic systems such as flip-flops, multivalued logic, and multistate memory could enhance computational power without increasing spatial requirements. Applying multivalued digits in data storage could exponentially increase memory capacity. Furthermore, we evaluate the pros and cons of MBLC and identify targets for future research in this Account.

Ball or tube: Flexible and amorphous nanotubes were generated with a palladium salt and a multidentate ligand having a tetrahedral structure (right). In contrast, regardless of the number of metal coordination sites, ligands with a two-dimensional geometry lead to the formation of spheres and their aggregates (left).

The electrochemical properties of a metallosupramolecular network that undergoes reversible redox chemistry on indium-tin oxide (ITO)-coated glass substrates have been investigated. The redox-active osmium complexes are electrochemically accessible even for films with a thickness > 15 nm. The electrochemical data correlates well with our previously observed self-propagating growth process, for which the electron density for the assemblies remains constant during film growth. Electron-transfer rate constants obtained by potential step chronoamperometry experiments suggest an exceptionally low attenuation factor, β, of 0.013 ± 0.001 Å^-1. However, the intrinsically porous nature of the assembly could be to a large extent or even entirely responsible for such a low value.

Here we report the first use of self-propagating molecule-based assemblies (SPMAs) as efficient electron-transporting layers for inverted organic photovoltaic (OPV) cells. P3HT-PCBM cells functionalized with optimized SPMAs exhibit power conversion efficiencies approaching 3.6% (open circuit voltage = 0.6 V) vs 1.5% and 2.4% for the bare ITO and Cs₂CO₃-coated devices, respectively. The dependence of cell response parameters on interlayer thickness is investigated, providing insight into how to further optimize device performance.

(Figure Presented) Designer materials: HOMO-LUMO engineering of coordination-based oligomers covalently bound to silicon or glass has been achieved by the use of a partially fluorinated chromophore (see graphic). The experimental and computationally derived physical chemical properties of these assemblies are compared to their non-fluorinated analogues.

"Figure Presented" Stolen identity: The molecular geometries of a series of cross-linkers that bear between one and four pyridyl moieties are expressed in the optical properties of AuNP assemblies (see picture). TEM analysis indicates that the molecular-level structural differences of the cross-linkers are also transferred at the submicrometer level in the formation of the AuNP assemblies.

Signal amplification has been demonstrated with surface-bound electrochromic complexes that can exist in one of two oxidation states (M ^2+/3+). Reaction of FeCl₃ with covalently immobilized Os²⁺ complexes on glass substrates converts the metal centers from one oxidation state to the other. The formed Fe²⁺ reduces a series of Ru³+-based monolayers. The absorption of light is coupled with the oxidation state of the complexes and provides the output for the monolayer-based device. The gain of the setup can be controlled by the addition of a Fe ²⁺ chelating ligand.

Assemblies with molecular-level organization based on organic chromophores and a bimetallic palladium complex are presented. A layer-by-layer strategy is employed by alternately coordinating vinylpyridine-terminated chromophores to the metal centers to form cationic oligomers. These new structures are formed from solution on quartz and silicon substrates functionalized with a covalently bound template layer. Twelve consecutive deposition steps result in structurally regular assemblies as demonstrated by linear increases in the ellipsometrically determined thickness and UV-vis optical absorption. The increase in thickness for each additional layer shows that the long-range order of the system is determined by the structure of the chromophores and by the square-planar geometry of the metal centers. Furthermore, the optical properties indicate that the conjugation length of the assembly component does not increase in the surface-bound oligomers with each additional deposition cycle. Structural communication is transferred via the system components, but they remain electronically isolated. This is supported by density functional theory (DFT) calculations.

A metallo-supramolecular network undergoes reversible redox chemistry on indium-tin oxide (ITO) coated glass substrates with concurrent color change. The switching time, long-term stability, and coloration efficiency are competitive with polymeric materials such as the industrially important PEDOT.

(Chemical Equation Presented) A layer of logic: A series of Boolean operations has been demonstrated with redox-active monolayers of osmium and ruthenium complexes on glass substrates. High stability, selective reactivity, and reversible redox-chemistry, coupled with significant optical changes, make these monolayers versatile logic gates. The electron-transfer-based systems are suitable as standalone systems or as operational parts in logic circuits.

Accelerated growth of a molecular-based material that is an active participant in its continuing self-propagated assembly has been demonstrated. This nonlinear growth process involves diffusion of palladium into a network consisting of metal-based chromophores linked via palladium.

The fluxional behavior of two analogous platinum complexes has been studied in solution by NMR spectroscopy to elucidate the reaction mechanism and to determine the activation parameters. This includes variable temperature NMR spectroscopy, 2D ¹H-¹H exchange spectroscopy, and spin saturation transfer measurements. A platinum moiety, Pt(PEt₃) ₂, translocates between two carbon-carbon double bonds of two vinylpyridine moieties bridged by an arene (i.e., phenyl, anthracene) at elevated temperatures. Magnetization transfer NMR experiments in the presence of free ligands unambiguously revealed an intramolecular pathway for the "phenyl" system. An intermolecular pathway is proposed for the "anthracene" complex.

Two cocrystals (4, 5) have been obtained with 1,3,5-tris[4-pyridyl(ethenyl) ]benzene (1), sym-triiodo-trifluorobenzene (2), and diiodo-tetrafluorobenzene (3), respectively. Cocrystal 4 contains both compounds 1 and 2 in a molecular ratio of 1:2, whereas cocrystal 5 contains both compounds 1 and 3 in a molecular ratio of 1:0.5 and the solvent used for crystallization, namely, chloroform. Both cocrystals (4, 5) contain N⋯I halogen bonds with distances of ∼80% of the sum of the van der Waals radii. Compound 1 forms four halogen bonding interactions in cocrystal 4, resulting in an infinite halogen-bonded network with two types of N⋯I halogen bonds, whereas in cocrystal 5, compound 1 forms only one type of halogen bonding interaction with compound 3.

Optical detection of parts-per-million (ppm) levels of CO by a structurally well-defined monolayer consisting of bimetallic rhodium complexes on glass substrates has been demonstrated.

The study of molecular electric properties is an intriguing, rapidly developing field in which technological and basic scientific challenges and developments are evolving. Nevertheless, understanding of the interplay of intermolecular interactions, substrate effects, and electrode contacts remains challenging. Here, we present noncontact chemically resolved electrical measurements (CREM) of halide-terminated molecular layers and a straightforward model for quantitative analysis of submolecular chemical site capacitance. We demonstrate that under low current densities, the main electronic effects can be accounted for by considering the (sub)molecular properties of the monolayers, whereas the excess potential due to charge injection can be described as site capacitance corresponding to chemically identifiable molecular sites.

Exposure of an osmium(II)-based monolayer on glass to organic solvents containing 0.36-116 ppm of NOBF₄ results in one-electron transfer from the covalently immobilized complexes to the inorganic analyte with concurrent optical changes. The NO⁺ induced oxidation of the monolayer can be detected optically with an of-the-self-UV/vis spectrophotometer (260-800 nm). The redox-based NO⁺ detection and quantification system can be reset with water within ∼20 s for at least 40 times. The reaction of the monolayer with a THF solution containing 5 ppm of NOBF ₄ follows pseudo first-order kinetics in the monolayer with ΔG_298K = 21.5 ±0.7 kcal/mol, ΔH = 9.5 ± 0.3 kcal/mol, and ΔS = -40.6 ±1.1 eu. The monolayer structure and properties have been resolved by electrochemical measurements and synchrotron X-ray reflectivity measurements in combination with density functional theory calculations (B3LYP/SDD level of theory).

Optical detection of parts-per-million (ppm) levels of NO₂ (1-10 ppm) and NO_x (800-2550 ppm) by a structurally well-defined monolayer consisting of osmium polypyridyl complexes on glass substrates has been demonstrated.

The optical O₂ recognition capability of a covalently assembled monolayer (CAM) of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin on silica-based substrates was studied at room temperature by both UV-vis and photoluminescence (PL) measurements. The optical properties of this robust monolayer setup appear to be highly sensitive to the O₂ concentration in N₂. Both UV-vis and PL measurements were used to study the porphyrin-oxygen interactions. The monolayer-based sensor exhibits a short response time and can be restored within seconds. The oxygen-induced luminescence quenching of the monolayer involves both ground and excited states. The proposed mechanism responsible for the luminescence quenching involves different kinds of interactions between the monolayer and O₂.

Reaction of a zerovalent platinum complex with a partially fluorinated stilbazole ligand results in direct Ar_f-Br oxidation addition. The exclusive formation of the Ar_f-Pt^II-Br complex does not proceed via η²-C=C coordination on the reaction coordinate or as a side-equilibrium prior to the observed C-Br bond activation. Ar_f-Br activation by Pt⁰ is the kinetically and most probably also the thermodynamically favorable process independent of the reaction temperature and solvent polarity. This is in stark contrast with the reactivity of isostructural nonfluorinated stilbazole systems, where there is a lower barrier for η²-C=C coordination than for Ar-Br oxidative addition with Pt⁰.

A study that illustrates the straightforward chemical modulation of the optical absorbance of a thermally monolayer in air by exploitation of redox properties of metal complex, is presented. The monolayers have been demonstrated as thermally robust, and 25 Os²⁺/Os³⁺ redox cycles. the solution-surface electron transfer process induced changes in the molecular properties of the monolayer have been monitored by UV/vis transmission spectroscopy. This siloxane-basaed monolayers and surface chemistry could open up new opportunities in interfacial engineering and the formation of monolayer-based memory devices.

The new dibranched, heterocyclic "push-pull" chromophores bis{1-(pyridin-4-yl)-2-[2-(N-methyl-pyrrol-5-yl)]ethane}methane (1), 1-(pyrid-4-yl)-2-(N-methyl-5-formylpyrrol-2-yl)ethylene (2), {1-(N-methylpyridinium-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane} {(1-(pyridin-4-yl)-2-[2-(N-methylpyrrol-5-yl)]ethane}-methane (3), N-methyl-2-[1-(N-methylpyrid-4-yl)ethen-2-yl]-5-[pyrid-4-yl]ethen-2-yl]pyrrole iodide (4), bis-{1-(N-methyl-4-pyridinio)-2-[2-(N-methylpyrrol-5-yl)]ethane} methane iodide (5), and N-methyl-2,5-[1-(N-methylpyrid-4-yl)ethen-2-yl]pyrrole iodide (6) have been synthesized and characterized. The neutral (1 and 2) and monomethyl salts (3 and 4) undergo chemisorptive reaction with iodobenzyl-functionalized surfaces to afford chromophore monolayers SA-1/SA-2 and SA-3/SA-4, respectively. Molecular structures and other physicochemical properties have been defined by ¹H NMR, optical spectroscopy, and XRD. Thin-film characterization by a variety of techniques (optical spectroscopy, specular X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and angle-dependent polarized second harmonic generation) underscore the importance of the chromophore molecular architecture as well as film growth method on film microstructure and optical/electrooptic response.

The title compound, a new fluorinated stilbazole-based chromophore combines hydrocarbon (HC) donor and perfluorocarbon (PFC) acceptor sites in one rigid-rod-type conjugated molecule. The bifunctional stilbazole derivative combines several possibilities of weak, medium, and strong intermolecular interactions leading to an attractive crystal packing. The chromophore shows strong pi-pi stacking behavior in solution, while its crystal structure consists of an infinite unimolecular network involving both interchain pi-pi stacking and (NBr)-Br-... halogen bonding. The latter Lewis acid-base interactions do not play a dominant role in controlling the solution optical properties. In the solid state, the molecules are aligned in one-dimensional infinite head-to-tail chains as a result of attractive halogen bonding. Adjacent chains are oriented in opposite directions.

Trimethoxy-[11-(2-nitrobenzyloxy)undecyl]silane (1) and trimethoxy-[17-(2-nitrobenzyloxy)heptadecyl]silane (2) have been used for the covalent assembly of siloxane-based photopatternable monolayers. Exposing the monolayers to UV light (312 ± 10 nm) results in the generation of reactive hydroxyl-terminated monolayers without affecting the film quality. The new monolayers, deprotection chemistry, and the effect of photoinduced headgroup lift-off on the monolayer microstructure have been studied in detail by a full complement of physicochemical techniques, including optical (UV-vis) spectroscopy, ellipsometry, aqueous contact angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), synchrotron X-ray reflectivity (XRR), and atomic force microscopy (AFM and AFM-force spectroscopy). AFM-force spectroscopy was used to analyze hydrogen-bond interactions as a function of the nature of the solid-liquid interface. AFM-force spectroscopy indicates a hydrogen-bond energy for photodeprotected monolayers of 8.2 kJ mol ^-1 (∼2 kcal mol ^-1). Scanning electron microscopy (SEM) revealed that treatment of photopatterned monolayers with ZnEt ₂ solutions resulted in well-defined ∼2 μm × 2 μm features of 10 Å thick ZnO layers.

(Chemical Equation Presented) Don't forget to write! Electrochemical charge storage in a ruthenium-based monolayer on a hydrophilic substrate (for example, indium tin oxide coated glass) produces redox switching of the optical properties of the system (see picture). (Graph Presented) This read/write process can be carried out at low voltage in air and monitored by UV/Vis spectrophotometry. This makes the monolayer system a suitable candidate for nonvolatile memory devices.

Do-it-yourself oxidation: The Rh^III complex 1 undergoes a self-oxidative coupling process, in which the phenolate oxygen atom serves as the oxidant, to give 2 and the hydride complex 3 (2:3 = 1:3). This reaction involves cleavage of a strong aryl-oxygen bond. X-ray analysis of 2 reveals that the two quinonoid C=O bonds are n²-coordinated to the metal centers.

Reaction of Nil(2) with the PCP-ligand {1-Et-2,6-((CH2PPr2)-Pr-/)(2)-C6H3} (1) results in selective activation of the strong sp(2)-sp(3) arylethyl bond to afford the aryl-nickel complex [Ni{2,6-((CH2PPr2)-Pr-i)(2)-C6H3}I] (2), whereas reaction of Nil(2) with {1,3,5-(CH3)(3)-2,6((CH2PPr2)-Pr-i)(2)-C6H} (4) leads to the formation of the benzylic complex [Ni{1-CH2-2,6-((CH2PPr2)-Pr-i)(2)-3,5-(CH3)(2)-C6H}I] (5) by selective C-H bond activation. Thermolysis of 5 results in formation of [Ni{2,6-((CH2PPr2)-Pr-i)(2)-3,5-(CH3)(2)-C6H}I] (6) by activation of the sp(2)-sp(3) C-C bond. The identity of the new 16-electron complexes 2 and 6 was confirmed by reaction of NiI2 with {1,3-((CH2PPr2)-Pr-i)(2)-C6H4 (3) and 11,3-(CH3)(2)-4,6-((CH2PPr2)-Pr-i)(2)-C6H2) (7), respectively, lacking the aryl-alkyl groups between the "phosphines arms" (alkyl = ethyl, methyl). Complexes 2 and 5 have been fully characterized by X-ray analysis. Nickel-based activation of an unstrained C-O single bond was observed as well. Reaction of the aryl-methoxy bisphosphine {1-OMe-2,6(CH2-(PPr2)-Pr-i)-C6H3 (8) with NiI2 results in the formation of the phenoxy complex [Ni{1-O-2,6-((CH2PPr2)-Pr-i)(2)-C6H3}I] (9) by selective sp(3)sp(3) C-O bond activation.

Halide abstraction from the 18 electron Ru(II) complex RuCl(CO)(2)[2,6-(CH2P'Bu-2)(2)C6H3] (2) with AgPF6 results in the exclusive formation of the cationic complex {Ru(CO)(2)[2,6-(CH2P'Bu)(2)C6H3]}+PF6-(3). The molecular structures of 2 and 3 were determined by complete single-crystal diffraction studies. X-ray crystallographic analysis of 3 reveals that the "open" coordination site is occupied by an agostic interaction between the metal center and an sp(3) C-H bond of a tert-butyl substituent. DFT gas phase calculations (B97-1/SDD) show the necessity of two sterically demanding tert-butyl substituents on one P donor atom for the agostic interaction to occur. The reaction of 3 with H-2 results in the quantitative conversion to {Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H4]}+PF6- (4) where the aromatic C-ipso-H bond is eta(2)-coordinated to the metal center. Treatment of the agostic complex 4 with Et3N results in the formation of the neutral complex Ru(H)(CO)(2)[2,6-((CH2PBu2)-Bu-t)(2)C6H3] (5). The mechanistic details of 3 + H-2 --> 4 were investigated by DFT calculations at the B97-1/SDB-cc-pVDZ//B97-1/SDD level of theory.

One common reaction that metallabenzene complexes undergo is the formation of cyclopentadienyl (Cp) complexes. Density functional theory (DFT) was used here to investigate the reaction mechanism. It was found that the reaction can proceed via a carbene migratory insertion class of C−C coupling. Cp complexes are found to be thermodynamically favored, except for the case of (C5H5Ir)(PH3)2Cl2 (1j) where the metallabenzene was favored. Isolation a rhodiabenzene of the type (C5H5Rh)(PH3)2Cl2 (1m) and a palladiabenzene, such as (C5H5Pd)Cp (1p), may be possible.

Bernard Shaw's pioneering research1-15 on cyclometalated phosphine-based pincer complexes has inspired many others.16-208 Although the first metal complexes with ligands 1 and 2 were reported about 30 years ago, it is still a hot and emerging topic covering all aspects of modern organometallic chemistry and touching other fields as well.

Nanometer-scale thick liquid films of poly(methylhydro-dimethyl)siloxane copolymer (PMDMS) deposited on hydrophilic and hydrophobic solid organic films have been studied using synchrotron X-ray specular reflectivity (XRR). The physico-chemical properties of liquid PMDMS at the interfacial level are controlled by the nature of the solid surface. Detailed analysis of the XRR-data revealed the formation of a low-density region in the liquid PMDMS film in the vicinity of the hydrophobic surface, whereas a densely packed molecular layer is formed at the liquid PMDMS-hydrophilic substrate interface. Non-covalent polymer chains are 'frozen' at the solid-liquid interfaces in the confined liquid films and interactions with the substrate surfaces (i.e. hydrogen bonding) are responsible for distinctly different density profiles.

Moieties that can be cross-linked appear to be the necessary feature of dendrimers that can be used to make poled polymers displaying high chromophore density, excellent electrooptic properties, and good thermal and temporal stability. Such materials could find use in optical telecommunications and public networking.

The refractive indices of self-assembled organic electro-optic superlattices can be tuned by intercalating high-Z optically transparent group 13 metal oxide sheets into the structures during the self-assembly process. Microstructurally regular acentricity and sizable electro-optic responses are retained in this straightforward synthetic procedure. This "one-pot" all wet-chemistry approach involves: i) layer-by-layer covalent self-assembly of intrinsically acentric multilayers of high-hyperpolarizability chromophores on inorganic oxide substrates, ii) protecting group cleavage to generate a large density of reactive surface hydroxyl sites, iii) self-limiting capping of each chromophore layer with octachlorotrisiloxane, iv) deposition of metal oxide sheets derived from THF solutions of Ga(OⁱC₃H₇)₃ or In(OⁱC₃H₇)₃, v) covalent capping of the resulting superlattices.

Traveling-wave electro-optic modulators based on chromophoric self-assembled superlattices (SASs) possessing intrinsically polar microstructures have been designed and fabricated. Although the thickness of the SAS layer is only ∼ 150 nm, a π-phase shift is clearly observed. From the measured V_π value, the effective electro-optic coefficient of the SAS film is determined to be ∼ 21.8 pm/V at an input wavelength of 1064 nm.

Oxidative addition of one of the strongest C-C bonds, Aryl-CF₃, to Rh(I) takes place upon treating 1-CF₃-2,6-(CH₂P(t)Bu₂)₂-C₆H₃ (1, PCP) with [RhClL₂]₂ (L = C₂H₄ or C₈H₁₄) in dioxane or toluene at elevated temperatures leading to quantitative formation of Rh(CF₃)(2,6- (CH₂P(t)Bu₂)₂-C₆H₃)Cl (2-Cl). The iodide analogue 2-I was prepared by reacting Rh(η¹-N₂)(2,6-(CH₂P(t)Bu₂)₂-C₆H₃) (3) with CF₃I at room temperature. ArCF₂-F bond cleavage was not observed in parallel to the C-C bond activation. Treating a dioxane solution of the thermally stable Rh(III)- CF₃ complexes 2-Cl,I with excess trifluoromethanesulfonic acid (HOTf) at room temperature resulted in CF bond cleavage and selective formation of the unique difluoromethylene-arenium complexes [Rh(1-CF₂-2,6-(CH₂P(t)Bu₂)₂- C₆H₃)X][OTf] (4; X = Cl, I) which were characterized spectroscopically by NMR, UV/vis, and FD-MS. No reaction was observed with HCl. Reaction of 2-Cl with BF₃ or Ph₃CBF₄ (trityl cation) also resulted in C-F bond cleavage to give [Rh(1-CF₂-2,6-(CH₂P(t)Bu₂)₂-C₆H₃)Cl]BF₄ (10).

An unusual, highly selective C-Si coupling reaction takes place upon treating the platinum(II) alkyl complex cis-(DIPPIDH)(DIPPID)PtMe (2) (DIPPIDH = α²-(diisopropyl-phosphino)isodurene, 1) with HSiR₃ (R₃ = Et₃, EtMe₂, Me₂Ph), leading to formation of MeSiR₃ and the hydrido complex trans-(DIPPIDH)(DIPPID)PtH (5). The overall process involves activation of a Si-H bond, reverse-cyclometalation of the phosphine ligand DIPPIDH (1), and C-Si elimination. The resulting thermally stable platinum(II) hydride complex 5 was independently prepared by thermolysis of a platinum(II) dihydride, trans-(DIPPIDH)₂PtH₂ (6), which was obtained by reaction of 2 with H₂. In the absence of silane, the cis complex 2 isomerizes thermally to trans-(DIPPIDH)(DIPPID)PtMe (3), which does not react productively with silanes. Our results indicate that opening of the metallacycle of 2 by benzylic C-H formation is kinetically preferred over formation of CH₄ or CH₃SiR₃.

Unprecedented oxidative addition of a strong, unstrained Ar-CF₃ bond to a metal complex in solution yields the new aryl-Rh^III-CF₃ complex 2.

The palladium-catalyzed vinylation of aryl halides, also known as the \u201cHeck reaction\u201d (eq 1) is a very useful synthetic method for the generation of carbon-carbon bonds1 that has found many applications.2 We report here a new type ofpalladium catalyst for this reaction which shows outstanding activities and yields. These catalysts are exceedingly thermally stable and are not sensitive to oxygen. Our evidence suggests that, contrary to the classical mechanism, in this case Pd(0) may not be involved as the active species. The issue of a possible Pd(II)/Pd(IV) cycle in Heck catalysis is currently under debate.3,4 Recently, highly efficient catalysis using cyclopalladated tri-otolylphosphine complexes3,5 and palladium carbene complexes6 has been reportedErrata: Page 11688, Table 1: The time/temp column of entry 1 (first row) should read 60/140.03/19/199

The coordination behavior prior to C-M bond formation of the chelating aromatic PCP substrate DPPMH (3; DPPMH = l,3-bis((diphenylphosphino)methylene)mesitylene) has been studied in order to determine the factors which control the complex formation of such ligands. Reacting 3 with (RCN)₂MCl₂ (R = Me, Ph; M = Pd, Pt) and (COD)PtX₂ (X = Cl, Me; COD = 1,5-cyclooctadiene) resulted in the formation of several 8- and 16-membered mono- and binuclear palladium(II) and platinum(II) macrocycles: trans-[(DPPMH)PdCl₂]₂ (5), trans-[(DPPMH)-PtCl₂]₂ (6), cis-(DPPMH)PtCl₂ (7), cis-(DPPMH)PtMe₂ (8), and cis-[(DPPMH)PtMe₂]₂ (9). Compounds 5-9 were fully characterized using NMR, FAB-MS, FD-MS, elemental analysis, and X-ray crystallography. Thermolysis of the bimetallic trans-[(DPPMH)PtCl₂]₂ (6) results in the formation of the monomeric cis-(DPPMH)PtCl₂ (7). The product formation depends on the neutral- (nitriles or COD) and anionic ligands (Cl and CH₃) of the metal precursor. The molecular structures of trans-[(DPPMH)PdCl₂]₂ (5) and cis-[(DPPMH)PtMe₂]₂ (9) have been determined by complete single-crystal diffraction studies. Crystal data for 5: monoclinic, space group P2₁ /n with a = 14.547(3) Å, b = 17.431(4) Å, c = 27.839 (5) Å, β= 99.56(2)°, V = 6961(3) ų, and Z = 4. The structure converged to R = 0.048 and R_w = 0.049. Crystal data for 9: monoclinic, space group P2₁/n with a = 19.187(4) Å, b = 19.189(4) Å c = 20.705(2) Å, β= 103.41(3)°, V = 7415(3) ų, and Z = 4. The structure refinement converged to R = 0.0977 and R_w = 0.2212.