Synthesis of Rationally Halogenated Buckybowls by Chemoselective Aromatic C−F Bond Activation

Tetra(peri-naphthylene)anthracene: A Near-IR Fluorophore with Four-Stage Amphoteric Redox Properties

A novel, benign synthetic strategy towards soluble tetra(peri-naphthylene)anthracene (TPNA) decorated with triisopropylsilylethynyl substituents has been established. The compound is perfectly stable under ambient conditions in air and features intense and strongly bathochromically shifted UV/vis absorption and emission bands reaching to near-IR region beyond 900 nm. Cyclic voltammetry measurements revealed four facilitated reversible redox events comprising two oxidations and two reductions. These remarkable experimental findings were corroborated by theoretical studies to identify the TPNA platform a particularly useful candidate for the development of functional near-IR fluorophores upon appropriate functionalization.

Planar π-extended cycloparaphenylenes featuring an all-armchair edge topology

The [n]cycloparaphenylenes ([n]CPPs)-n para-linked phenylenes that form a closed-loop-have attracted substantial attention due to their unique cyclic structure and highly effective para-conjugation leading to a myriad of fascinating electronic and optoelectronic properties. However, their strained topology prevents the π-extension of CPPs to convert them either into armchair nanobelts or planarized CPP macrocycles. Here we successfully tackle this long-standing challenge and present the bottom-up synthesis and characterization of atomically precise in-plane π-extended [12]CPP on Au(111) by low-temperature scanning probe microscopy and spectroscopy combined with density functional theory. The planar π-extended CPP is a nanographene with an all-armchair edge topology. The exclusive para-conjugation at the periphery yields delocalized electronic states and the planarization maximizes the overlap of p orbitals, which both reduce the bandgap compared to conventional CPPs. Calculations predict ring currents and global aromaticity in the doubly charged system. The intriguing planar ring topology and unique electronic properties make planar π-extended CPPs promising quantum materials.

Acenaphthenoannulation Induced by the Dual Lewis Acidity of Alumina

We have discovered a dual (i. e., soft and hard) Lewis acidity of alumina that enables rapid one‐pot π‐extension through the activation of terminal alkynes followed by C−F activation. The tandem reaction introduces an acenaphthene fragment – an essential moiety of geodesic polyarenes. This reaction provides quick access to elusive non‐alternant polyarenes such as π‐extended buckybowls and helicenes through three‐point annulation of the 1‐(2‐ethynyl‐6‐fluorophenyl)naphthalene moiety. The versatility of the developed method was demonstrated by the synthesis of unprecedented structural fragments of elusive geodesic graphene nanoribbons.

Bowl-Shaped Symmetric and Non-symmetric Bis-functionalized Indacenopicenes

Regioselective arrangement of two groups of orthogonal reactivity in bowl-shaped as-indaceno[3,2,1,8,7,6-pqrstuv]picene (Idpc) was key for the synthesis of hetero-bis-functionalized Idpc derivatives. Halogen and methyl groups were positioned at specific positions in the rim area of Idpc at an early stage during the synthesis by functionalization of suitable precursors. Regioselective functionalization of the bowl's rim was then finally achieved either via consecutive Cu(I)-catalyzed azide-alkyne coupling (CuAAC) and C-C cross-coupling reactions or by C-C cross-coupling alone, giving access to either symmetric or non-symmetric bis-functionalized Idpc derivatives. The self-aggregation behavior of 9c in solution was investigated by recording a series of concentration-dependent NMR spectra. The aggregation constant of 9c was determined by a nonlinear least-squares treatment of the ¹H NMR shift data to be 2.9 ± 0.2 M^-1, and the formation of dimers was found to be the prevailing process.

Carbon Origami via an Alumina-Assisted Cyclodehydrofluorination Strategy

The synthesis of pristine non-planar nanographenes (NGs) via a cyclodehydrofluorination strategy is reported and the creation of highly strained systems via alumina-assisted C-F bond activation is shown. Steric hindrance could execute an alternative coupling program leading to rare octagon formation offering access to elusive non-classical NGs. The combination of two alternative ways of folding could lead to the formation of various 3D NG objects, resembling the Japanese art of origami. The power of the presented "origami" approach is proved by the assembly of 12 challenging nanographenes that are π-isoelectronic to planar hexabenzocoronene but forced out of planarity.

In situ laser annealing as pathway for the metal free synthesis of tailored nanographenes

Tailored synthesis of nanographenes, and especially graphene nanoribbons (GNR), has been achieved on metal substrates via a bottom-up approach from organic precursors, which paves the way to their application in nanoelectronics and optoelectronics. Since quantum confinement in nanographenes leads to the creation of peculiar band structures, strongly influenced by their topological characteristics, it is important to be able to exactly engineer them in order to precisely tune their electronic, optical and magnetic properties. However practical application of these materials requires post-synthesis transfer to insulating substrates. Recently, cyclodehydrofluorination of fluorinated organic precursors has been shown to be a promising pathway to achieve metal-free bottom-up synthesis of nanographenes. Here we present how to apply in situ laser annealing to induce cyclodehydrofluorination leading to nanographene formation directly on non-metallic surfaces. In this work, we analyze the changes in the Raman fingerprint of the fluorinated precursor tetrafluoro-diphenyl-quinquephenyl (TDQ) during the laser annealing process in high vacuum (HV), demonstrating that both heating and photo-induced processes influence the cyclization process. Hence, in situ laser annealing allows not only to influence chemical reactions, but also to have a fast and contact-free monitoring of the reaction products. Optimization of the laser annealing process adds a new level of control in the tailored synthesis of nanographenes on non-metallic substrates. This is a very promising pathway to unravel the full application potential of nanographenes in general and GNR in particular, enabling a fast optimization of precursor molecules and substrate geometry engineered for specific applications.

Geodesic-Planar Conjugates: Substituted Buckybowls-Synthesis, Photoluminescence and Electrochemistry

C-C cross coupling products of bowl-shaped as-indaceno[3,2,1,8,7,6-pqrstuv]picene (Idpc) and different planar arenes and ethynyl-arenes were synthesized. Photoluminescence as well as electrochemical properties of all products were investigated and complemented by time-dependent quantum chemical calculations. UV/Vis spectroelectrochemistry investigations of the directly linked (Idpc)2 indicated the absence of any intramolecular charge-transfer transition of intermittently formed (Idpc)2 .- . All coupling products showed fluorescence. Ferrocene-1-yl-Idpc was structurally characterized by X-ray diffraction and is a rare example of a ferrocene-containing buckybowl exhibiting luminescence.

A Hexabenzocoronene-Based Helical Nanographene

A synthetic route towards a novel hexabenzocoronene-based helical nanographene motif was developed. A hexaphenylbenzene precursor was therefore designed, which cannot undergo, due to steric restrictions, a complete planarization reaction. This precursor was transformed under oxidative cyclodehydrogenation conditions to a π-extended [5]helicene, which was fully characterized including X-ray diffraction analysis.

as-Indaceno[3,2,1,8,7,6-ghijklm]terrylene as a near-infrared absorbing C70-fragment

Carbon and hydrogen are fundamental components of organic molecules and a fascinating plethora of functions can be generated using these two elements. Yet, realizing attractive electronic structures only by using carbon and hydrogen remains challenging. Herein, we report the synthesis and properties of the C₇₀ fragment as-indaceno[3,2,1,8,7,6-ghijklm]terrylene, which exhibits near-infrared (NIR) absorption (up to ca. 1300 nm), even though this molecule consists of only 34 carbon and 14 hydrogen atoms. A remarkably small highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap is confirmed by electrochemical measurement and theoretical calculations. Furthermore, as-indacenoterrylene is stable despite the absence of peripheral substituents, which contrasts with the cases of other NIR-absorbing hydrocarbons such as diradicaloids and antiaromatic molecules. The results of this study thus offer fundamental insights into the design of hydrocarbons with a small band gap.

A unified statistical RRKM approach to the fragmentation and autoneutralization of metastable molecular negative ions of hexaazatrinaphthylenes

For the compounds promising for use as n-type semiconductors in organic electronics and energy storage devices, hexaazatrinaphthylene (HATNA) and its derivative hexamethoxy-hexaazatrinaphthylene (HMHATA), the monomolecular processes occurring under the exposure of molecules to low-energy (0-15 eV) free electrons were studied by means of resonant electron capture negative ion mass spectrometry. Resonant electron attachment results in the formation of eminently long-lived molecular negative ions (MNIs) in an abnormally wide range of incident electron energy (E_e) from 0 to 5-7 eV. For both compounds, this observation serves as an indication of the strong electron-accepting properties and high stability of MNIs against electron autodetachment. A weak yield of the only fragment NIs, dehydrogenated anions, was detected for HATNA at E_e > 6 eV. MNIs of HMHATA are less stable to dissociative decay because of the presence of weakly bound terminal substituents. This is evidenced by the mass spectral observation of intense fragmentation occurring above E_e≈ 1 eV and leading to a loss of up to 3 methyl groups as the E_e increases. A series of metastable NI peaks observed in the mass spectra testify to the delayed and sequential nature of fragmentation. Based on the principles of statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory, the theoretical model of dissociative decay of NIs was developed and then adopted to quantify the rates of ground-state anion decay via electron autodetachment. The experimentally measured electron autodetachment lifetimes and fragmentation rates were best reproduced by the model at molecular adiabatic electron affinities preset to 2.15 eV for HATNA and 1.88 eV for HMHATA, in reasonable agreement with the quantum chemical DFT PBE/3ζ predictions.

Alumina-promoted oxodefluorination

A simple protocol for the clean preparation of heterocyclic compounds containing dibenzofuran's core via oxodefluorination of fluoroarenes on activated γ-Al₂O₃ is reported. Alumina can be considered as a reliable oxygen source enabling one-pot substitution of fluorine atoms and yielding benzoannulated furan derivatives. The corresponding C–F bond activation is selective towards less stable C–Br/C–I and occurs under metal- and solvent-free conditions.

A simple protocol for the clean preparation of heterocyclic compounds containing dibenzofuran's core via oxodefluorination of fluoroarenes on activated γ-Al₂O₃ is reported.

Towards Nonalternant Nanographenes through Self-Promoted Intramolecular Indenoannulation Cascade by C-F Bond Activation

Large polycyclic aromatic hydrocarbons (PAHs) containing pentagons represent an important class of compounds that are considered to be superior materials in future nano-electronic applications. From this perspective, the development of synthetic approaches to large PAHs and nanographenes (NGs) is a matter of great importance. In this context indenoannulation appears to be the most practical way to introduce pentagons into NGs. Here we report that alumina-mediated C-F bond activation is an attractive tool for the synthesis of non-alternant NGs bearing several pentagons. The unique nature of the reaction leads to a rather counter-intuitive outcome and allows considering each previous aryl-aryl coupling as a promoter of the following one, despite the continuous increase in the strain energy. Thus, the presented strategy combines both facile synthesis and significant yields for large nonalternant PAHs and NGs.

Unusual Fusion of α-Fluorinated Benzophenones under McMurry Reaction Conditions

By exposure of α-fluorinated benzophenones to McMurry reaction conditions, we have observed the remarkable formation of 9,10-diphenylanthracene derivatives. This unexpected transformation necessitates the cleavage of the exceptionally stable aromatic C-F bond under mild McMurry conditions. In this work, the condensation of several related fluorinated benzo- and acetophenones has been investigated, which allow us to propose a domino-like fusion mechanism for this unusual transformation. The scope and limitations of the fluorine-promoted benzophenone fusion are subsequently discussed.

Fluorine-programmed nanozipping to tailored nanographenes on rutile TiO2 surfaces

The rational synthesis of nanographenes and carbon nanoribbons directly on nonmetallic surfaces has been an elusive goal for a long time. We report that activation of the carbon (C)–fluorine (F) bond is a reliable and versatile tool enabling intramolecular aryl-aryl coupling directly on metal oxide surfaces. A challenging multistep transformation enabled by C–F bond activation led to a dominolike coupling that yielded tailored nanographenes directly on the rutile titania surface. Because of efficient regioselective zipping, we obtained the target nanographenes from flexible precursors. Fluorine positions in the precursor structure unambiguously dictated the running of the “zipping program,” resulting in the rolling up of oligophenylene chains. The high efficiency of the hydrogen fluoride zipping makes our approach attractive for the rational synthesis of nanographenes and nanoribbons directly on insulating and semiconducting surfaces.

The Rolling-Up of Oligophenylenes to Nanographenes by a HF-Zipping Approach

Intramolecular aryl-aryl coupling is the key transformation in the rational synthesis of nanographenes and nanoribbons. In this respect the C-F bond activation was shown to be a versatile alternative enabling the synthesis of several unique carbon-based nanostructures. Herein we describe an unprecedentedly challenging transformation showing that the C-F bond activation by aluminum oxide allows highly effective domino-like C-C bond formation. Despite the flexible nature of oligophenylene-based precursors efficient regioselective zipping to the target nanostructures was achieved. We show that fluorine positions in the precursor structure unambiguously dictate the "running of the zipping-program" which results in rolling-up of linear oligophenylene chains around phenyl moieties yielding target nanographenes. The high efficiency of zipping makes this approach attractive for the synthesis of unsubstituted nanographenes which are difficult to obtain in pure form by other methods.