The design of a spin imbalance within the crystallographic
unit
cell of bottom-up engineered 1D graphene nanoribbons (GNRs) gives
rise to nonzero magnetic moments within each cell. Here, we demonstrate
the bottom-up assembly and spectroscopic characterization of a one-dimensional
Kondo spin chain formed by a chevron-type GNR (cGNR) physisorbed on
Au(111). Substitutional nitrogen core doping introduces a pair of
low-lying occupied states per monomer within the semiconducting gap
of cGNRs. Charging resulting from the interaction with the gold substrate
quenches one electronic state for each monomer, leaving behind a 1D
chain of radical cations commensurate with the unit cell of the ribbon.
Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal
the signature of a Kondo resonance emerging from the interaction of S = 1/2 spin centers in each monomer core with itinerant
electrons in the Au substrate. STM tip lift-off experiments locally
reduce the effective screening of the unpaired radical cation being
lifted, revealing a robust exchange coupling between neighboring spin
centers. First-principles DFT-LSDA calculations support the presence
of magnetic moments in the core of this GNR when it is placed on Au.
Metallic graphene
nanoribbons (GNRs) represent a critical component
in the toolbox of low-dimensional functional materials technology
serving as 1D interconnects capable of both electronic and quantum
information transport. The structural constraints imposed by on-surface
bottom-up GNR synthesis protocols along with the limited control over
orientation and sequence of asymmetric monomer building blocks during
the radical step-growth polymerization have plagued the design and
assembly of metallic GNRs. Here, we report the regioregular synthesis
of GNRs hosting robust metallic states by embedding a symmetric zero-mode
(ZM) superlattice along the backbone of a GNR. Tight-binding electronic
structure models predict a strong nearest-neighbor electron hopping
interaction between adjacent ZM states, resulting in a dispersive
metallic band. First-principles density functional theory-local density
approximation calculations confirm this prediction, and the robust,
metallic ZM band of olympicene GNRs is experimentally corroborated
by scanning tunneling spectroscopy.
We herein report the reactivity and regioselectivity of 2-pyrenyl as a coupling unit in Scholl reactions. On the basis of the Scholl reactions of hexaarylbenzene substrates, we have found that pyrenyl units are preferably oxidized over naphthyl and phenyl units under appropriate Scholl reaction conditions, allowing divergent synthesis through a highly controllable intramolecular coupling sequence. The C1 and C3 positions of 2-pyrenyl unit are found as the favorable sites for intramolecular coupling while C4 is not reactive to allow further coupling. The reactivity and regioselectivity pattern can be explained by the spin density distribution, which shows that carbon-carbon bonds form preferably at sites with higher positive spin density. Guided by these findings, we successfully synthesized a double helicene and a sextuple helicene through the controlled Scholl reactions of 2-pyrenyl units.
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