Carbon allotropes built from rings of two-coordinate atoms, known as cyclo[n]carbons, have fascinated chemists for many years, but until now they could not be isolated or structurally characterized, due to their high reactivity. We generated cyclo[18]carbon (C18) using atom manipulation on bilayer NaCl on Cu (111) at 5 Kelvin by eliminating carbon monoxide from a cyclocarbon oxide molecule C24O6. Characterization of cyclo[18]carbon by high-resolution atomic force microscopy revealed a polyynic structure with defined positions of alternating triple and single bonds. The high reactivity of cyclocarbon and cyclocarbon oxides allows covalent coupling between molecules to be induced by atom manipulation, opening an avenue for the synthesis of other carbon allotropes and carbon-rich materials from the coalescence of cyclocarbon molecules.
Cyclo[18]carbon (C
18
, a molecular carbon allotrope)
can be synthesized by dehalogenation of a bromocyclocarbon precursor,
C
18
Br
6
, in 64% yield, by atomic manipulation
on a sodium chloride bilayer on Cu(111) at 5 K, and imaged by high-resolution
atomic force microscopy. This method of generating C
18
gives
a higher yield than that reported previously from the cyclocarbon
oxide C
24
O
6
. The experimental images of C
18
were compared with simulated images for four theoretical
model geometries, including possible bond-angle alternation:
D
18
h
cumulene,
D
9
h
polyyne,
D
9
h
cumulene, and
C
9
h
polyyne. Cumulenic structures, with (
D
9
h
) and without (
D
18
h
) bond-angle alternation, can be excluded. Polyynic
structures, with (
C
9
h
) and without (
D
9
h
)
bond-angle alternation, both show a good agreement with the experiment
and are challenging to differentiate.
Determination
of the molecular structures of petroporphyrins has
been crucial to understand the diagenetic pathways and maturation
of petroleum. However, these studies have been hampered by their structural
complexity and the challenges associated with their isolation. In
comparison to the skeletal macrocyclic structures, much less is known
about the substitutions, which are more sensitive to the maturation
and diagenesis pathways. While these isolated vanadyl petroporphyrins
largely consist of etioporphyrin and deoxophylloerythroetioporphyrin
as expected, surprisingly, we find evidence that one or a few β
hydrogens are present in petroporphyrins of low carbon numbers using
a combination of ultraviolet–visible spectroscopy, Fourier
transform ion cyclotron resonance mass spectrometry, and non-contact
atomic force microscopy. Petroporphyrins with β hydrogens were
not anticipated on the basis of their biological precursors. The data
support dealkylation under catagenesis but not transalkylation or
random alkylation of the β and meso positions, despite the fact
that more complex porphyrin structures are formed.
A route to generate cyclacenes by on‐surface synthesis is explored. We started by synthesizing two tetraepoxycyclacenes by sequences of Diels–Alder cycloadditions. Subsequently, these molecules were deposited onto Cu(111) and scanning‐tunneling‐microscopy(STM)‐based atom manipulation was employed to dissociate the oxygen atoms. Atomic force microscopy (AFM) with CO‐functionalized tips enabled the detailed characterization of the reaction products and revealed that, at most, two oxygens per molecule could be removed. Importantly, our experimental results suggest that the generation of cyclacenes by the described route might be possible for larger epoxycyclacenes.
Soot emitted from
incomplete combustion of hydrocarbon fuels contributes
to global warming and causes human disease. The mechanism by which
soot nanoparticles form within hydrocarbon flames is still an unsolved
problem in combustion science. Mechanisms proposed to date involving
purely chemical growth are limited by slow reaction rates, whereas
mechanisms relying on solely physical interactions between molecules
are limited by weak intermolecular interactions that are unstable
at flame temperatures. Here, we show evidence for a reactive π-diradical
aromatic soot precursor imaged using non-contact atomic force microscopy.
Localization of π-electrons on non-hexagonal rings was found
to allow for Kekulé aromatic soot precursors to possess a triplet
diradical ground state. Barrierless chain reactions are shown between
these reactive sites, which provide thermally stable aromatic rim-linked
hydrocarbons under flame conditions. Quantum molecular dynamics simulations
demonstrate physical condensation of aromatics that survive for tens
of picoseconds. Bound internal rotors then enable the reactive sites
to find each other and become chemically cross-linked before dissociation.
These species provide a rapid, thermally stable chain reaction toward
soot nanoparticle formation and could provide molecular targets for
limiting the emission of these toxic combustion products.
Atomic force microscopy
(AFM) as well as scanning tunneling microscopy
induced light emission (STM-LE) are, each on their own, powerful tools
used to investigate a large variety of properties of single molecules
adsorbed on a surface. However, accessing both structural information
by AFM as well as optical information by STM-LE on the same molecule
so far remains elusive. We present a combined high-resolution AFM
and STM-LE study on single metal-oxide phthalocyanines. Using atomic
manipulation, the molecules can be deliberately reduced. We demonstrate
structure elucidation and adsorption geometry determination of single
molecules with atomic resolution combined with optical characterization
by STM-LE and the possibility of investigating the change in a molecule’s
exciton emission intensity by a chemical reaction.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.