Corannulene-Helicene Hybrids: Chiral π-Systems Comprising Both Bowl and Helical Motifs

Enhancing corannulene chemiluminescence, electrochemiluminescence and photoluminescence by means of an azabora-helicene to slow down its bowl inversion

Aromatic system extension of corannulene (Cor) is a synthetic challenge to access non-planar polyaromatic hydrocarbons (PAHs). Herein, we report the design and synthesis of azaborahelicene corannulene 1 through hybridization of an azabora[5] helical structure and subsequent luminescence studies. Significant enhancement in chemiluminescence (CL), electroluminescence (ECL) and photoluminescence (PL) is achieved compared to those of pristine Cor. Specifically, hybrid 1 shows a notable augmentation in absolute luminescence quantum efficiencies: 25-fold for CL, up to 23-fold for ECL with BPO as a coreactant, and 30-fold for PL, respectively, compared to those of pristine Cor. Intriguingly, the blue light emission observed in all three luminescence types suggests the presence of a single excited state. As revealed by variable-temperature (VT) 1H NMR experiments, the bowl inversion frequency apparently decelerates by the steric effect of the helix motif in 1, which could contribute to the enhanced luminescent properties by reducing excited energy losses non-radiatively through fewer molecular motions; these enhanced luminescence observations could be categorized alongside the aggregation induced emission (AIE) and crystallization-induced emission (CIE) phenomena. This work not only provides fundamental insights into improved luminescence quantum efficiencies via strategic modulation of the molecular structure and geometry, but the work also reveals Cor's inherent potential to build efficient blue-light emitting materials and devices.

Buckybowl and its chiral hybrids featuring eight-membered rings and helicene units

Here we report the synthesis of a novel buckybowl (7) with a high bowl-to-bowl inversion barrier (ΔG‡ = 38 kcal mol-1), which renders the rate of inversion slow enough at room temperature to establish two chiral polycyclic aromatic hydrocarbons (PAHs). By strategic fusion of eight-membered rings to the rim of 7, the chiral hybrids 8 and 9 are synthesized and display helicity and positive and negative curvature, allowing the enantiomers to be configurationally stable and their chiroptical properties are thoroughly examined. Computational and experimental studies reveal the enantiomerization mechanisms for the chiral hybrids and demonstrate that the eight-membered ring strongly affects the conformational stability. Because of its static and doubly curved conformation, 9 shows a high binding affinity towards C60. The OFET performance of 7-9 could be tuned and the hybrids show ambipolar characteristics. Notably, the 9·C60 cocrystal exhibits well-balanced ambipolar performance with electron and hole mobilities of up to 0.19 and 0.11 cm2 V-1 s-1, respectively. This is the first demonstration of a chiral curved PAH and its complex with C60 for organic devices. Our work presents new insight into buckybowl-based design of PAHs with configurational stability and intriguing optoelectronic properties.

Chalcogen-doped, (seco)-hexabenzocoronene-based nanographenes: synthesis, properties, and chalcogen extrusion conversion

A series of chalcogen-doped nanographenes (NGs) and their oxides are described. Their molecular design is conceptually based on the insertion of different chalcogens into the hexa-peri-hexabenzocoronene (HBC) backbone. All the NGs adopt nonplanar conformations, which would show better solubility compared to planar HBC. Except for the oxygen-doped, saddle-shaped NG, the insertion of large chalcogens like sulfur and selenium leads to a seco-HBC-based, helical geometry. All the three-dimensional structures are unambiguously confirmed by single-crystal X-ray diffractometry. Their photophysical properties including UV-vis absorption, fluorescence, chiroptical, charge distribution, and orbital gaps are investigated experimentally or theoretically. The properties of each structure are significantly affected by the doped chalcogen and its related oxidative state. Notably, upon heating or adding an acid, the selenium-doped NG or its oxide undergoes a selenium extrusion reaction to afford seco-HBC or HBC quantitatively, which can be treated as precursors of hydrocarbon HBCs.

Circularly polarized luminescence of helicenes: A data-informed insight

Helicenes are an interesting scaffold for chiroptical properties and in particular circularly polarized luminescence (CPL). In this short review, we collect the luminescence (g_lum ) and absorption (g_abs ) dissymmetry factors associated to the first Cotton effect of the electronic circular dichroism (ECD) spectrum. Considering the data for 170 [n]-helicenes (n = 4-11), overall we found reasonable correlations between g_lum and g_abs . Despite a few notable exceptions, this would confirm a similarity in the stereochemistry of the ground and emitting excited states for most helicenes. These results may be useful in rationalizing chiroptical data and help chemists in designing new helicene structures with the desired CPL properties.

[1,4]Diazocine-Embedded Electron-Rich Nanographenes with Cooperatively Dynamic Skeletons

Curved nanographenes (NGs) are emerging as promising candidates for organic optoelectronics, supramolecular materials, and biological applications. Here we report a distinctive type of curved NGs bearing a [1,4]diazocine core that is fused with four pentagonal rings. This is formed by Scholl-type cyclization of two adjacent carbazole moieties through an unusual diradical cation mechanism followed by C-H arylation. Owing to the strain in the unique 5-5-8-5-5-membered ring skeleton, the resulting NG adopts an interesting concave-convex cooperatively dynamic structure. By peripheral π-extension, a helicene moiety with fixed helical chirality can be further mounted to modulate the vibration of the concave-convex structure, through which the distant bay region of the curved NG inherits the chirality of the helicene moiety in a reversed fashion. The [1,4]diazocine-embedded NGs show typical electron-rich characteristics and form charge transfer complexes with tunable emissions with a series of electron acceptors. The relatively protruding armchair edge also allows the fusion of three NGs into a C₂ symmetric triple diaza[7]helicene which reveals a subtle balance of fixed and dynamic chirality.

Donor-Acceptor Tribenzotriquinacene-Based Molecular Wizard Hats Bearing Three ortho-Benzoquinone Units

Two π-extended bay-bridged tribenzotriquinacenes ("TBTQ wizard hats") 12 and 16 bearing three mutually conjugated, alternating veratrole-type and ortho-benzoquinone units were synthesized. The electronic properties of these complementarily arranged, nonplanar push-pull systems are affected by the fusion with the rigid, C₃ -symmetric TBTQ core to a different extent, as revealed by X-ray structural analysis, UV-vis spectroscopy and cyclovoltammetry. The combination of three quinone units within the original TBTQ core and three veratrole-type bay bridging units in 12 gives rise to a more efficiently π-conjugated chromophore, as reflected by the shallower shape of wizard hat and its absorption in the visible up to 750 nm in comparison to 16. Congener 12 contains an aromatic 18-π electron system in contrast to the cross-conjugated analog 16. X-ray structure analysis of the precursor dodecaether 15 revealed the formation of a cage-like supramolecular dimer, in which the peripheral dioxane-type ether groups interlace by twelve noncovalent C-H⋅⋅⋅⋅⋅O bonds.

An Unrevealed Molecular Function of Corannulene Buckybowl Glycoconjugates in Selective Tumor Annihilation by Targeting the Cancer-Specific Warburg Effect

The biomedical application of corannulene π-bowls is historically limited by low solubility and bioavailability despite the potential in their unique electronic properties for new functional materials. Herein, the unexpected role and molecular mechanism of Corranulene π-bowls are uncovered in biomedical applications as an effective anticancer agent for Warburg effect mediated selective tumor targeting. The corannulene triazolyl monosaccharides Cor-sugars exhibit highly potent cytotoxicity against human cancer cells and effectively inhibit xenograft growth of hyperglycolytic tumors. Particularly, the galactose-conjugated Cor-gal exhibits superior in vivo anticancer efficacy in A549 tumor models with outstanding safety profile compared to doxorubicin. Moreover, the combined treatment of Cor-gal with immune checkpoint inhibitor results in an effective synergy in treating H460 human lung carcinoma. An uptake mechanism study reveals that Cor-sugars exploit tumor-specific glucose transporter glucose transporter 1 (GLUT1) for targeted cell delivery and intra-tumoral accumulation through the cancer-specific Warburg effect. Their significant anticancer activity is attributed to multiphasic DNA-binding and cell cycle alteration effects. This study uncovers new molecular properties of corannulene buckybowl and enabling their potential new applications in biomedical engineering.

Corannulene-based acenes

Synthesis of diacenes still represents a considerable challenge due to their poor stability and low solubility.

Infinitene: A Helically Twisted Figure-Eight [12]Circulene Topoisomer

New forms of molecular nanocarbon particularly looped polyarenes adopting various topologies contribute to the fundamental science and practical applications. Here we report the synthesis of an infinity-shaped polyarene, infinitene (1) (cyclo[c.c.c.c.c.c.e.e.e.e.e.e]dodecakisbenzene), comprising consecutively fused 12-benzene rings forming an enclosed loop with a strain energy of 60.2 kcal·mol^-1. Infinitene (1) represents a topoisomer of still-hypothetical [12]circulene, and its scaffold can be formally visualized as the outcome of the "stitching" of two homochiral [6]helicene subunits by both their ends. The synthetic strategy encompasses transformation of a rationally designed dithiacyclophane to cyclophadiene through the Stevens rearrangement and pyrolysis of the corresponding S,S'-bis(oxide) followed by the photocyclization. The structure of 1 is a unique hybrid of helicene and circulene with a molecular formula of C₄₈H₂₄, which can be regarded as an isomer for kekulene, [6,6]carbon nanobelt ([6,6]CNB), and [12]cyclacene. Infinitene (1) is a bench-stable yellow solid with green fluorescence and soluble to common organic solvents. Its figure-eight molecular structure was unambiguously confirmed by X-ray crystallography. The scaffold of 1 is significantly compressed as manifested by a remarkably shortened distance (3.152-3.192 Å) between the centroids of two π-π stacked central benzene rings and the closest C···C distance of 2.920 Å. Fundamental photophysical properties of 1 were thoroughly elucidated by UV-vis absorption and fluorescence spectroscopic studies and density functional theory calculations. Its configurational stability enabled separation of the corresponding enantiomers (P,P) and (M,M) by a chiral HPLC. Circular dichroism (CD) and circularly polarized luminescence (CPL) measurements revealed that 1 has moderate |g_CD| and |g_CPL| values.

Azepine- or Oxepine-embedded Double Saddle-Helix Nanographenes

The azepine- and oxepine-embedded polycyclic aromatic hydrocarbons (PAH) 1-3, as the hexa-peri-hexabenzocoronene (HBC)-based nanographenes (NG) were designed and synthesized by Diels-Alder reaction of cyclic alkene with tetrachlorothiophene-S,S-dioxide, followed by Suzuki-Miyaura cross-coupling and Scholl-type cyclodehydrogenation. Due to the strained seven-membered ring and the inherent structural pattern, heteroatom-doped NGs 1-3 show C_s symmetrical, double saddle-helix hybrid conformation, which represents a new shape for HBC based nanographenes. The calculation studies reveal the low aromaticity of the 8π heterocycles themselves and the heterocycles also decrease the electron delocalization of benzenes surrounding them. Dynamics-based calculation suggests the C_s symmetry would maintain druing the saddle-inversion process. Meanwhile, we show property perturbation by doping with different heteroatoms.

Five-Fold Symmetric Pentaindolo- and Pentakis(benzoindolo)Corannulenes: Unique Structural Dynamics Derived from the Combination of Helical and Bowl Inversions

Peripherally π-extended corannulenes bearing quintuple azahelicene units, 10 and 11, were prepared and their dynamic behaviors were studied experimentally and theoretically. The fused corannulenes were synthesized from sym-pentabromocorannulene in three steps. X-Ray diffraction analysis for 10 displayed a conformer possessing a P(M) bowl chirality and a PPMPM (PMPMM) helical chirality, which was found to be the most stable conformer(s). Variable-temperature NMR measurements of 10 and 11 revealed that their structural isomers can be interconvertible in solution, depending on the steric congestion around the helical scaffolds. Automated search for conformers in the equilibrium and transition states by Artificial Force Induced Reaction (AFIR) method revealed their interconversion networks, including bowl-inversion and helical-inversion. This analysis indicated that the co-existing corannulene and azahelicene moieties influence the conformational dynamics, which leads to mitigation of the activation energy barriers for isomerization.

Synthesis and derivatization of hetera-buckybowls

Buckybowls have concave and convex surfaces with distinct π-electron cloud distribution, and consequently they show unique structural and electronic features as compared to planar aromatic polycycles. Doping the π-framework of buckybowls with heteroatoms is an efficient scheme to tailor inherent properties, because the nature of heteroatoms plays a pivotal role in the structural and electronic characteristics of the resulting hetera-buckybowls. The design, synthesis, and derivatization of hetera-buckybowls open an avenue for obtaining fascinating organic entities not only of fundamental importance but also of promising applications in optoelectronics. In this review, we summarize the advances in hetera-buckybowl chemistry, particularly the synthetic strategies toward these scaffolds.

Effect of Fusion Manner of Concave Molecules on the Properties of Resulting Nanoboats

A boat-shaped compound, which can be viewed as the fusion of two concave molecules with crossed quinacridone and indolocarbazole throughout, was synthesized and characterized. The investigation determined that the fusion manner of two concave molecules has little influence on the molecular curvature and aromaticity when compared with its congener containing crossed indolocarbazole throughout. The situation of carbonyl groups is critical in adjusting the electronic structure and physicochemical properties due to the fixed position of nitrogen atoms.

Corannulene-based nanographene containing helical motifs

The synthesis and structural analyses of corannulene-PAH hybrids 1 with a [4] helicene subunit and 2 with a [7] helicene subunit have been reported.

Coronenohelicenes with Dynamic Chirality

The synthesis of a new type of chiral and dynamic nonplanar aromatics containing a combination of fused perylene-based coronenes and helicenes is reported. Either one or two helicene moieties were fused to the bay regions of an extended perylene core. The target compounds contain either identical or two different helicene building blocks. The combination with two helicene units leads to six different isomers, including two pairs of enantiomers and two meso forms. The experimental determination of the isomerization barriers the corresponding double [5]-helicenes revealed activation energies of Ea =24.81 and 25.38 kcal mol-1 , which is slightly above the barrier of the parent [5]-helicene. Resolution of all possible regio- and stereoisomers allowed for the systematic investigation of the chiroptical properties. They revealed remarkable dissymmetry factors Igabs I of up to 1.2×10-2 , which mirror the synergy between the strong absorbing perylenes and the inherent chirality of helicenes.

Tuning the Properties of Corannulene-Based Polycyclic Aromatic Hydrocarbons by Varying the Fusing Positions of Corannulene

The selective fusions with pyrene derivative to the rim and flank bonds of corannulene generated 4 and 7, respectively, which underwent a Scholl reaction to provide novel distorted PAHs CORA-1 and CORA-2, consisting of corannulene and dibenzocoronene units with different connections between them. The studies revealed that the properties of these PAHs are highly dependent on the fusing positions of corannulene.

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.

Helical Nanographenes Containing an Azulene Unit: Synthesis, Crystal Structures, and Properties

Three unprecedented helical nanographenes (1, 2, and 3) containing an azulene unit are synthesized. The resultant helical structures are unambiguously confirmed by X‐ray crystallographic analysis. The embedded azulene unit in 2 possesses a record‐high twisting degree (16.1°) as a result of the contiguous steric repulsion at the helical inner rim. Structural analysis in combination with theoretical calculations reveals that these helical nanographenes manifest a global aromatic structure, while the inner azulene unit exhibits weak antiaromatic character. Furthermore, UV/Vis‐spectral measurements reveal that superhelicenes 2 and 3 possess narrow energy gaps (2: 1.88 eV; 3: 2.03 eV), as corroborated by cyclic voltammetry and supported by density functional theory (DFT) calculations. The stable oxidized and reduced states of 2 and 3 are characterized by in‐situ EPR/Vis–NIR spectroelectrochemistry. Our study provides a novel synthetic strategy for helical nanographenes containing azulene units as well as their associated structures and physical properties.

Synthesis of a Tetracorannulene-perylenediimide That Acts as a Selective Receptor for C60 over C70

We report the use of a tetraborylated perylenediimide as starting material for the preparation of a tetracorannulene-perylenediimide that is able to bind up to two fullerene-C₆₀ molecules by host-guest molecular recognition with preference over C₇₀. Titration with fullerene-C₆₀ is followed by a dramatic shift of the aromatic signals in ¹H NMR and an initial increase in the fluorescence of the system. By this simple mechanism, fluorogenic sensing of fullerene-C₆₀ is easily accomplished by an unprecedented fluorescent turn-on mechanism.

Mechanical Stabilization of Helical Chirality in a Macrocyclic Oligothiophene

We introduce a design principle to stabilize helically chiral structures from an achiral tetrasubstituted [2.2]paracyclophane by integrating it into a macrocycle. The [2.2]paracyclophane introduces a three-dimensional perturbation into a nearly planar macrocyclic oligothiophene. The resulting helical structure is stabilized by two bulky substituents installed on the [2.2]paracyclophane unit. The increased enantiomerization barrier enabled the separation of both enantiomers. The synthesis of the target helical macrocycle 1 involves a sequence of halogenation and cross-coupling steps and a high-dilution strategy to close the macrocycle. Substituents tuning the energy of the enantiomerization process can be introduced in the last steps of the synthesis. The chiral target compound 1 was fully characterized by NMR spectroscopy and mass spectrometry. The absolute configurations of the isolated enantiomers were assigned by comparing the data of circular dichroism spectroscopy with TD-DFT calculations. The enantiomerization dynamics was studied by dynamic HPLC and variable-temperature 2D exchange spectroscopy and supported by quantum-chemical calculations.

Enantiopure Aromatic Saddles Bearing the Fenestrindane Core

The synthesis of enantiomerically pure, configurationally stable fenestrindane-based polyaromatic compounds with saddle-like structures is reported. Seven racemic fenestrane synthetic precursors were first screened by chiral HPLC for resolvability into enantiomers. Among the three resolvable precursors, a tribenzofenestrene derivative was resolved on a semipreparative scale, and the absolute configuration of the more slowly eluting enantiomer was established by X-ray crystallography. The enantiopure tribenzofenestrenes were then separately converted, in six steps, to the saddle-shaped fenestrindane derivatives in optically pure form. The two enantiomeric pairs of saddles were characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry, and circular dichroism spectroscopy. All new compounds reported herein represent the first enantiopure non-natural carbocyclic fenestranes isolated to date.

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.

Enantiopure distorted ribbon-shaped nanographene combining two-photon absorption-based upconversion and circularly polarized luminescence

Herein we describe a distorted ribbon-shaped nanographene exhibiting unprecedented combination of optical properties in graphene-related materials, namely upconversion based on two-photon absorption (TPA-UC) together with circularly polarized luminescence (CPL). The compound is a graphene molecule of ca. 2 nm length and 1 nm width with edge defects that promote the distortion of the otherwise planar lattice. The edge defects are an aromatic saddle-shaped ketone unit and a [5]carbohelicene moiety. This system is shown to combine two-photon absorption and circularly polarized luminescence and a remarkably long emission lifetime of 21.5 ns. The [5]helicene is responsible for the chiroptical activity while the push-pull geometry and the extended network of sp2 carbons are factors favoring the nonlinear absorption. Electronic structure theoretical calculations support the interpretation of the results.

Synthesis of distorted nanographenes containing seven- and eight-membered carbocycles

We highlight recent progress in bottom-up synthesis of well-defined distorted polyaromatic hydrocarbons with saddle shapes containing heptagonal and octagonal carbocycles.

Spiro-fused bis-hexa-peri-hexabenzocoronene

A spiro-fused hexa-peri-hexabenzocoronene dimer is first prepared, which is confirmed by X-ray crystallography, exhibiting reversible redox property.