Nanosized Ball Joints Constructed from C60 and Tribenzotriquinacene Sockets: Synthesis, Component Self-Assembly and Structural Investigations

Tridentate Lewis Acids Based on Tribenzotriquinacene Chalices

Chalice-shaped tridentate poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Stannylation of the alkyne units, attached via phenyl-spacers to the benzhydrylic positions to the TBTQ scaffold, with Me2NSnMe3 afforded the trimethyltin substituted TBTQ derivative. Replacement of these tin functions with other elements resulted in rigid boron- and aluminium-functionalised PLAs. More flexible PLAs were obtained by hydrometallation reactions of the terminal alkyne groups of 4b,8b,12b-tris((ethynyl)phenyl)tribenzotriquinacene. The resulting poly-Lewis acids were tested for their acceptor abilities in host-guest experiments with suitable bases. Preliminary tests with pyridine led to the synthesis of a large tridentate base with three pyridyl groups attached to a TBTQ backbone. Complexation of this Lewis base with the PLAs resulted in the formation of aggregates, which were studied in solution in more detail by 1H DOSY NMR experiments regarding their size. Further experiments were performed with the tridentate bases 1,4,7-trimethyl-1,4,7-triazacyclononane and tris((dimethylphosphino)methyl)phenylsilane.

Molecular bowls for inclusion complexation of toxic anticancer drug methotrexate

We describe the preparation and study of novel cavitands, molecular bowls 16+ and 26+, as good binders of the anticancer drug methotrexate (MTX). Molecular bowls are comprised of a curved tribenzotriquinacene (TBTQ) core conjugated to three macrocyclic pyridinium units at the top. The cavitands are easily accessible via two synthetic steps from hexabromo-tribenzotriquinacene in 25% yield. As amphiphilic molecules, bowls 16+ and 26+ self-associate in water by the nucleation-to-aggregation pathway (NMR). The bowls are preorganized, having a semi-rigid framework comprising a fixed bottom with a wobbling pyridinium rim (VT NMR and MD). Further studies, both experimental (NMR) and computational (DFT and MCMM), suggested that a folded MTX occupies the cavity of bowls wherein it forms π-π, C-H-π, and ion pairing intermolecular contacts but also undergoes desolvation to give stable binary complexes (μM) in water. Moreover, a computational protocol is introduced to identify docking pose(s) of MTX inside molecular bowls from NMR shielding data. Both molecular bowls have shown in vitro biocompatibility with liver and kidney cell lines (MTS assay). As bowl 26+ is the strongest binder of MTX reported to date, we envision it as an excellent candidate for further studies on the way toward developing an antidote capable of removing MTX from overdosed cancer patients.

Hexadentate poly-Lewis acids based on the bowl-shaped tribenzotriquinacene

Hexadentate poly-Lewis acids (PLA) based on the bowl-shaped tribenzotriquinacene (TBTQ) have been synthesised. The introduction of three n-propyl groups into the benzhydrylic positions of the TBTQ backbone has significantly increased the solubility of the subsequently derived compounds. Semi-flexible PLAs containing boron and aluminium were obtained by hydrometallation of the corresponding 2,3,6,7,10,11-hexaalkynyl-TBTQ. Other rigid hexadentate PLAs were synthesised by stannylation of the corresponding alkyne units with Me3SnNMe2 followed by tin-element exchange reactions. The Lewis acidity of these PLAs was investigated in host-guest experiments with pyridine. Further experiments with bidentate bases showed correlations between their flexibility, their Lewis basicity and the complexation behaviour towards the synthesised PLAs. Addition of bis((dimethylphosphino)methyl)dimethylsilane (BisPhos) to solutions of the rigid alkynyl PLAs led to the formation of 3 : 1 adducts. Single crystal X-ray diffraction was used to further elucidate the host-guest connectivtiy. In addition, a sixfold pnictogen-bonding donor was synthesised by tin-antimony exchange.

A Molecular Cage Accessed by Threefold Click Reaction of a C3v-Symmetric Triazido-Functionalized Tribenzotriquinacene

The hitherto unknown hexakis(halomethyl)-functionalized tribenzotriquinacenes (TBTQs) 9 and 10 were synthesized from the key 4b,8b,12b-tribromo-TBTQ derivative 6 by an improved route in 67% overall yield. Extension of the bowl-shaped framework of 9 or 10 by threefold condensation with propargylamine or 2-azidoethylamine afforded the corresponding TBTQ-trialkyne 11 and TBTQ-triazide 12, respectively. While attempts to construct bis-TBTQ cages, including homodimerization of 11 and heterocoupling of 11 with 12, were unsuccessful, triazide 12 was found to undergo threefold [3 + 2]-cycloaddition with 3-ethynylaniline and phloroglucinol tripropargyl ether under click chemistry conditions. The latter reaction enabled facile capping of the TBTQ bowl to give the novel cage compound 5 in 22% yield.

pH-Responsive fluorescent supramolecular nanoparticles based on tetraphenylethylene-labelled chitosan and a six-fold carboxylated tribenzotriquinacene

We synthesized a new tetraphenylethylene-modified chitosan bioconjugate, CS-TPE, that shows the aggregation-induced emission effect. It can self-assemble into fluorescent polymeric nanoparticles in an aqueous solution at pH 5.3 either alone or with the water-soluble bowl-shaped six-fold carboxylated tribenzotriquinacene derivative TBTQ-C₆ via host-guest binding. The spherical nanoparticles formed by CS-TPE amphiphiles or TBTQ-C₆/CS-TPE supra-amphiphiles disintegrated under alkaline stimulation at pH 10.4 and the dispersion of the aggregates after the collapse in the presence of TBTQ-C₆ was greatly improved. In addition, the fluorescence of CS-TPE was significantly enhanced by introducing TBTQ-C₆, and remained relatively stable with variations in pH for both CS-TPE and TBTQ-C₆/CS-TPE. Such pH-responsive supramolecular spherical nanoparticles with stable fluorescence emission based on CS-TPE or TBTQ-C₆/CS-TPE may find applications in various fields, including the development of visual oral drug delivery systems.

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.

Synthesis of a new water-soluble hexacarboxylated tribenzotriquinacene derivative and its competitive host–guest interaction for drug delivery

A new water-soluble hexacarboxylated tribenzotriquinacene derivative (TBTQ-CB6) was synthesized and used as a supramolecular drug carrier to load the model anticancer drugs dimethyl viologen (MV) and doxorubicin (DOX) via host–guest interactions. The drugs could be effectively released by spermine (SM), a molecule overexpressed in cancer cells, through host–guest competitive substitution since TBTQ-CB6 has a stronger binding affinity toward SM than MV and DOX. The host–guest interactions of the complexes of TBTQ-CB6 with MV, DOX and SM were investigated by NMR spectroscopy and fluorescence spectroscopy. The association stoichiometry of the complexes of TBTQ-CB6 with MV, DOX, and SM was found to be 1:1 with association constants of K_a = (7.67 ± 0.34) × 10⁴ M⁻¹, K_a = (6.81 ± 0.33) × 10⁴ M⁻¹, and K_a = (5.09 ± 0.98) × 10⁵ M⁻¹, respectively. The competitive substitution process was visualized by NMR titration. This novel TBTQ-based host–guest drug delivery system may have potential use in supramolecular chemotherapy.

Rational Molecular Design of Azaacene-Based Narrowband Green-Emitting Fluorophores: Modulation of Spectral Bandwidth and Vibronic Transitions

A series of green-emitting fluorophores based on a tetra-azaacene core is synthesized by introducing nitrile substituents at different positions. Their molecular structure-optical property relationship [i.e., vibronic transitions in photoluminescence (PL) and electroluminescence (EL) spectra] is investigated to obtain a sharp emission where the vibronic peak ν_0-0 should be intensified by suppressing ν_0-n (n = 1, 2, 3...) transitions. The intensity ratios (I_0-1/I_0-0) of the ν_0-1 and ν_0-0 vibronic transitions in the PL spectra of DBBNP, DBBNP2CN1, and DBBNP2CN2 in hexane are 1.13, 0.80, and 0.67, respectively. Theoretical calculations explain that the CN substitution at positions 8 and 13 in DBBNP2CN2 induces a uniform charge distribution and reduces the Huang-Rhys factors (HRFs) of the vibrational normal modes coupled to the electronic transition. The organic light-emitting diode (OLED) fabricated with DBBNP2CN2 shows a narrower green EL emission at 518 nm with a smaller bandwidth (50 nm) than those of devices adopting DBBNP or DBBNP2CN1. The careful modification of the molecular structures and positions of substituents enables us to reduce the HRFs of vibrations to achieve a narrow emission bandwidth with decreased I_0-1/I_0-0, which suggests a design strategy to develop narrowband organic fluorophores to improve the color purity for wide-gamut OLED displays.

Synthesis of Enantiopure Hydrocarbon Cages Based on an Optically Resolved C3-Symmetric Triaminotribenzotriquinacene

The synthesis of the enantiomerically pure, D₃-symmetric covalent hydrocarbon cages (+)-(M,M)-4 and (-)-(P,P)-4 bearing two C₃-symmetrically functionalized tribenzobenzotriquinacene (TBTQ) vertices is reported. The enantiomerically pure TBTQ building blocks (+)-(M)-5 and (-)-(P)-5 were prepared via the diastereomeric TBTQ triamides obtained by use of both Boc-d- and Boc-l-phenylglycine as chiral auxiliaries.

Water-soluble host-guest complexes between fullerenes and a sugar-functionalized tribenzotriquinacene assembling to microspheres

A sugar-functionalized water-soluble tribenzotriquinacene derivative bearing six glucose residues, TBTQ-(OG) 6 , was synthesized and its interaction with C60 and C70-fullerene in co-organic solvents and aqueous solution was investigated by fluorescence spectroscopy and ultraviolet-visible spectroscopy. The association stoichiometry of the complexes TBTQ-(OG) 6 with C60 and TBTQ-(OG) 6 with C70 was found to be 1:1 with binding constants of K a = (1.50 ± 0.10) × 105 M-1 and K a = (2.20 ± 0.16) × 105 M-1, respectively. The binding affinity between TBTQ-(OG) 6 and C60 was further verified by Raman spectroscopy. The geometry of the complex of TBTQ-(OG) 6 with C60 deduced from DFT calculations indicates that the driving force of the complexation is mainly due to the hydrophobic effect and to host-guest π-π interactions. Hydrophobic surface simulations showed that TBTQ-(OG) 6 and C60 forms an amphiphilic supramolecular host-guest complex, which further assembles to microspheres with diameters of 0.3-3.5 μm, as determined by scanning electron microscopy.

A covalent organic cage compound acting as a supramolecular shadow mask for the regioselective functionalization of C60

A trigonal-bipyramidal covalent organic cage compound serves as an efficient host to form stable 1 : 1-complexes with C₆₀ and C₇₀. Fullerene encapsulation has been comprehensively studied by NMR and UV/Vis spectroscopy, mass spectrometry as well as single-crystal X-ray diffraction. Exohedral functionalization of encapsulated C₆₀via threefold Prato reaction revealed high selectivity for the symmetry-matched all-trans-3 addition pattern.

The taming of the Prato reaction: a covalent organic cage compound serves as a supramolecular template for the regioselective functionalization of C₆₀.

Highly stable fullerene-based porous molecular crystals with open metal sites

The synthesis of conventional porous crystals involves building a framework using reversible chemical bond formation, which can result in hydrolytic instability. In contrast, porous molecular crystals assemble using only weak intermolecular interactions, which generally do not provide the same environmental stability. Here, we report that the simple co-crystallization of a phthalocyanine derivative and a fullerene (C₆₀ or C₇₀) forms porous molecular crystals with environmental stability towards high temperature and hot aqueous base or acid. Moreover, by using diamond anvil cells and synchrotron single-crystal measurements, stability towards extreme pressure (>4 GPa) is demonstrated, with the stabilizing fullerene held between two phthalocyanines and the hold tightening at high pressure. Access to open metal centres within the porous molecular co-crystal is demonstrated by in situ crystallographic analysis of the chemisorption of pyridine, oxygen and carbon monoxide. This suggests strategies for the formation of highly stable and potentially functional porous materials using only weak van der Waals intermolecular interactions.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

One-electron oxidation of the compound shown shows no evidence for intervalence charge transfer in the macrocylic ligand radical product. In contrast, related [{Pt(L)}₃(μ₃-ctc˙)]⁺ (H₆ctc = cyclotricatechylene), exhibits class II mixed valency.

Switching the Statistical C3 /C1 Ratio in the Threefold Aromatic Substitution of Tribenzotriquinacenes towards the C3 Isomer

Tribenzotriquinacene (TBTQ) is a bowl-shaped molecule that has been widely used as a molecular building block in supramolecular and materials chemistry. Especially C₃ -symmetric threefold-substituted TBTQs are interesting for these purposes. Until now a general and selective synthetic approach to those C₃ -symmetric products was lacking, mainly because the typically used electrophilic aromatic substitution reactions of the parent TBTQ hydrocarbons produce predominantly the C₁ isomer over the C₃ isomer (3:1 statistical ratio). Herein we introduce a threefold borylation of TBTQ with the C₃ isomer as the main product (2.6:1 C₃ /C₁ ratio). The borylated TBTQ can be converted in good yields into other C₃ -symmetric TBTQs, thus allowing straightforward synthetic access to new building blocks for supramolecular and materials chemistry.

A Polyaromatic Nano-nest for Hosting Fullerenes C60 and C70

A "Janus" type of hexa- cata-hexabenzocoronene with three triptyceno subunits fused symmetrically on the periphery of coronene has been synthesized using a covalent self-assembly strategy. The triptyceno subunits form a nanosized nest on one side of the aromatic plane with space-matching fullerenes such as C₆₀ and C₇₀ to afford shape-complementary supramolecular complexes. The formation of the complexes in solution was confirmed by ¹H NMR and fluorescence titration. Four complexes with C₆₀ or C₇₀ were obtained and studied by single-crystal X-ray diffraction analysis. In the crystal structure, the host shows a proper tunability to adjust its conformation in accordance with the shape of the guest. The different stoichiometric ratios and various stacking patterns of the complexes suggest the diversity of this nonplanar polyaromatic host in complexation with fullerenes.

Biconcave and Convex-Concave Tribenzotriquinacene Dimers

A new chiral tribenzotriquinacene bearing an ortho-bromoaniline nucleus was synthesized and optically resolved. The individual enantiomers, the absolute configuration of which was confirmed by single-crystal X-ray structure analysis, were stereoselectively converted into the same pyrazine-fused syn-bis-TBTQ derivative by chirality-assisted Buchwald-Hartwig amination. The corresponding diastereomeric anti-dimer was obtained alongside the syn-dimer from the racemic sample under similar reaction conditions. X-ray structure analysis of the dimers confirmed the mutual biconcave and convex-concave configuration of their TBTQ moieties and the preservation of the orthogonal orientation of the indane wings within each of them.

Porphyrin Arch-Tapes: Synthesis, Contorted Structures, and Full Conjugation

Porphyrin tapes possessing meso-meso β-β β-β triple direct linkages have been targets of extensive studies because of their fully conjugated characteristic π-electronic networks. In this paper, we report porphyrin arch-tapes that bear additional carbonyl group(s) or methylene group(s) inserted between one of the β-β linkage(s) of the porphyrin tapes. The carbonyl-inserted porphyrin arch-tapes were efficiently synthesized by double fusion reactions of β-to-β carbonyl-bridged porphyrin oligomers with DDQ and Sc(OTf)₃, and were converted to the methylene-bridged porphyrin arch-tapes via Luche reduction with NaBH₄ and CeCl₃ followed by ionic hydrogenation with HBF₄·OEt₂ and BH₃·NEt₃. While the conventional porphyrin tapes display rigid and planar structures and low solubilities, these porphyrin arch-tapes show remarkably contorted structures, flexible conformations, and improved solubilities because of the presence of the incorporated seven-membered ring(s). Interestingly, the methylene-inserted arch-tapes exhibited conjugative electronic interactions that were comparable to those of porphyrin tapes probably owing to through-space interaction in the contorted conformations. The carbonyl-inserted arch-tapes displayed distinctly larger conjugative interactions owing to an active involvement of the carbonyl group(s) in the electronic conjugation. A similar trend was observed in the nonlinear optical properties, as evidenced by their two-photon absorption cross sections. Furthermore, as a benefit of the contorted structures, these porphyrin arch-tapes can catch C₆₀ fullerene effectively. Naturally, the electron-rich methylene-bridged arch-tapes exhibited larger association constants than the electron-deficient carbonyl-bridged arch-tapes. Among these arch-tapes, a methylene-bridged syn-Ni(II) porphyrin trimer recorded the largest association constant of (1.5 ± 0.4) × 10⁷ M^-1 in toluene at 25 °C.

Poly-Lewis-acids based on bowl-shaped tribenzotriquinacene

Tribenzotriquinacene is a versatile bowl-shaped hydrocarbon and a useful building block for the synthesis of poly-Lewis-acids. First derivatives bearing Lewis-acidic groups are reported.

β,β-(1,4-Dithiino)subporphyrin Dimers Capturing Fullerenes with Large Association Constants

β,β-(1,4-Dithiino)subporphyrin dimers 7-syn and 7-anti were synthesized by the nucleophilic aromatic substitution reaction of 2-bromo-3-(4-methoxyphenylsulfonyl)subporphyrin 4 with 2,3-dimercaptosubporphyrin 5 under basic conditions followed by axial arylation. Additions of C60 or C70 to a dilute solution of 7-anti (ca. 10(-6)  m) in toluene did not cause appreciable UV/Vis spectral changes, while similar additions to a concentrated solution (ca. 10(-3)  m) resulted in precipitation of complexes. In contrast, dimer 7-syn captured C60 and C70 in different complexation stoichiometries in toluene; a 1:1 manner and a 2:1 manner, respectively, with large association constants; Ka =(1.9±0.2)×10(6)  m(-1) for C60 @7-syn, and K1 =(1.6±0.5)×10(6) and K2 =(1.8±0.9)×10(5)  m(-1) for C70 @(7-syn)2 . These association constants are the largest for fullerenes-capture by bowl-shaped molecules reported so far. The structures of C60 @7-anti, C70 @7-anti, C60 @7-syn, and C70 @7-syn have been determined by single-crystal X-ray diffraction analysis.

Centrohexaindane: six benzene rings mutually fixed in three dimensions - solid-state structure and six-fold nitration

The solid-state molecular structure of centrohexaindane (), a unique hydrocarbon comprising six benzene rings clamped to each other in three dimensions around a neopentane core, and the molecular packing in crystals of ·CHCl3 are reported. The molecular Td-symmetry and the Cartesian orientation of the six indane wings of in the solid state have been confirmed. The course and limitation of electrophilic aromatic substitution of are demonstrated for the case of nitration. Based on nitration experiments of a lower congener of , tribenzotriquinacene , the six-fold nitrofunctionalisation of has been achieved in excellent yield, giving four constitutional isomers, two nonsymmetrical ( and ) and two C3-symmetrical ones ( and ), all of which contain one single nitro group in each of the six benzene rings. The relative yields of the four isomers (∼3 : 1 : 1 : 3) point to a random electrophilic attack of the electrophiles at the twelve formally equivalent outer positions of the aromatic periphery of , suggesting electronic independence of its six aromatic π-electron systems. In turn, the pronounced conformational rigidity of the centrohexacyclic framework of enables the unequivocal structural identification of the isomeric hexanitrocentrohexaindanes by (1)H NMR spectroscopy.

Tuning the depth of bowl-shaped phosphine hosts: capsule and pseudo-cage architectures in host-guest complexes with C60 fullerene

Bowl-shaped phosphine molecules, whose bowl geometry can be controlled by a variation of the axial substituent, were synthesized, and used as host molecules to encapsulate C60. Host molecules with relatively shallow bowls formed a chiral capsule, while hosts with deeper bowls formed an achiral pseudo-cage.

Twofold fused concave hosts containing two phosphorus atoms: modules for the sandwich-type encapsulation of fullerenes in variable cavities

The design and synthesis of extended concave host P2 by fusion of two concave phosphorus-containing units is reported. Co-crystallization of P2 and the fullerene guests C₆₀ and C₇₀ afforded the 2 : 1 host-guest complexes (P2)₂ ⊃ C₆₀ and (P2)₂ ⊃ C₇₀, in which the two concave surfaces of P2 encapsulate the convex surface of the fullerenes in a sandwich fashion. Interestingly, the orientation of the two P2 molecules with respect to each other was observed to be flexible, resulting in the formation of a variety of cavity shapes. MALDI-TOF mass, NMR, and UV-vis absorption spectra supported the formation of host-guest complexes between P2 and the fullerenes in solution. The affinity of P2, containing two phosphorus atoms, towards fullerenes was significantly enhanced relative to P1 with one phosphorus atom.

Almost enclosed buckyball joints: synthesis, complex formation, and computational simulations of pentypticene-extended tribenzotriquinacene

We report the synthesis of a tribenzotriquinacene-based (TBTQ) receptor (3) for C60 fullerene, which is extended by pentiptycene moieties to provide an almost enclosed concave ball bearing. The system serves as a model for a self-assembling molecular rotor with a flexible and adapting stator. Unexpectedly, nuclear magnetic resonance spectroscopic investigations reveal a surprisingly low complex stability constant of K1 =213±37 M(-1) for [C60 ⊂3], seemingly inconsistent with the previously reported TBTQ systems. Molecular dynamics (MD) simulations have been conducted for three different [C60 ⊂TBTQ] complexes to resolve this. Because of the dominating dispersive interactions, the binding energies increase with the contact area between guest and host, however, only for rigid host structures. By means of free-energy calculations with an explicit solvent model it can be shown that the novel flexible TBTQ receptor 3 binds weakly because of hampering entropic contributions.

Phosphorus-containing chiral molecule for fullerene recognition based on concave/convex interaction

A C3-symmetric chiral concave molecule having a phosphorus atom at the center was synthesized, and its enantiomers were resolved. The chiral concave shape and absolute structure of the concave molecules were revealed by X-ray analysis. The concave molecule exhibited intense chiroptical properties with a large anisotropy, which was derived from molecular orbitals delocalized to the side chains. In the co-crystal with pristine C60, four of the enantiopure concave molecules perfectly wrapped the surface of C60. MALDI-TOF mass, NMR, and circular dichromism spectra also supported the concave/convex interaction between the concave molecule and fullerene. These results suggest that the phosphorus-containing molecule with a concave shape plays an important role as a chiral host molecule for C60.

C3-symmetrical tribenzotriquinacene derivatives: optical resolution through cryptophane synthesis and supramolecular self-assembly into nanotubes

Based on a regioselective tris-formylation of a tribenzotriquinacene (TBTQ) hydrocarbon, the racemic C(3)-symmetrical TBTQ-trialdehyde and the corresponding TBTQ-trimethanol were synthesized along with their C(1)-isomers. Conversion of the C(3)-trialdehyde to three diastereomeric TBTQ-based cryptophanes occurring in high yield enabled the preparation of the optically pure C(3)-symmetrical TBTQ-trialdehydes and the determination of their absolute configuration. The racemic C(3)-symmetrical TBTQ-trimethanol was found to form several stable nanotubular aggregates in the solid state.