Investigation of Cycloparaphenylenes (CPPs) and their Noncovalent Ring-in-Ring and Fullerene-in-Ring Complexes by (Matrix-Assisted) Laser Desorption/Ionization and Density Functional Theory

[n]Cycloparaphenylenes ([n]CPPs) with n=5, 8, 10 and 12 and their noncovalent ring-in-ring and [m]fullerene-in-ring complexes with m=60, 70 and 84 have been studied by direct and matrix-assisted laser desorption ionization ((MA)LDI) and density-functional theory (DFT). LDI is introduced as a straightforward approach for the sensitive analysis of CPPs, free from unwanted decomposition and without the need of a matrix. The ring-in-ring system of [[10]CPP⊃[5]CPP]^+. was studied in positive-ion MALDI. Fragmentation and DFT indicate that the positive charge is exclusively located on the inner ring, while in [[10]CPP⊃C₆₀]^+. it is located solely on the outer nanohoop. Positive-ion MALDI is introduced as a new sensitive method for analysis of CPP⊃fullerene complexes, enabling the detection of novel complexes [[12]CPP⊃C_{60, 70 and 84}]^+. and [[10]CPP⊃C₈₄]^+.. Selective binding can be observed when mixing one fullerene with two CPPs or vice versa, reflecting ideal size requirements for efficient complex formation. Geometries, binding and fragmentation energies of CPP⊃fullerene complexes from DFT calculations explain the observed fragmentation behavior.     

[n]Cycloparaphenylene-Pillar[5]arene Bismacrocycles: Their Circularly Polarized Luminescence and Multiple Guest Recognition Properties

Both pillar[n]arenes (P[n]As) and [n]cycloparaphenylenes ([n]CPPs) play an important role in supramolecular chemistry. Herein, we report the precise synthesis of two multifunctional bismacrocycles [n]CPP-P[5]A by integrating P[5]A into the [n]CPP backbone. The photoluminescence quantum yield (Φ_F) of the bismacrocycles was found to show a dramatic increase relative to the corresponding [n]CPPs. The chiral enantiomers (pR)/(pS)-[8]CPP-P[5]A were successfully isolated by chiral HPLC, and showed promising properties of circularly polarized luminescence (g_lum≈0.02). In addition, [n]CPP-P[5]A bismacrocycles are capable of binding pyridinium salts and fullerene derivatives with high affinity and specificity within the two distinct cavities. Transient absorption studies showed that photo-induced electron transfer occurs in [10]CPP-P[5]A⊃C₆₀ complex. Our results suggest that [n]CPP-P[5]A are potentially useful in CPL-active materials, multiple guest recognition and supramolecular polymer preparation.     

Structural deformation and host–guest properties of doubly-reduced cycloparaphenylenes, [n]CPPs2− (n = 6, 8, 10, and 12)

Chemical reduction of several cycloparaphenylenes (CPPs) ranging in size from [8]CPP to [12]CPP has been investigated with potassium metal in THF. The X-ray diffraction characterization of the resulting doubly-reduced [n]CPPs provided a unique series of carbon nanohoops with increasing dimensions and core flexibility for the first comprehensive structural analysis. The consequences of electron acquisition by a [n]CPP core have been analyzed in comparison with the neutral parents. The addition of two electrons to the cyclic carbon framework of [n]CPPs leads to the characteristic elliptic core distortion and facilitates the internal encapsulation of sizable cationic guests. Molecular and solid-state structure changes, alkali metal binding and unique size-dependent host abilities of the [n]CPP²⁻ series with n = 6–12 are discussed. This in-depth analysis opens new perspectives in supramolecular chemistry of [n]CPPs and promotes their applications in size-selective guest encapsulation and chemical separation.

The series of doubly-reduced cycloparaphenylenes (CPPs) with increasing dimensions and flexibility shows the size-dependent structural changes and enhanced host abilities.     

Intra‐ and Intermolecular Dispersion Interactions in [n]Cycloparaphenylenes: Do They Influence Their Structural and Electronic Properties?

Cycloparaphenylenes (CPPs) are nanosized structures with unique isolated and bulk properties, and are synthetic targets for the template‐driven bottom‐up synthesis of carbon nanotubes. Thus, a systematic understanding of the supramolecular order at the nanoscale is of utmost relevance for molecular engineering. In this study, it is found that intramolecular noncovalent (dispersion) interactions must be taken into account for obtaining accurate estimates of the structural and optoelectronic properties of [n]CPP compounds, and their influence as the number of repeat units increases from n=4 to n=12 is also analyzed, both in the gas phase and in solution. The supramolecular self‐assembly, for which both intra‐ and intermolecular noncovalent interactions are relevant, of [6]CPP is also investigated by calculating the binding energies of dimers taken along several crystal directions. These are also used to estimate the cohesive energy of the crystal, which is compared to the value obtained by means of dispersion‐corrected DFT calculations using periodic boundary conditions. The reasonable agreement between both computational strategies points towards a first estimate of the [6]CPP cohesive energy of around 50 kcal mol^?1.     

Electronic Communication between two [10]cycloparaphenylenes and Bis(azafullerene) (C59N)2 Induced by Cooperative Complexation

The complex of [10]cycloparaphenylene ([10]CPP) with bis(azafullerene) (C₅₉N)₂ is investigated experimentally and computationally. Two [10]CPP rings are bound to the dimeric azafullerene giving [10]CPP?(C₅₉N)₂?[10]CPP. Photophysical and redox properties support an electronic interaction between the components especially when the second [10]CPP is bound. Unlike [10]CPP?C₆₀, in which there is negligible electronic communication between the two species, upon photoexcitation a partial charge transfer phenomenon is revealed between [10]CPP and (C₅₉N)₂ reminiscent of CPP‐encapsulated metallofullerenes. Such an alternative electron‐rich fullerene species demonstrates C₆₀‐like ground‐state properties and metallofullerene‐like excited‐state properties opening new avenues for construction of functional supramolecular architectures with organic materials.     

Synthesis,Characterization, and Computational Investigation of Bright Orange‐Emitting Benzothiadiazole [10]Cycloparaphenylene

Conjugated aromatic macrocycles are attractive due to their unique photophysical and optoelectronic properties. In particular, the cyclic radially oriented π‐system of cycloparaphenylenes (CPPs) gives rise to photophysical properties unlike any other small molecule or carbon nanomaterial. CPPs have tunable emission, possess large extinction coefficients, wide effective Stokes shifts, and high quantum yields. However, accessing bright CPPs with emissions beyond 500 nm remains difficult. Herein, we present a novel and bright orange‐emitting CPP‐based fluorophore showing a dramatic 105 nm red‐shift in emission and striking 237 nm effective Stokes shift while retaining a large quantum yield of 0.59. We postulate, and experimentally and theoretically support, that the quantum yield remains large due to the lack of intramolecular charge transfer.     

Noncovalent Modification of Cycloparaphenylene by Catenane Formation Using an Active Metal Template Strategy**

The active metal template (AMT) strategy is a powerful tool for the formation of mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes, allowing the synthesis of a variety of MIMs, including π-conjugated and multicomponent macrocycles. Cycloparaphenylene (CPP) is an emerging molecule characterized by its cyclic π-conjugated structure and unique properties. Therefore, diverse modifications of CPPs are necessary for its wide application. However, most CPP modifications require early stage functionalization and the direct modification of CPPs is very limited. Herein, we report the synthesis of a catenane consisting of [9]CPP and a 2,2′-bipyridine macrocycle as a new CPP analogue that contains a reliable synthetic scaffold enabling diverse and concise post-modification. Following the AMT strategy, the [9]CPP-bipyridine catenane was successfully synthesized through Ni-mediated aryl-aryl coupling. Catalytic C−H borylation/cross-coupling and metal complexation of the bipyridine macrocycle moiety, an effective post-functionalization method, were also demonstrated with the [9]CPP-bipyridine catenane. Single-crystal X-ray structural analysis revealed that the [9]CPP-bipyridine catenane forms a tridentated complex with an Ag ion inside the CPP ring. This interaction significantly enhances the phosphorescence lifetime through improved intermolecular interactions.     

Synthesis of C60/[10]CPP-Catenanes by Regioselective,Nanocapsule-Templated Bingel Bis-Addition

The addition of two unsymmetric malonate esters to the Buckminster fullerene C₆₀ can lead to 22 spectroscopically distinguishable isomeric products and therefore represents a formidable synthesis challenge. In this work, we achieve 87 % selectivity for the formation of a single (in,out-trans-3) isomer by combining three approaches: (i) we use a starting material, in which the two malonates are covalently connected (tether approach); (ii) we form the strong supramolecular complex of C₆₀ with the shape-persistent [10]CPP macrocycle (template approach) and (iii) we embed this complex further within a self-assembled nanocapsule (shadow mask approach). Variation of the spacer chain shed light on the limitations of the approach and the ring dynamics in the unusual [2]catenanes were studied in silico with atomistic resolution. This work significantly widens the scope of mechanically interlocked architectures comprising cycloparaphenylenes (CPP).     

Energies of charge transfer and supramolecular interactions of some mono O-substituted calix[6]arenes with [60]fullerene by absorption spectrometric method

Equilibria for the formation of supramolecular complexes of [60]fullerene with a series of mono O-substituted calix[6]arenes, namely: (i) 37-methoxy-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (1), (ii) 37-allyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (2), (iii) 37-phenacyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (3), (iv) 37-ethylester-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (4) and (v) 37-benzyl-38,39,40,41,42-pentahydroxy-5,11,17,23,29,35-hexa(4-tert-butyl)calix[6]arene (5) have been studied in CCl4 medium by absorption spectroscopic technique. The stoichiometry has been found to be 1:1 ([60]fullerene:calix[6]arene) in each case. An absorption band due to charge transfer (CT) transition is observed in each case in the visible region. The vertical ionisation potentials (I(D)(v)) of all the calix[6]arenes under study have been estimated utilising CT transition energy. The experimental I(D)(v) values also yield a good estimate of the electron affinity of [60]fullerene. The degrees of CT in the ground state of the complexes have been found to be very low (about 0.15%). Resonance energy of the complexes have been estimated. Thermodynamic parameters for the supramolecular complex formation of [60]fullerene with mono O-substituted calix[6]arene receptors are reported. It is observed that among the calix[6]arenes under the present study, only 1 and 4 form inclusion complexes with [60]fullerene. This has also been substantiated by theoretical calculation using PM3 method. Thus presence of one substituent group (of different types) on the lower rim of the calix[6]arene molecule has been shown to govern the host-guest complexation process.     

A Supramolecular Photosynthetic Model Made of a Multiporphyrinic Array Constructed around a C60 Core and a C60–Imidazole Derivative

The photophysical properties of a supramolecular fullerene–porphyrin ensemble resulting from the self‐assembly of a pyrrolidinofullerene–imidazole derivative ( F1 ) with a multimetalloporphyrin array constructed around a hexasubstituted fullerene core ( F(ZnP)₁₂ ) have been investigated. The fullerene hexa‐adduct core of the host system does not play any active role in the cascade of photoinduced events of the supramolecular ensemble, indeed no intercomponent photoinduced processes could be observed in host F(ZnP)₁₂ . In contrast, upon axial coordination with the monosubstituted fullerene guest F1 , a quantitative quenching of the fluorescence signal of the metalloporphyrins was observed for the supramolecular complex [F(ZnP)₁₂(F1) _n ] both in polar and nonpolar solvents. In toluene, the supramolecular ensemble exhibits a charge transfer emission centered around 930 nm, suggesting the occurrence of intramolecular face‐to‐face interactions of F1 with neighboring metalloporphyrin moieties within the self‐assembled photoactive array. This mechanism is supported by the fact that a one order of magnitude increase in the binding constant was observed for the supramolecular complex [F(ZnP)₁₂(F1) _n ] when compared with a reference system lacking the pyrrolidinofullerene unit. In benzonitrile, a long‐lived charge‐separated state (τ=0.3 μs) has been detected for the supramolecular adduct.     

Combined experimental and theoretical studies on the photophysical properties of cycloparaphenylenes

We studied the UV-vis absorption and fluorescence in solution/solid states of [n]cycloparaphenylene ([n]CPP: n = 9, 12, 14, 15, and 16), and conducted theoretical studies to better understand the experimental results. The representative experimental findings include (i) the most intense absorption maxima (λ(abs1)) display remarkably close values (338-339 nm), (ii) the longest-wavelength absorption maxima (λ(abs2)) are blue-shifted with increasing the ring size (395 → 365 nm), (iii) the emission maxima (λ(em)) are blue-shifted with increasing the ring size (494 → 438 nm for longest-wavelength maxima), (iv) the fluorescent quantum yields (Φ(F)) in solution are high (0.73-0.90), (v) the fluorescence lifetimes (τ(s)) of [9]- and [12]CPP are 10.6 and 2.2 ns, respectively, and (vi) the Φ(F) values slightly increase in polymer matrix but significantly decrease in the crystalline state. According to TD-DFT calculations, the longest-wavelength absorption (λ(abs2)) corresponds to a forbidden HOMO → LUMO transition and the most intense absorption (λ(abs1)) corresponds to degenerate HOMO - 1 → LUMO and HOMO → LUMO + 1 transitions with high oscillator strength. The interesting and counterintuitive optical properties of CPPs (constant λ(abs1) and blue shift of λ(abs2)) could be ascribed mainly to the ring-size effect in frontier molecular orbitals (in particular the increase of the HOMO-LUMO gap as the number of benzene rings increases). On the basis of comparative calculations using hypothetical model geometries, we conclude that the unique behavior of HOMO and LUMO of CPPs is due mainly to their lack of a conjugation length dependence in combination with a significant bending effect (particularly to HOMO) and a torsion effect (particularly to LUMO).     

Cycloparaphenylenes and Their Catenanes: Complex Macrocycles Unveiled by Ion Mobility Mass Spectrometry

The insoluble product mixture obtained from cycloparaphenylene (CPP) synthesis from Suzuki coupling and reductive aromatization was analyzed. Traditional mass spectrometry suggests a homologous series of macrocycles with 12 to 84 phenylene units. Ion‐mobility mass spectrometry, however, unravels an unexpected complexity of isomers with identical chemical formula, but different topologies. Whereas macrocycles containing up to 30 phenylene units show only one structure, the homologue with 36 phenylene units forms at least four different isomers with significant molecular size differences. They can be assigned to catenanes composed of CPPs with 2×18 and 12+24 phenylene units together with the ordinary [36]CPP macrocycle. Most likely, a trefoil knot of the CPP with 36 moieties is also present. For the first time, catenanes can be elucidated in a simple reaction mixture by analyzing their ions in the gas phase, an analysis which lies beyond the scope of traditional analytical methods.     

Partial Charge Transfer in the Shortest Possible Metallofullerene Peapod,La@C82⊂[11]Cycloparaphenylene

[11]Cycloparaphenylene ([11]CPP) selectively encapsulates La@C₈₂ to form the shortest possible metallofullerene–carbon nanotube (CNT) peapod, La@C₈₂?[11]CPP, in solution and in the solid state. Complexation in solution was affected by the polarity of the solvent and was 16 times stronger in the polar solvent nitrobenzene than in the nonpolar solvent 1,2‐dichlorobenzene. Electrochemical analysis revealed that the redox potentials of La@C₈₂ were negatively shifted upon complexation from free La@C₈₂. Furthermore, the shifts in the redox potentials increased with polarity of the solvent. These results are consistent with formation of a polar complex, (La@C₈₂)^δ??[11]CPP^δ+, by partial electron transfer from [11]CPP to La@C₈₂. This is the first observation of such an electronic interaction between a fullerene pea and CPP pod. Theoretical calculations also supported partial charge transfer (0.07) from [11]CPP to La@C₈₂. The structure of the complex was unambiguously determined by X‐ray crystallographic analysis, which showed the La atom inside the C₈₂ near the periphery of the [11]CPP. The dipole moment of La@C₈₂ was projected toward the CPP pea, nearly perpendicular to the CPP axis. The position of the La atom and the direction of the dipole moment in La@C₈₂?[11]CPP were significantly different from those observed in La@C₈₂?CNT, thus indicating a difference in orientation of the fullerene peas between fullerene–CPP and fullerene–CNT peapods. These results highlight the importance of pea–pea interactions in determining the orientation of the metallofullerene in metallofullerene–CNT peapods.     

A Modular Synthesis of Substituted Cycloparaphenylenes

Herein, we report a modular synthesis providing access to substituted cycloparaphenylenes (CPPs) of different sizes. A key synthon introducing two geminal ester units was efficiently prepared by [2+2+2] cycloaddition. This building block can be conveniently converted to macrocyclic precursors controlling the ring size of the final CPP. Efficient reductive aromatization through single-electron transfer provided the substituted nanohoops in a straightforward manner. The tBu ester substitution pattern enables a tube-like arrangement in the solid-state governed by van der Waals interactions that exhibits one of the tightest packings of CPPs in tube direction, thus opening new avenues in the crystal design of CPPs.     

Intracellular uptake and phototoxicity of 3(1),3(2)-didehydrophytochlorin-fullerene hexaadducts

C(60)-fullerene derivatives are potential building blocks in modular carrier systems for selective tumor targeting. In [5:1] fullerene hexakis adducts, one position can be occupied by an addressing unit (e.g. monoclonal antibody) while the other five positions are suitable for dendrimers or spacers loaded with several drug moieties. This article reports intracellular uptake and phototoxicity of three fullerene hexakis adducts coupled with a different number of photosensitizers: a bis(3(1),3(2)-didehydrophytochlorin)-fullerene [5:1]-hexaadduct (FHP1), a fullerene [5:1]-hexaadduct with six 3(1),3(2)-didehydrophytochlorin groups (FHP6) and a fullerene [6:0]-hexaadduct that carries 12 3(1),3(2)-didehydrophytochlorin units (FHP12). The most promising complex, the hexa-3(1),3(2)-didehydrophytochlorin fullerene hexaadduct FHP6, was also compared with its fullerene-free analogous derivative P6. It was found that the extent of intracellular uptake is influenced by both nanomolecular size and asymmetry (amphiphilicity) of the fullerene complexes. The degree and mechanism of phototoxicity was found to depend on intracellular concentrations and singlet oxygen quantum yields.     

The Precise Synthesis of Phenylene‐Extended Cyclic Hexa‐peri‐hexabenzocoronenes from Polyarylated [n]Cycloparaphenylenes by the Scholl Reaction

The longitudinal extension of cycloparaphenylenes (CPP) towards ultrashort carbon nanotubes (CNTs) is essential for the solution based bottom‐up synthesis of CNTs. Herein, the longitudinal extension of the CPP skeleton by the introduction of hexaphenylbenzene units towards polyarylated [n]CPPs is described. Further, the applicability of the Scholl reaction to selectively form graphenic sidewalls is demonstrated. The ring size and substitution patterns of the polyarylated [n]CPPs were varied to overcome strain‐induced side reactions during the oxidative cyclodehydrogenation and cyclic para‐hexa‐peri‐hexabenzocoronene trimers ([3]CHBCs) were selectively obtained. This concept is envisioned as an access to ultrashort carbon nanotubes subject to the condition that further benzene rings with the right connectivity will be inserted.     

Solvent-free self-assembly of C60 and cucurbit[7]uril using high-speed vibration milling

The synthesis of a cucurbit[7]uril-[60]fullerene supramolecular complex using a simple, green, and efficient pathway is reported for the first time. In the complex, which was found to be of the 1:2 type, the compounds interact weekly with each other. Since the complexation can be achieved by a solid-solid reaction without solvent both the waste and contact with harmful solvents can be reduced to a minimum. A significant increase of reaction rate and yield compared to the heterogeneous complexation was observed.     

Supramolecular [60]Fullerene Liquid Crystals Formed By Self‐Organized Two‐Dimensional Crystals

Fullerene‐based liquid crystalline materials have both the excellent optical and electrical properties of fullerene and the self‐organization and external‐field‐responsive properties of liquid crystals (LCs). Herein, we demonstrate a new family of thermotropic [60]fullerene supramolecular LCs with hierarchical structures. The [60]fullerene dyads undergo self‐organization driven by π–π interactions to form triple‐layer two‐dimensional (2D) fullerene crystals sandwiched between layers of alkyl chains. The lamellar packing of 2D crystals gives rise to the formation of supramolecular LCs. This design strategy should be applicable to other molecules and lead to an enlarged family of 2D crystals and supramolecular liquid crystals.     

Size Dependence of [n]Cycloparaphenylenes (n=5–12) in Electrochemical Oxidation

The oxidation processes of [n]cycloparaphenylenes ([n]CPPs) (n=5–12) were systematically investigated by cyclic and rotating disk electrode voltammetry. All CPPs underwent pseudo‐reversible two‐electron oxidation irrespective of ring size, forming the corresponding radical cations and then dications. The results were in sharp contrast to those observed for linear oligoparaphenylenes, which only undergo one‐electron oxidation. The difference in the first and second oxidation potentials in the CPP oxidation was affected by the ring size and became more significant as the decrease of CPP size. In other words, while the first oxidation from neutral CPP to the radical cation occurred faster as the size of CPP becomes smaller, the second oxidation from the radical cation to dication exhibited opposite size dependence.     

NMR spectrometric studies of complexation of [60]fullerene with series of meso-tetraphenylporphyrins

Extensive 1H NMR spectrometric studies have been done to gain information on the nature of molecular interactions of the supramolecular complexes of [60]fullerene with a series of meso-tetraphenylporphyrins, namely, meso-5,10,15,20-tetraphenylporphyrin (1), meso-5,10,15,20-tetra-2-bromophenyl-porphyrin (2) and meso-5,10,15,20-tetra-2-chlorophenyl-porphyrin (3) in toluene medium. [60]Fullerene has been shown to form 1:1 adducts with the above series of meso-tetraphenylporphyrins. Formation constants (K) for all the complexes have been determined from the systematic variation of the NMR chemical shifts of beta proton of the porphyrin in presence of [60]fullerene. It has been observed that 3 acts as a better donor in forming supramolecular complex with [60]fullerene.