Synthesis, Structure, and Resolution of Exceptionally Twisted Pentacenes

Electrochemical Continuous-Flow Scholl Reaction toward Polycyclic Aromatic Hydrocarbons

The preparation of polycyclic aromatic hydrocarbons (PAHs) by the Scholl reaction is typically performed by using superstoichiometric oxidants. Herein, we develop an electrochemical continuous-flow Scholl reaction to access PAHs that features a reduction in the use of supporting electrolytes and easy scale-up without changing the reaction conditions and setups. This reaction allows the synthesis of distorted PAHs containing three [5]helicene units that possess intriguing electronic and optical properties.

Helitwistacenes-Combining Lateral and Longitudinal Helicity Results in Solvent-Induced Inversion of Circularly Polarized Light

Helicity is expressed differently in ortho- and para-fused acenes-helicenes and twistacenes, respectively. While the extent of helicity is constant in helicenes, it can be tuned in twistacenes, and the handedness of flexible twistacenes is often determined by more rigid helicenes. Here, we combine helicenes with rigid twistacenes consisting of a tunable degree of twisting, forming helitwistacenes. While the X-ray structures reveal that the connection does not affect the helicity of each moiety, their electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra are strongly affected by the helicity of the twistacene unit, resulting in solvent-induced sign inversion. ROESY NMR and TD-DFT calculations support this observation, which is explained by differences in the relative orientation of the helicene and twistacene moieties.

Solid-State Generation of Diarylisonaphthofuran and Its Mechanochemical Diels-Alder Reaction with Epoxynaphthalene

A solid-state method was developed for generating diarylisonaphthofurans from 1,3-diaryl-1,3-dihydronaphthofuranols. The generated reactive molecules were stable in the solid state and could be stored without any extra precautions. X-ray diffraction analysis revealed a typical quinoidal structure. Furthermore, the mechanochemical Diels-Alder reaction of 1,3-diarylisonaphthofurans with epoxynaphthalenes afforded synthetically attractive diepoxypentacenes.

Helical fused 1,2:8,9-dibenzozethrene oligomers with up to 201° end-to-end twist: "one-pot" synthesis and chiral resolution

Twisted polyarenes with persistent chirality are desirable but their synthesis has remained a challenge. In this study, we present a "one-pot" synthesis of 1,2:8,9-dibenzozethrene (DBZ) and its vertically fused dimers and trimers using nickel-catalyzed cyclo-oligomerization reactions. X-ray crystallographic analysis confirmed highly twisted helical structures that consist of equal parts left- and right-handed enantiomers. Notably, the end-to-end twist between the terminal anthracene units measured 66°, 130°, and 201° for the DBZ monomer, dimer, and trimer, respectively, setting a new record among twisted polyarenes. Furthermore, the chiral resolution by HPLC yielded two enantiomers for the fused DBZ dimer and trimer, both of which maintained stable configurations and showed absorption dissymmetry factors of around 0.008-0.009. Additionally, their optical and electrochemical properties were investigated, which exhibited a chain-length dependence.

Di-2,7-pyrenidecaphyrin(1.1.0.0.0.1.1.0.0.0) and Its Bis-Organopalladium Complexes: Synthesis and Chiroptical Properties

A non-aromatic expanded carbaporphyrinoid, incorporating two built-in 2,7-pyrenylene moieties was synthesized. The intrinsically labile structure was demonstrated by proton-triggered conformational changes between the figure-of-eight and quasi-Möbius conformers. Upon treatment with Pd(OAc)₂ , the reaction produces two bis-Pd^II complexes with distinct coordination modes. Metal coordination serves to fix the macrocyclic frameworks with the net result that both bis-Pd^II complexes could be resolved by high performance liquid chromatography (HPLC) on a chiral stationary phase. The isolated enantiomers showed persistent chiroptical properties as evidenced by the intense response in the circular dichroism (CD) spectra and the record high absorption dissymmetry factors (g_abs of up to 0.038) seen in the near-infrared spectral region. Moreover, the mutual interconversion of these two Pd^II complexes was found to be stereospecific and to favor the more stable isomers under weakly acidic conditions.

Heptagon-Embedded π-Expanded Thieno- and N-Methylpyrrolo-Pyridazines with Substantial Out-of-Plane Dipole Moment

Herein, we describe the synthesis and characterization of fully fused tetraphenylthieno[3,4-d]pyridazine 1 and N-methylpyrrolo[3,4-d]pyridazine 2 with two embedded seven-membered rings. Owing to the incorporated heptagon, 1 and 2 exhibited C_s-symmetric saddle conformations in the solid state with mean plane deviation around 0.38 Å. π-Expanded thienopyridazine 1 showed a one-dimensional (1-D) columnar packing along the b axis with net dipole moment aligning perpendicular to the b axis in the polar crystal system Pc. On the other hand, 2 formed a partially π-stacked brick-work structure. In addition to the C_s-symmetric saddle conformations found in the crystals, density functional theory (DFT) calculation found C₂-symmetric twisted conformations of both 1 and 2 close in energy to the saddle conformations. The barrier of conformational interconversion was calculated to be 32 (1) and 31 kJ·mol^-1 (2), and the interconversion occurs fast even at -60 °C as evidenced by variable-temperature (VT)-NMR studies. While 1 and 2 have moderately curved structures, optical and electrochemical studies revealed effective π-conjugation over the fused diphenylene units, which is also supported by DFT calculation. As the result of the intrinsic large dipole moment of thieno- and pyrrolo-pyridazines and the notably curved structure, 1 (2) has a substantial out-of-plane dipole moment of 2.0 (3.3) D in the saddle conformations.

Isoacenofuran: a novel quinoidal building block for efficient access to high-ordered polyacene derivatives

Herein, a simple and practical method for generating isoacenofuran, a new π-extended quinoidal building block, was developed. A three-step protocol involving double nucleophilic additions of alkynyllithiums to acene-2,3-dicarbaldehyde, mono-oxidation, and acid-promoted cyclization enables the generation of the target molecule, which is trapped by a dienophile to produce highly condensed acenequinones. Further transformations by double nucleophilic additions of alkynyllithium to hexacenequinone, followed by reductive aromatization, produce tetraalkynylhexacenes with a remarkably higher stability than that of the previously reported substituted hexacenes.

Synthesis of Decaarylanthracene with Nine Different Substituents

A synthesis of decaarylanthracene with nine different substituents has been accomplished by a coupling/ring-transformation strategy. The oxidation of tetraarylthiophenes with four different substituents to the corresponding thiophene S-oxides and a [4 + 2] cycloaddition with a double benzyne precursor afforded a multiply arylated naphthalene derivative. Subsequently, the naphthalene derivative was converted into a naphthalyne, and then a [4 + 2] cycloaddition of another thiophene S-oxide provided decaarylanthracenes with nine different aryl groups.

Of Twists and Curves: Electronics, Photophysics, and Upcoming Applications of Non-Planar Conjugated Organic Molecules

Non-planar conjugated organic molecules (NPCOMs) contain π-conjugation across their length and also exhibit asymmetry in their conformation. In other words, certain molecular fragments in NPCOMs are either twisted or curved out of planarity. This conformational asymmetry in NPCOMs leads to non-uniform charge-distribution across the molecule, with important photophysical and electronic consequences such as altered thermodynamic stability, chemical reactivity, as well as materials properties. Majorly, NPCOMs can be classified as having either Fused or Rotatable architectures. NPCOMs have been the focus of significant scientific attention in the recent past due to their exciting photophysical behavior that includes intramolecular charge-transfer (ICT), thermally activated delayed fluorescence (TADF) and long-lived charge-separated states. In addition, they also have many useful materials characteristics such as biradical character, semi-conductivity, dynamic conformations, and mechanochromism. As a result, rational design of NPCOMs and mapping their structure-property correlations has become imperative. Researchers have executed conformational changes in NPCOMs through a variety of external stimuli such as pH, temperature, anions-cations, solvent, electric potential, and mechanical force in order to tailor their photophysical, optoelectronic and magnetic properties. Converging to these points, this review highlights the lucrative electronic features, photophysical traits and upcoming applications of NPCOMs by a selective survey of the recent scientific literature.

Dodecatwistarene Imides with Zigzag-Twisted Conformation for Organic Electronics

1D nonplanar graphene nanoribbons generally have three possible conformers: helical, zigzag, and mixed conformations. Now, a kind of 1D nonplanar graphene nanoribbon, namely dodecatwistarene imides featuring twelve linearly fused benzene rings, was obtained by bottom-up synthesis of palladium-catalyzed Stille coupling and C-H activation. Single-crystal X-ray diffraction analyses revealed that it displays a zigzag-twisted conformation caused by steric hindrance between imide groups and neighboring annulated benzene rings with the pendulum angle of 53°. This conformation is very stable and could not convert into other conformations even when heated up to 250 °C for 6 h. Despite of the highly twisted topology, organic field-effect transistor based on it exhibits electron mobility up to 1.5 cm²  V^-1  s^-1 after annealing.

BF3-Etherate-catalyzed tandem reaction of 2-formylarylketones with electron-rich arenes/heteroarenes: an assembly of isobenzofurans

An efficient and BF₃·Et₂O-catalyzed chemoselective synthesis of diversified 1,3-diarylisobenzofuran in a high yield has been described.

Extremely twisted and bent pyrene-fused N-heterocyclic germylenes

The first examples of pyrene-fused Janus-type N-heterocyclic germylenes (NHGe) are reported. Remarkably, the pyrene linker and the germanium containing rings are extremely twisted, with "twist angles" up to 64°. Coordination of a Lewis base modifies the twisting of pyrene to an overall bent core (141° bend angle).

The Consequences of Twisting Nanocarbons: Lessons from Tethered Twisted Acenes

The properties of polycyclic aromatic hydrocarbons are determined by their size, shape, and functional groups. Equally important is their curvature, since deviation from planarity can affect their optical, electronic, and magnetic properties and also induce chirality. Acenes, which can be viewed as one-dimensional nanocarbons, are often twisted out of planarity. Although twisting is expected to affect the above-mentioned properties, it is often overlooked. This Account focuses on helically locked twistacenes (twisted acenes) having different twist angles and the effect of twisting on their electronic and optical properties. Various synthetic approaches to inducing backbone twist in acenes are discussed, with a focus on the introduction of a diagonal tether across the core, as this minimizes confounding substituent effects. Using such tethered acenes as our model, we then discuss the effects of twisting the aromatic core on twistacene properties. Electronic properties. Increasing the degree of twist only slightly affects the HOMO and LUMO energy levels. Twisting leads to a small increase in the HOMO level and a decrease in the LUMO level, which produces an overall decrease in the HOMO-LUMO gap. Optical properties. As the degree of twist increases, a slight bathochromic shift is observed in the absorption spectra, in accordance with the decrease in the HOMO-LUMO gap. The fluorescence quantum efficiency and the fluorescence lifetime also decrease. This is likely to be related to an increasing rate of intersystem crossing, which arises from increased spin-orbit coupling. In addition, computational studies indicate that the S⁰-T¹ energy gap decreases with increasing twist. Chiroptical properties. Increased twisting results in a larger Cotton effect and anisotropy factor, with the anisotropy factors of Ant-Cn being higher than those of longer helicenes. The parallel orientation of electric and magnetic transition dipole moments in twistacenes underlies this behavior and renders them as excellent chiroptical materials. The same trend is observed for the radical cations of twistacenes, which absorb in the NIR spectral region. Conjugation and delocalization. Twisting the anthracene radical cation up to 40° (13° per benzene ring) does not significantly affect spin delocalization, with the EPR spectra of twistacene radical cations showing that only slight localization occurs. This is in line with computational studies, which show only a small decrease in π-overlap for large acene twist. Overall, modifying the length of the tether in diagonally tethered acenes allows chemists to control core twist and to induce chirality. Twisting affects key optical, electronic, and chiroptical properties of acenes. Consequently, controlling the twist angle can improve the future design of nanocarbons with desired properties.

Radical cations of twisted acenes: chiroptical properties and spin delocalization

We introduce the first series of enantiopure twistacene radical cations, which form reversibly upon chemical or electrochemical oxidation. Their vis-NIR chiroptical properties (Cotton effect and anisotropy factor) increase systematically with the backbone twist. The hyperfine constants observed by EPR demonstrate significant spin delocalization even for large backbone twist angles.

Helically Locked Tethered Twistacenes

Twisting linear acenes out of planarity affects their electronic and optical properties, and induces chirality. However, it is difficult to isolate the effect of twisting from the substituent effect. Moreover, many twistacenes (twisted acenes) readily racemize in solution. Here, we introduce a series of twistacenes having an anthracene backbone diagonally tethered by an n-alkyl bridge, which induces a twist of various angles. This allows us to systematically monitor the effect of twisting on electronic and optical properties. We find that absorption is bathochromically shifted with increasing twist, while fluorescence quantum efficiency drops dramatically. The tethered twistacenes were isolated to their enantiomerically pure form, displaying strong chiroptical properties and anisotropy factor ( g-value). No racemization was observed even upon prolonged heating, rendering these tethered twistacenes suitable as enantiopure helical building units for π-conjugated backbones.

Reductive Aromatization/Dearomatization and Elimination Reactions to Access Conjugated Polycyclic Hydrocarbons, Heteroacenes, and Cumulenes

Acenes, heteroacenes, conjugated polycyclic hydrocarbons, and polycyclic aromatic hydrocarbons (collectively referred to in this review as conjugated polycyclic molecules, CPMs) have fascinated chemists since they were first isolated and synthesized in the mid 19th century. Most recently, these compounds have shown significant promise as the active components in organic devices (e.g., solar cells, thin-film transistors, light-emitting diodes, etc.), and, since 2001, a plethora of publications detail synthetic strategies to produce CPMs. In this review, we discuss reductive aromatization, reductive dearomatization, and elimination/extrusion reactions used to form CPMs. After a brief discussion on early methods to synthesize CPMs, we detail the use of reagents used for the reductive (de)aromatization of precursors containing 1,4-diols/diethers, including SnCl₂ and iodide (I^- ). Extension of these methods to carbomers and cumulenes is briefly discussed. We then describe low-valent metal species used to reduce endoxides to CPMs, and discuss the methods to directly reduce acenediones and acenones to the respective acene. In the final section, we describe methods used to affect aromatization to the desired CPM via extrusion of small, volatile molecules.

Synthesis, physical properties, and chemistry of donor–acceptor-substituted pentacenes

Pentacenes bearing electron-donating and (or) -withdrawing groups, namely methoxy-, dialkylamino-, and nitroaryl moieties, are synthesized to afford polarized pentacenes. The optical, electrochemical, and chemical properties of these derivatives are explored. The cycloaddition reaction of selected derivatives with tetracyanoethylene (TCNE) is explored, and the experimental results are rationalized on the basis calculations using density functional theory (DFT). X-ray crystallographic data provides insight into the molecular structure and intermolecular interactions present in the solid state.

Synthesis and physical properties of triphenylamine-functionalized twistacenes: blue-emitting fluorophores

Two novel triphenylamine-decorated twistacenes SNPy and DNPy have been synthesized and characterized.

The Hairpin Furans: Easily Prepared Hybrids of Helicenes and Twisted Acenes

The thermal reaction of a cyclopentadienone with 6,6'-dimethoxy-5,5'-binaphthoquinone gives, in a single step, a molecular ribbon consisting of two twisted aromatic systems fused to a heteropentahelicene. Such molecules constitute a new class of chiral polycyclic aromatic compounds, the "hairpin furans".

A dinaphtho[8,1,2-cde:2′,1′,8′-uva]pentacene derivative and analogues: synthesis, structures, photophysical and electrochemical properties

Dinaphtho[8,1,2-cde:2′,1′,8′-uva]pentacene with a scorpion-shaped structure has a twisted configuration, but shows marked intermolecular π–π interactions, strong electronic delocalization, and a small HOMO–LUMO bandgap.

Synthesis and chiroptical properties of arylimines of cholest-4-ene-3,6-dione

In the hope of generating a new class of materials with large optical rotations, various arylamines were condensed with cholest-4-ene-3,6-dione (2) to give 3-arylimino steroids. These compounds possess moderately high specific rotations ([α](D) ~300-800) and strong circular dichroism. One such derivative, 3-(4-methoxyphenylimino)cholest-4-en-6-one (3), crystallizes as the (E)-imine and upon dissolution undergoes mutarotation to an equilibrium mixture of (E)- and (Z)-isomers with a half-life of approximately 1 h at room temperature, as judged by both NMR spectroscopy and polarimetry.

Synthesis, characterization, and bipolar transporting behavior of a new twisted polycyclic aromatic hydrocarbon: 1',4'-diphenyl-naphtho-(2'.3':1.2)-pyrene-6'-nitro-7'-methyl carboxylate

An asymmetric twistacene, 1',4'-diphenyl-naphtho-(2'.3':1.2)-pyrene-6'-nitro-7'-methyl carboxylate (tetracene 2), was synthesized by using benzyne-trapping chemistry. Its structure, determined by X-ray crystallography, confirmed that this material has a twisted topology with torsion angles as high as 23.8(3) degrees. Organic light-emitting devices using tetracene 2 as either charge-transporting materials or emitters have been fabricated. The results indicate that this material has bipolar transporting behavior in these devices.

Oligomers and Polymers Based on Pentacene Building Blocks

Functionalized pentacene derivatives continue to provide unique materials for organic semiconductor applications. Although oligomers and polymers based on pentacene building blocks remain quite rare, recent synthetic achievements have provided a number of examples with varied structural motifs. This review highlights recent work in this area and, when possible, contrasts the properties of defined-length pentacene oligomers to those of mono- and polymeric systems.