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

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

Site-Specific Substitutional Boron Doping of Semiconducting Armchair Graphene Nanoribbons

A fundamental requirement for the development of advanced electronic device architectures based on graphene nanoribbon (GNR) technology is the ability to modulate the band structure and charge carrier concentration by substituting specific carbon atoms in the hexagonal graphene lattice with p- or n-type dopant heteroatoms. Here we report the atomically precise introduction of group III dopant atoms into bottom-up fabricated semiconducting armchair GNRs (AGNRs). Trigonal-planar B atoms along the backbone of the GNR share an empty p-orbital with the extended π-band for dopant functionality. Scanning tunneling microscopy (STM) topography reveals a characteristic modulation of the local density of states along the backbone of the GNR that is superimposable with the expected position and concentration of dopant B atoms. First-principles calculations support the experimental findings and provide additional insight into the band structure of B-doped 7-AGNRs

Concentration Dependence of Dopant Electronic Structure in Bottom-up Graphene Nanoribbons

Bottom-up fabrication techniques enable atomically precise integration of dopant atoms into the structure of graphene nanoribbons (GNRs). Such dopants exhibit perfect alignment within GNRs and behave differently from bulk semiconductor dopants. The effect of dopant concentration on the electronic structure of GNRs, however, remains unclear despite its importance in future electronics applications. Here we use scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of bottom-up synthesized <i>N</i> = 7 armchair GNRs featuring varying concentrations of boron dopants. First-principles calculations of freestanding GNRs predict that the inclusion of boron atoms into a GNR backbone should induce two sharp dopant states whose energy splitting varies with dopant concentration. Scanning tunneling spectroscopy experiments, however, reveal two broad dopant states with an energy splitting greater than expected. This anomalous behavior results from an unusual hybridization between the dopant states and the Au(111) surface, with the dopant–surface interaction strength dictated by the dopant orbital symmetry