38 research outputs found
B<sub>2</sub>N<sub>2</sub>âDibenzo[<i>a</i>,<i>e</i>]pentalenes: Effect of the BN Orientation Pattern on Antiaromaticity and Optoelectronic Properties
Two BN units were
embedded in dibenzoÂ[<i>a</i>,<i>e</i>]Âpentalene
with different orientation patterns, which significantly
modulated its antiaromaticity and optoelectronic properties. Importantly,
the vital role of the BN orientation in conjugated molecules with
more than one BN unit was demonstrated for the first time. This work
indicates a large potential of the BN/CC isosterism for the development
of new antiaromatic systems and highlights the importance of precise
control of the BN substitution patterns in conjugated materials
Synthesis of Stable Nanographenes with OBO-Doped Zigzag Edges Based on Tandem Demethylation-Electrophilic Borylation
A tandem demethylation-aryl
borylation strategy was developed to
synthesize OBO-doped tetrabenzoÂ[<i>a,f,j,o</i>]Âperylenes
(namely âbistetracenesâ) and tetrabenzoÂ[<i>bc,ef,kl,no</i>]Âcoronenes (namely âperitetracenesâ). The OBO-doped
bistetracene analogues exhibited excellent stability and strong fluorescence,
in contrast to the unstable all-carbon bistetracene. Single-crystal
X-ray analysis for OBO-doped bistetracene revealed a twisted double
[5]Âhelicene structure, indicating that this synthesis is applicable
to new heterohelicenes. Importantly, cyclodehydrogenation of the bistetracene
analogues successfully produced the unprecedented heteroatom-doped
peritetracenes, which opened up a new avenue to periacene-type nanographenes
with stable zigzag edges
Synthesis of Stable Nanographenes with OBO-Doped Zigzag Edges Based on Tandem Demethylation-Electrophilic Borylation
A tandem demethylation-aryl
borylation strategy was developed to
synthesize OBO-doped tetrabenzoÂ[<i>a,f,j,o</i>]Âperylenes
(namely âbistetracenesâ) and tetrabenzoÂ[<i>bc,ef,kl,no</i>]Âcoronenes (namely âperitetracenesâ). The OBO-doped
bistetracene analogues exhibited excellent stability and strong fluorescence,
in contrast to the unstable all-carbon bistetracene. Single-crystal
X-ray analysis for OBO-doped bistetracene revealed a twisted double
[5]Âhelicene structure, indicating that this synthesis is applicable
to new heterohelicenes. Importantly, cyclodehydrogenation of the bistetracene
analogues successfully produced the unprecedented heteroatom-doped
peritetracenes, which opened up a new avenue to periacene-type nanographenes
with stable zigzag edges
A Shape-Persistent Polyphenylene Spoked Wheel
A shape-persistent polyphenylene
with a âspoked wheelâ
structure was synthesized as a subunit of an unprecedented two-dimensional
polyphenylene that we name graphenylene. The synthesis was carried
out through a sixfold intramolecular Yamamoto coupling of a dodecabromo-substituted
dendritic polyphenylene precursor, which had a central hexaphenylbenzene
unit as a template. Characterizations by NMR spectroscopy and matrix-assisted
laser ionization time-of-flight mass spectrometry provided an unambiguous
structural proof for the wheel-like molecule with a molar mass of
3815.4 g/mol. Remarkably, scanning tunneling microscopy visualization
clearly revealed the defined spoked wheel structure of the molecule
with six internal pores
Graphene Nanoribbons as Low Band Gap Donor Materials for Organic Photovoltaics: Quantum Chemical Aided Design
Graphene nanoribbons (GNRs) are strips of graphene cut along a specific direction that feature peculiar electronic and optical properties owing to lateral confinement effects. We show here by means of (time-dependent) density functional theory calculations that GNRs with properly designed edge structures fulfill the requirements in terms of electronic level alignment with common acceptors (namely, C<sub>60</sub>), solar light harvesting, and singletâtriplet exchange energy to be used as low band gap semiconductors for organic photovoltaics
Synthesis, Structure, and Chiroptical Properties of a Double [7]Heterohelicene
The synthesis of
11a,25a-dibora-11,Â12,Â25,Â26-tetraoxatetranaphthoÂ[1,2-<i>a</i>:2â˛,1â˛-<i>f</i>:1âł,2âł-<i>j</i>:2â´,1â´-<i>o</i>]Âperylene, a double
[7]Âheterohelicene containing OBO units, has been achieved via tandem
demethylation-borylation, representing the highest double helicene
reported thus far with all six-membered rings. Single-crystal X-ray
analysis clearly demonstrated a significantly twisted structure with
the terminal aromatic rings overlapping at both ends, giving the first
example of a double helicene with intramolecular Ď-layers. Such
structural features resulted in a high theoretical isomerization barrier
of 45.1 kcal/mol, which is the highest value for all the double helicenes
ever reported, rendering the achieved molecule with high chiral stability.
The (<i>P</i>,<i>P</i>)- and (<i>M</i>,<i>M</i>)-isomers were separated by chiral HPLC and the
chiroptical properties were investigated, revealing opposite circular
dichroism responses
Unexpected Scholl Reaction of 6,7,13,14-Tetraarylbenzo[<i>k</i>]tetraphene: Selective Formation of Five-Membered Rings in Polycyclic Aromatic Hydrocarbons
Cyclodehydrogenation is a versatile
reaction that has enabled the
syntheses of numerous polycyclic aromatic hydrocarbons (PAHs). We
now describe a unique Scholl reaction of 6,7,13,14-tetraarylbenzoÂ[<i>k</i>]Âtetraphene, which âunexpectedlyâ forms five-membered
rings accompanying highly selective 1,2-shift of aryl groups. The
geometric and optoelectronic nature of the resulting bistetracene
analogue with five-membered rings is comprehensively investigated
by single-crystal X-ray, NMR, UVâvis absorption, and cyclic
voltammetry analyses. Furthermore, a possible mechanism is proposed
to account for the selective five-membered-ring formation with the
rearrangement of the aryl groups, which can be rationalized by density
functional theory (DFT) calculations. The theoretical results suggest
that the formation of the bistetracene analogue with five-membered
rings is kinetically controlled while an âexpectedâ
product with six-membered rings is thermodynamically more favored.
These experimental and theoretical results provide further insights
into the still controversial mechanism of the Scholl reaction as well
as open up an unprecedented entry to extend the variety of PAHs by
programing otherwise unpredictable rearrangements during the Scholl
reaction
Strong ExcitonâPhoton Coupling in a Nanographene Filled Microcavity
DibenzoÂ[<i>hi,st</i>]Âovalene (DBOV)î¸a quasi-zero-dimensional
ânanographeneâî¸displays strong, narrow, and well-defined
optical-absorption transitions at room temperature. On placing a DBOV-doped
polymer film into an optical microcavity, we demonstrate strong coupling
of the 0 â 0Ⲡelectronic transition to a confined cavity
mode, with a coupling energy of 126 meV. Photoluminescence measurements
indicate that the polariton population is distributed at energies
approximately coincident with the emission of the DBOV, indicating
a polariton population via an optical pumping mechanism
Free-Standing Monolayer Two-Dimensional Supramolecular Organic Framework with Good Internal Order
Utilizing dynamic self-assembly and
self-sorting to obtain large-area,
molecularly precise monolayered structures represents a promising
approach toward two-dimensional supramolecular organic frameworks
(2D SOF) or 2D supramolecular polymers. So far, related approaches
suffer from small domain sizes, fragility and weak long-range internal
order. Here we report on the self-assembly of a hostâguest
enhanced donorâacceptor interaction, consisting of a trisÂ(methoxynaphthyl)-substituted
truxene spacer, and a naphthalene diimide substituted with <i>N</i>-methyl viologenyl moieties as donor and acceptor monomers,
respectively, in combination with cucurbit[8]Âuril as host monomer
toward monolayers of an unprecedented 2D SOF. Featuring orthogonal
solubility, the participating molecules self-assemble at a liquidâliquid
interface, yielding exceptionally large-area, insoluble films, which
were analyzed by transmission electron microscopy, atomic force microscopy
and optical microscopy to be monolayers with a thickness of 1.8 nm,
homogeneously covering areas up to 0.25 cm<sup>2</sup>, and featuring
the ability to be free-standing over holes of 10 Îźm<sup>2</sup>. Characterization with ultravioletâvisible absorption spectroscopy,
solid-state nuclear magnetic resonance spectroscopy, infrared spectroscopy,
and grazing incidence wide-angle X-ray scattering allowed for confirmation
of a successful complexation of all three monomers toward an internal
long-range order and gave indications to an expected hexagonal superstructure.
Our results extend the existing variety of two-dimensional soft nanomaterials
by a versatile supramolecular approach, whereas the possibility of
varying the functional monomers is supposed to open adaptability to
different applications like membranes, sensors, molecular sieves,
and optoelectronics
Ultrafast Photoconductivity of Graphene Nanoribbons and Carbon Nanotubes
We present a comparative study of
the ultrafast photoconductivity
in two different forms of one-dimensional (1D) quantum-confined graphene
nanostructures: structurally well-defined semiconducting graphene
nanoribbons (GNRs) fabricated by a âbottom-upâ chemical
synthesis approach and semiconducting carbon nanotubes (CNTs) with
a similar bandgap energy. Transient photoconductivities of both materials
were measured using time-resolved terahertz spectroscopy, allowing
for contact-free measurements of complex-valued photoconductivity
spectra with subpicosecond time-resolution. We show that, while the
THz photoresponse seems very different for the two systems, a single
model of free carriers experiencing backscattering when moving along
the long axis of the CNTs or GNRs provides a quantitative description
of both sets of results, revealing significantly longer carrier scattering
times for CNTs (ca. 150 fs) than for GNRs (ca. 30 fs) and in turn
higher carrier mobilities. This difference can be explained by differences
in band structures and phonon scattering and the greater structural
rigidity of CNTs as compared to GNRs, minimizing the influence of
bending and/or torsional defects on the electron transport