Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization

Alkyne cross-metathesis of molybdenum carbyne complex [TolCMo­(OCCH₃(CF₃)₂)₃]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (>98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)­pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo­[<i>a</i>,<i>e</i>]­[8]­annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followed by ring-opening polymerization of strained ε-caprolactone

Highly Selective Molybdenum ONO Pincer Complex Initiates the Living Ring-Opening Metathesis Polymerization of Strained Alkynes with Exceptionally Low Polydispersity Indices

The pseudo-octahedral molybdenum benzylidyne complex [TolCMo­(ONO)­(OR)]<b>·</b>KOR (R = CCH₃(CF₃)₂) <b>1</b>, featuring a stabilizing ONO pincer ligand, initiates the controlled living polymerization of strained dibenzocyclooctynes at <i>T</i> > 60 °C to give high molecular weight polymers with exceptionally low polydispersities (PDI ∼ 1.02). Kinetic analyses reveal that the growing polymer chain attached to the propagating catalyst efficiently limits the rate of propagation with respect to the rate of initiation (<i>k</i>_p/<i>k</i>_i ∼ 10^–3). The reversible coordination of KOCCH₃(CF₃)₂ to the propagating catalyst prevents undesired chain-termination and -transfer processes. The ring-opening alkyne metathesis polymerization with <b>1</b> has all the characteristics of a living polymerization and enables, for the first time, the controlled synthesis of amphiphilic block copolymers via ROAMP

Regioselective Carbyne Transfer to Ring-Opening Alkyne Metathesis Initiators Gives Access to Telechelic Polymers

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)­benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-C₆H₄CMo­(OC­(CH₃)­(CF₃)₂)₃] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo­[<i>a</i>,<i>e</i>]­[8]­annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (<i>k</i>_i) over the rate of propagation (<i>k</i>_p) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear <i>poly</i>-(<i>o</i>-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible addition–fragmentation chain-transfer (RAFT) initiator complex. ROAMP followed by RAFT polymerization yields hybrid <i>poly</i>-(<i>o</i>-phenylene ethynylene)-<i>block</i>-<i>poly</i>-(methyl acrylate) block copolymers

Highly Selective Molybdenum ONO Pincer Complex Initiates the Living Ring-Opening Metathesis Polymerization of Strained Alkynes with Exceptionally Low Polydispersity Indices

The pseudo-octahedral molybdenum benzylidyne complex [TolCMo­(ONO)­(OR)]<b>·</b>KOR (R = CCH₃(CF₃)₂) <b>1</b>, featuring a stabilizing ONO pincer ligand, initiates the controlled living polymerization of strained dibenzocyclooctynes at <i>T</i> > 60 °C to give high molecular weight polymers with exceptionally low polydispersities (PDI ∼ 1.02). Kinetic analyses reveal that the growing polymer chain attached to the propagating catalyst efficiently limits the rate of propagation with respect to the rate of initiation (<i>k</i>_p/<i>k</i>_i ∼ 10^–3). The reversible coordination of KOCCH₃(CF₃)₂ to the propagating catalyst prevents undesired chain-termination and -transfer processes. The ring-opening alkyne metathesis polymerization with <b>1</b> has all the characteristics of a living polymerization and enables, for the first time, the controlled synthesis of amphiphilic block copolymers via ROAMP

Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization

Molybdenum carbyne complexes [RCMo­(OC­(CH₃)­(CF₃)₂)₃] featuring a mesityl (R = Mes) or an ethyl (R = Et) substituent initiate the living ring-opening alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo­[<i>a</i>,<i>e</i>]­[8]­annulene, to yield fully conjugated poly­(<i>o</i>-phenylene ethynylene). The difference in the steric demand of the polymer end-group (Mes vs Et) transferred during the initiation step determines the topology of the resulting polymer chain. While [MesCMo­(OC­(CH₃)­(CF₃)₂)₃] exclusively yields linear poly­(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] results in cyclic polymers ranging in size from <i>n</i> = 5 to 20 monomer units. Kinetic studies reveal that the propagating species emerging from [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] undergoes a highly selective intramolecular backbiting into the butynyl end-group

Initiator Control of Conjugated Polymer Topology in Ring-Opening Alkyne Metathesis Polymerization

Molybdenum carbyne complexes [RCMo­(OC­(CH₃)­(CF₃)₂)₃] featuring a mesityl (R = Mes) or an ethyl (R = Et) substituent initiate the living ring-opening alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo­[<i>a</i>,<i>e</i>]­[8]­annulene, to yield fully conjugated poly­(<i>o</i>-phenylene ethynylene). The difference in the steric demand of the polymer end-group (Mes vs Et) transferred during the initiation step determines the topology of the resulting polymer chain. While [MesCMo­(OC­(CH₃)­(CF₃)₂)₃] exclusively yields linear poly­(<i>o</i>-phenylene ethynylene), polymerization initiated by [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] results in cyclic polymers ranging in size from <i>n</i> = 5 to 20 monomer units. Kinetic studies reveal that the propagating species emerging from [EtCMo­(OC­(CH₃)­(CF₃)₂)₃] undergoes a highly selective intramolecular backbiting into the butynyl end-group

Super-Resolution Imaging of Clickable Graphene Nanoribbons Decorated with Fluorescent Dyes

The functional integration of atomically defined graphene nanoribbons (GNRs) into single-ribbon electronic device architectures has been limited by access to nondestructive high-resolution imaging techniques that are both compatible with common supports such as Si or Si/SiO₂ wafers and capable of resolving individual ribbons in dilute samples. Conventional techniques such as scanning probe (AFM, STM) or electron microscopy (SEM, TEM) have been restricted by requisite sample preparation techniques that are incompatible with lithographic device fabrication. Here we report the design and synthesis of ultralong (∼10 μm) cove-type GNRs (cGNRs) featuring azide groups along the edges that can serve as a universal handle for late-stage functionalization with terminal alkynes. Copper-catalyzed click-chemistry with Cy5 fluorescent dyes gives rise to cGNRs decorated along the edges with fluorescent tags detectable by optical microscopy. The structures of individual dye-functionalized cGNRs spin-coated from a dilute solution onto transparent and opaque insulating substrates were resolved using diffraction-limited fluorescence microscopy and super-resolution microscopy (SRM) imaging techniques. Analysis of SRM images reveals an apparent width of cGNRs in the range 40–50 nm and lengths in excess of 10 μm, the longest GNRs imaged to date. Isolated cGNRs can even be distinguished from bundles and larger aggregates as long as the center-to-center distance is greater than the apparent width

Bidentate Phenoxides as Ideal Activating Ligands for Living Ring-Opening Alkyne Metathesis Polymerization

We describe here a well-behaved initiator for ring-opening alkyne metathesis polymerization (ROAMP) of dibenzocyclooctynes. The reaction produces living polymers with low polydispersities and predictable molecular weights. We activate the well-known alkyne metathesis precatalyst, [(N­(<i>t</i>Bu)­Ar)₃MoCCH₂CH₃], with phenolic ligands that have σ-electron donating substituents. We show that the chelating ability of these ligands as well as the nature of the propagating molybdenum center have dramatic effects on the outcome of the polymerization reaction

Synergistic Enhancement of Electrocatalytic CO₂ Reduction with Gold Nanoparticles Embedded in Functional Graphene Nanoribbon Composite Electrodes

Regulating the complex environment accounting for the stability, selectivity, and activity of catalytic metal nanoparticle interfaces represents a challenge to heterogeneous catalyst design. Here we demonstrate the intrinsic performance enhancement of a composite material composed of gold nanoparticles (AuNPs) embedded in a bottom-up synthesized graphene nanoribbon (GNR) matrix for the electrocatalytic reduction of CO₂. Electrochemical studies reveal that the structural and electronic properties of the GNR composite matrix increase the AuNP electrochemically active surface area (ECSA), lower the requisite CO₂ reduction overpotential by hundreds of millivolts (catalytic onset > −0.2 V versus reversible hydrogen electrode (RHE)), increase the Faraday efficiency (>90%), markedly improve stability (catalytic performance sustained over >24 h), and increase the total catalytic output (>100-fold improvement over traditional amorphous carbon AuNP supports). The inherent structural and electronic tunability of bottom-up synthesized GNR-AuNP composites affords an unrivaled degree of control over the catalytic environment, providing a means for such profound effects as shifting the rate-determining step in the electrocatalytic reduction of CO₂ to CO, and thereby altering the electrocatalytic mechanism at the nanoparticle surface

Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps <i>via</i> covalent self-assembly of a new species of molecular precursors that yields <i>n</i> = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these <i>n</i> = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for <i>n</i> = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of <i>n</i> = 13 armchair GNRs that is associated with hydrogen-terminated sp²-hybridized carbon atoms at the zigzag termini