Pyrene-Fused <i>s</i>‑Indacene

One antiaromatic polycyclic hydrocarbon (PH) with and without solubilizing <i>tert</i>-butyl substituents, namely <i>s</i>-indaceno­[2,1-<i>a</i>:6,5-<i>a</i>′]­dipyrene (IDPs), has been synthesized by a four-step protocol. The IDPs represent the longitudinal, <i>peri</i>-extension of the indeno­[1,2-<i>b</i>]­fluorene skeleton towards a planar 40 π-electron system. Their structures were unambiguously confirmed by X-ray crystallographic analysis. The optoelectronic properties were studied by UV/vis absorption spectroscopy and cyclic voltammetry. These studies revealed that <i>peri</i>-fusion renders the IDP derivatives with a narrow optical energy gap of 1.8 eV. The maximum absorption of IDPs is shifted by 160 nm compared to the parent indenofluorene. Two quasi-reversible oxidation as well as reduction steps indicate an excellent redox behavior attributed to the antiaromatic core. Formation of the radical cation and the dication was monitored by UV/vis absorption spectroscopy during titration experiments. Notably, the fusion of <i>s</i>-indacene with two pyrene moieties lead to IDPs with absorption maxima approaching the near infrared (NIR) regime

Pyrene-Fused <i>s</i>‑Indacene

One antiaromatic polycyclic hydrocarbon (PH) with and without solubilizing <i>tert</i>-butyl substituents, namely <i>s</i>-indaceno­[2,1-<i>a</i>:6,5-<i>a</i>′]­dipyrene (IDPs), has been synthesized by a four-step protocol. The IDPs represent the longitudinal, <i>peri</i>-extension of the indeno­[1,2-<i>b</i>]­fluorene skeleton towards a planar 40 π-electron system. Their structures were unambiguously confirmed by X-ray crystallographic analysis. The optoelectronic properties were studied by UV/vis absorption spectroscopy and cyclic voltammetry. These studies revealed that <i>peri</i>-fusion renders the IDP derivatives with a narrow optical energy gap of 1.8 eV. The maximum absorption of IDPs is shifted by 160 nm compared to the parent indenofluorene. Two quasi-reversible oxidation as well as reduction steps indicate an excellent redox behavior attributed to the antiaromatic core. Formation of the radical cation and the dication was monitored by UV/vis absorption spectroscopy during titration experiments. Notably, the fusion of <i>s</i>-indacene with two pyrene moieties lead to IDPs with absorption maxima approaching the near infrared (NIR) regime

Fused Dibenzo[<i>a</i>,<i>m</i>]rubicene: A New Bowl-Shaped Subunit of C₇₀ Containing Two Pentagons

Total synthetic approaches of fullerenes are the holy grail for organic chemistry. So far, the main attempts have focused on the synthesis of the buckminsterfullerene C₆₀. In contrast, access to subunits of the homologue C₇₀ remains challenging. Here, we demonstrate an efficient bottom-up strategy toward a novel bowl-shaped polycyclic aromatic hydrocarbons (PAH) C34 with two pentagons. This PAH represents a subunit for C₇₀ and of other higher fullerenes. The bowl-shaped structure was unambiguously determined by X-ray crystallography. A bowl-to-bowl inversion for a C₇₀ fragment in solution was investigated by dynamic NMR analysis, showing a bowl-to-bowl inversion energy (Δ<i>G</i>^⧧) of 16.7 kcal mol^–1, which is further corroborated by DFT calculations

Persulfurated Coronene: A New Generation of “Sulflower”

We report the first synthesis of a persulfurated polycyclic aromatic hydrocarbon (PAH) as a next-generation “sulflower.” In this novel PAH, disulfide units establish an all-sulfur periphery around a coronene core. The structure, electronic properties, and redox behavior were investigated by microscopic, spectroscopic and electrochemical methods and supported by density functional theory. The sulfur-rich character of persulfurated coronene renders it a promising cathode material for lithium–sulfur batteries, displaying a high capacity of 520 mAh g^–1 after 120 cycles at 0.6 C with a high-capacity retention of 90%

Termini of Bottom-Up Fabricated Graphene Nanoribbons

Atomically precise graphene nanoribbons (GNRs) can be obtained via thermally induced polymerization of suitable precursor molecules on a metal surface. This communication discusses the atomic structure found at the termini of armchair GNRs obtained via this bottom-up approach. The short zigzag edge at the termini of the GNRs under study gives rise to a localized midgap state with a characteristic signature in scanning tunneling microscopy (STM). By combining STM experiments with large-scale density functional theory calculations, we demonstrate that the termini are passivated by hydrogen. Our results suggest that the length of nanoribbons grown by this protocol may be limited by hydrogen passivation during the polymerization step

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons

Monitoring the On-Surface Synthesis of Graphene Nanoribbons by Mass Spectrometry

We present a mass spectrometric approach to characterize and monitor the intermediates of graphene nanoribbon (GNR) formation by chemical vapor deposition (CVD) on top of Au(111) surfaces. Information regarding the repeating units, lengths, and termini can be obtained directly from the surface sample by a modified matrix-assisted laser desorption/ionization (MALDI) method. The mass spectrometric results reveal ample oxidative side reactions under CVD conditions that can be drastically diminished by the introduction of protective H₂ gas at ambient pressure. Simultaneously, the addition of hydrogen extends the lengths of the oligophenylenes and thus the final GNRs. Moreover, the prematurely formed cyclodehydrogenation products during the oligomer growth can be assigned by the mass spectrometric technique. The obtained mechanistic insights provide valuable information for optimizing and upscaling the bottom-up fabrication of GNRs. Given the important role of GNRs as semiconductors, the mass spectrometric analysis provides a readily available tool to characterize and improve their structural perfection

Toward Full Zigzag-Edged Nanographenes: <i>peri</i>-Tetracene and Its Corresponding Circumanthracene

Zigzag-edged nanographene with two rows of fused linear acenes, called as n-<i>peri</i>-acene (n-PA), is considered as a potential building unit in the arena of organic electronics. n-PAs with four (<i>peri</i>-tetracene, <b>4-PA</b>), five (<i>peri</i>-pentacene, <b>5-PA</b>) or more benzene rings in a row have been predicted to show open-shell character, which would be attractive for the development of unprecedented molecular spintronics. However, solution-based synthesis of open-shell n-PA has thus far not been successful because of the poor chemical stability. Herein we demonstrated the synthesis and characterization of the hitherto unknown <b>4-PA</b> by a rational strategy in which steric protection of the zigzag edges playing a pivotal role. The obtained <b>4-PA</b> possesses a singlet biradical character (<i>y</i>₀ = 72%) and exhibits remarkable persistent stability with a half-life time (<i>t</i>_1/2) of ∼3 h under ambient conditions. UV–vis–NIR and electrochemical measurements reveal a narrow optical/electrochemical energy gap (1.11 eV) for <b>4-PA</b>. Moreover, the bay regions of <b>4-PA</b> enable the efficient 2-fold Diels–Alder reaction, yielding a novel full zigzag-edged circumanthracene

Toward Full Zigzag-Edged Nanographenes: <i>peri</i>-Tetracene and Its Corresponding Circumanthracene

Zigzag-edged nanographene with two rows of fused linear acenes, called as n-<i>peri</i>-acene (n-PA), is considered as a potential building unit in the arena of organic electronics. n-PAs with four (<i>peri</i>-tetracene, <b>4-PA</b>), five (<i>peri</i>-pentacene, <b>5-PA</b>) or more benzene rings in a row have been predicted to show open-shell character, which would be attractive for the development of unprecedented molecular spintronics. However, solution-based synthesis of open-shell n-PA has thus far not been successful because of the poor chemical stability. Herein we demonstrated the synthesis and characterization of the hitherto unknown <b>4-PA</b> by a rational strategy in which steric protection of the zigzag edges playing a pivotal role. The obtained <b>4-PA</b> possesses a singlet biradical character (<i>y</i>₀ = 72%) and exhibits remarkable persistent stability with a half-life time (<i>t</i>_1/2) of ∼3 h under ambient conditions. UV–vis–NIR and electrochemical measurements reveal a narrow optical/electrochemical energy gap (1.11 eV) for <b>4-PA</b>. Moreover, the bay regions of <b>4-PA</b> enable the efficient 2-fold Diels–Alder reaction, yielding a novel full zigzag-edged circumanthracene

Synthesis of NBN-Type Zigzag-Edged Polycyclic Aromatic Hydrocarbons: 1,9-Diaza-9a-boraphenalene as a Structural Motif

A novel class of dibenzo-fused 1,9-diaza-9a-boraphenalenes featuring zigzag edges with a nitrogen–boron–nitrogen bonding pattern named NBN-dibenzophenalenes (NBN-DBPs) has been synthesized. Alternating nitrogen and boron atoms impart high chemical stability to these zigzag-edged polycyclic aromatic hydrocarbons (PAHs), and this motif even allows for postsynthetic modifications, as demonstrated here through electrophilic bromination and subsequent palladium-catalyzed cross-coupling reactions. Upon oxidation, as a typical example, NBN-DBP <b>5a</b> was nearly quantitatively converted to σ-dimer <b>5a-2</b> through an open-shell intermediate, as indicated by UV–vis–NIR absorption spectroscopy and electron paramagnetic resonance spectroscopy corroborated by spectroscopic calculations, as well as 2D NMR spectra analyses. In situ spectroelectrochemistry was used to confirm the formation process of the dimer radical cation <b>5a-2</b>^•+. Finally, the developed new synthetic strategy could also be applied to obtain π-extended NBN-dibenzoheptazethrene (NBN-DBHZ), representing an efficient pathway toward NBN-doped zigzag-edged graphene nanoribbons