B₂N₂‑Dibenzo[<i>a</i>,<i>e</i>]pentalenes: Effect of the BN Orientation Pattern on Antiaromaticity and Optoelectronic Properties

Two BN units were embedded in dibenzo­[<i>a</i>,<i>e</i>]­pentalene 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 tetrabenzo­[<i>a,f,j,o</i>]­perylenes (namely “bistetracenes”) and tetrabenzo­[<i>bc,ef,kl,no</i>]­coronenes (namely “peritetracenes”). 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]­helicene 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 tetrabenzo­[<i>a,f,j,o</i>]­perylenes (namely “bistetracenes”) and tetrabenzo­[<i>bc,ef,kl,no</i>]­coronenes (namely “peritetracenes”). 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]­helicene 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 “spoked wheel” 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, C₆₀), solar light harvesting, and singlet–triplet 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-11,­12,­25,­26-tetraoxatetranaphtho­[1,2-<i>a</i>:2′,1′-<i>f</i>:1″,2″-<i>j</i>:2‴,1‴-<i>o</i>]­perylene, a double [7]­heterohelicene 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 π-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-tetraarylbenzo­[<i>k</i>]­tetraphene, which “unexpectedly” 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, UV–vis 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 “expected” 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 Exciton–Photon Coupling in a Nanographene Filled Microcavity

Dibenzo­[<i>hi,st</i>]­ovalene (DBOV)a quasi-zero-dimensional “nanographene”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 → 0′ electronic 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 host–guest enhanced donor–acceptor interaction, consisting of a tris­(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]­uril as host monomer toward monolayers of an unprecedented 2D SOF. Featuring orthogonal solubility, the participating molecules self-assemble at a liquid–liquid 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 cm², and featuring the ability to be free-standing over holes of 10 μm². Characterization with ultraviolet–visible 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 “bottom-up” 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