Ultrafast Au(III)-Mediated Arylation of Cysteine.

Through mechanistic work and rational design, we have developed the fastest organometallic abiotic Cys bioconjugation. As a result, the developed organometallic Au(III) bioconjugation reagents enable selective labeling of Cys moieties down to picomolar concentrations and allow for the rapid construction of complex heterostructures from peptides, proteins, and oligonucleotides. This work showcases how organometallic chemistry can be interfaced with biomolecules and lead to a range of reactivities that are largely unmatched by classical organic chemistry tools.

Carrier density and delocalization signatures in doped carbon nanotubes from quantitative magnetic resonance.

High-performance semiconductor materials and devices are needed to supply the growing energy and computing demand. Organic semiconductors (OSCs) are attractive options for opto-electronic devices, due to their low cost, extensive tunability, easy fabrication, and flexibility. Semiconducting single-walled carbon nanotubes (s-SWCNTs) have been extensively studied due to their high carrier mobility, stability and opto-electronic tunability. Although molecular charge transfer doping affords widely tunable carrier density and conductivity in s-SWCNTs (and OSCs in general), a pervasive challenge for such systems is reliable measurement of charge carrier density and mobility. In this work we demonstrate a direct quantification of charge carrier density, and by extension carrier mobility, in chemically doped s-SWCNTs by a nuclear magnetic resonance approach. The experimental results are verified by a phase-space filling doping model, and we suggest this approach should be broadly applicable for OSCs. Our results show that hole mobility in doped s-SWCNT networks increases with increasing charge carrier density, a finding that is contrary to that expected for mobility limited by ionized impurity scattering. We discuss the implications of this important finding for additional tunability and applicability of s-SWCNT and OSC devices.

Efficient end-group functionalization and diblock copolymer synthesis via Au(III) polymer reagents.

Herein, we describe the synthesis of bench-stable organometallic Au(III) terminated polymer reagents. These reagents mediate the chemoselective S-arylation of thiol-containing small molecules and polymers to yield functionalized mono-telechelic polymers and diblock copolymers, respectively. These transformations proceed rapidly within minutes and produce conjugates in quantitative conversion, making this strategy a robust addition to the polymer functionalization toolbox.

Organometallic S-arylation Reagents for Rapid PEGylation of Biomolecules.

Bioconjugation techniques for biomolecule-polymer conjugation are numerous; however, slow kinetics and steric challenges generally necessitate excess reagents or long reaction times. Organometallic transformations are known to circumvent these issues; yet, harsh reaction conditions, incompatibility in aqueous media, and substrate promiscuity often limit their use in a biological context. The work reported herein demonstrates a facile and benign organometallic Au(III) S-arylation approach that enables the synthesis of poly(ethylene glycol) monomethyl ether (mPEG)-protein conjugates with high efficiency. Isolable and bench-stable 2, 5, and 10 kDa mPEG-Au(III) reagents were synthesized via oxidative addition into terminal aryl iodide substituents installed on mPEG substrates with a (Me-DalPhos)Au(I)Cl precursor. Reaction of the isolable mPEG-Au(III) oxidative addition complexes with a cysteine thiol on a biomolecule resulted in facile and selective cysteine arylation chemistry, forging covalent S-aryl linkages and affording the mPEG-biomolecule conjugates. Notably, low polymer reagent loadings were used to achieve near quantitative conversion at room temperature in 1 min due to the rapid kinetics and high chemoselectivity of this Au-based bioconjugation approach. Therefore, this work represents an important addition to the protein-polymer conjugation chemical toolbox.

An Organometallic Gold(III) Reagent for 18F Labeling of Unprotected Peptides and Sugars in Aqueous Media.

The 18F labeling of unprotected peptides and sugars with a Au(III)-[18F]fluoroaryl complex is reported. The chemoselective method generates 18F-labeled S-aryl bioconjugates in an aqueous environment in 15 min with high radiochemical yields and displays excellent functional group tolerance. This approach utilizes an air and moisture stable, robust organometallic Au(III) complex and highlights the versatility of designer organometallic reagents as efficient agents for rapid radiolabeling.

Superatomic solid solutions.

In atomic solids, substitutional doping of atoms into the lattice of a material to form solid solutions is one of the most powerful approaches to modulating its properties and has led to the discovery of various metal alloys and semiconductors. Herein we have prepared solid solutions in hierarchical solids that are built from atomically precise clusters. Two geometrically similar metal chalcogenide clusters, Co6Se8(PEt3)6 and Cr6Te8(PEt3)6, were combined as random substitutional mixture, in three different ratios, in a crystal lattice together with fullerenes. This does not alter the underlying crystalline structure of the [cluster][C60]2 material, but it influences its electronic and magnetic properties. All three solid solutions showed increased electrical conductivities compared with either the Co- or Cr-based parent material, substantially so for two of the Co:Cr ratios (up to 100-fold), and lowered activation barriers for electron transport. We attribute this to the existence of additional energy states arising from the materials' structural heterogeneity, which effectively narrow transport gaps.

Cyclopropenylidenes as Strong Carbene Anchoring Groups on Au Surfaces.

The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC-CO2 as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.

Gold-Carbon Contacts from Oxidative Addition of Aryl Iodides.

Aryl halides are ubiquitous functional groups in organic chemistry, yet despite their obvious appeal as surface-binding linkers and as precursors for controlled graphene nanoribbon synthesis, they have seldom been used as such in molecular electronics. The confusion regarding the bonding of aryl iodides to Au electrodes is a case in point, with ambiguous reports of both dative Au-I and covalent Au-C contacts. Here we form single-molecule junctions with a series of oligophenylene molecular wires terminated asymmetrically with iodine and thiomethyl to show that the dative Au-I contact has a lower conductance than the covalent Au-C interaction, which we propose occurs via an in situ oxidative addition reaction at the Au surface. Furthermore, we confirm the formation of the Au-C bond by measuring an analogous series of molecules prepared ex situ with the complex AuI(PPh3) in place of the iodide. Density functional theory-based transport calculations support our experimental observations that Au-C linkages have higher conductance than Au-I linkages. Finally, we demonstrate selective promotion of the Au-C bond formation by controlling the bias applied across the junction. In addition to establishing the different binding modes of aryl iodides, our results chart a path to actively controlling oxidative addition on an Au surface using an applied bias.

Nickel Phosphinidene Molecular Clusters from Organocyclophosphine Precursors.

We report a new family of nickel phosphinidene molecular clusters synthesized from the reaction of bis(1,5-cyclooctadiene)nickel(0) ([Ni(cod)2 ]) with organocyclophosphine and trialkylphosphine. We found that [Ni(cod)2 ] cleaves the organocyclophosphine P-P bonds to generate phosphinidene groups, establishing the cyclic molecules as valuable precursors for making charge-neutral molecular clusters passivated by two-electron donor capping ligands. The formation of the cluster core structure is controlled by the bulkiness of the precursor and of the capping ligand. As a demonstration of this new cluster-forming reaction, we describe three clusters with different core nuclearity and degree of ligation: Ni12 (PMe)10 (PEt3 )8 , Ni8 (PMe)6 (PMe3 )8 , and Ni8 (PiPr)6 (PMe3 )6 . In addition, we show that the larger cluster, Ni12 (PMe)10 (PEt3 )8 , can be used as a low temperature single-source molecular precursor to the catalytically active nickel phosphide phase Ni2 P.

Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy.

N-heterocyclic carbenes (NHCs) bind very strongly to transition metals due to their unique electronic structure featuring a divalent carbon atom with a lone pair in a highly directional sp2-hybridized orbital. As such, they can be assembled into monolayers on metal surfaces that have enhanced stability compared to their thiol-based counterparts. The utility of NHCs to form such robust self-assembled monolayers (SAMs) was only recently recognized and many fundamental questions remain. Here we investigate the structure and geometry of a series of NHCs on Au(111) using high-resolution X-ray photoelectron spectroscopy and density functional theory calculations. We find that the N-substituents on the NHC ring strongly affect the molecule-metal interaction and steer the orientation of molecules in the surface layer. In contrast to previous reports, our experimental and theoretical results provide unequivocal evidence that NHCs with N-methyl substituents bind to undercoordinated adatoms to form flat-lying complexes. In these SAMs, the donor-acceptor interaction between the NHC lone pair and the undercoordinated Au adatom is primarily responsible for the strong bonding of the molecules to the surface. NHCs with bulkier N-substituents prevent the formation of such complexes by forcing the molecules into an upright orientation. Our work provides unique insights into the bonding and geometry of NHC monolayers; more generally, it charts a clear path to manipulating the interaction between NHCs and metal surfaces using traditional coordination chemistry synthetic strategies.

In Situ Formation of N-Heterocyclic Carbene-Bound Single-Molecule Junctions.

Self-assembled monolayers (SAMs) formed using N-heterocyclic carbenes (NHCs) have recently emerged as thermally and chemically ultrastable alternatives to those formed from thiols. The rich chemistry and strong σ-donating ability of NHCs offer unique prospects for applications in nanoelectronics, sensing, and electrochemistry. Although stable in SAMs, free carbenes are notoriously reactive, making their electronic characterization challenging. Here we report the first investigation of electron transport across single NHC-bound molecules using the scanning tunneling microscope-based break junction (STM-BJ) technique. We develop a series of air-stable metal NHC complexes that can be electrochemically reduced in situ to form NHC-electrode contacts, enabling reliable single-molecule conductance measurements of NHCs under ambient conditions. Using this approach, we show that the conductance of an NHC depends on the identity of the single metal atom to which it is coordinated in the junction. Our observations are supported by density functional theory (DFT) calculations, which also firmly establish the contributions of the NHC linker to the junction transport characteristics. Our work demonstrates a powerful method to probe electron transfer across NHC-electrode interfaces; more generally, it opens the door to the exploitation of surface-bound NHCs in constructing novel, functionalized electrodes and/or nanoelectronic devices.

Phonon Speed, Not Scattering, Differentiates Thermal Transport in Lead Halide Perovskites.

Thermal management plays a critical role in the design of solid state materials for energy conversion. Lead halide perovskites have emerged as promising candidates for photovoltaic, thermoelectric, and optoelectronic applications, but their thermal properties are still poorly understood. Here, we report on the thermal conductivity, elastic modulus, and sound speed of a series of lead halide perovskites MAPbX3 (X = Cl, Br, I), CsPbBr3, and FAPbBr3 (MA = methylammonium, FA = formamidinium). Using frequency domain thermoreflectance, we find that the room temperature thermal conductivities of single crystal lead halide perovskites range from 0.34 to 0.73 W/m·K and scale with sound speed. These results indicate that regardless of composition, thermal transport arises from acoustic phonons having similar mean free path distributions. A modified Callaway model with Born von Karmen-based acoustic phonon dispersion predicts that at least ∼70% of thermal conductivity results from phonons having mean free paths shorter than 100 nm, regardless of whether resonant scattering is invoked. Hence, nanostructures or crystal grains with dimensions smaller than 100 nm will appreciably reduce thermal transport. These results are important design considerations to optimize future lead halide perovskite-based photovoltaic, optoelectronic, and thermoelectric devices.

Stepwise ligand exchange for the preparation of a family of mesoporous MOFs.

A stepwise ligand exchange strategy is utilized to prepare a series of isoreticular bio-MOF-100 analogues. Specifically, in situ ligand exchange with progressively longer dicarboxylate linkers is performed on single crystalline starting materials to synthesize products with progressively larger mesoporous cavities. The new members of this series of materials, bio-MOFs 101-103, each exhibit permanent mesoporosity and pore sizes ranging from ~2.1-2.9 nm and surface areas ranging from 2704 to 4410 m(2)/g. The pore volume for bio-MOF 101 is 2.83 cc/g. Bio-MOF-102 and 103 have pore volumes of 4.36 and 4.13 cc/g, respectively. Collectively, these data establish this unique family of MOFs as one of the most porous reported to date.