Supramolecular complexation studies of [60]fullerene with calix[4]naphthalenes—a reinvestigation

Estimations of equilibrium or association constant (K_ASSOC) values reported by many other groups for the supramolecular complexation between [60]fullerene (‘C₆₀’) with different macrocyclic hosts, in solvents such as toluene or carbon disulfide, for example, is often conducted by UV-vis absorption and/or ¹H NMR spectroscopy. In this paper, the complexation behaviour of two calix[4]naphthalene hosts with C₆₀ in toluene and carbon disulfide has been re-examined, using both of these methods. An analysis is presented of the data newly obtained, in light of recent advances and understanding published by others of the limitations of, in particular, the absorption spectroscopic methods. The discussion presented is also intended to aid those who may be unfamiliar with the nuances and limitations of the analytic models involving C₆₀ supramolecular complexation. Also presented is a general mechanism for C₆₀ supramolecular complexation studies, which lay the groundwork for further experiments.     

Photoinduced charge transfer in porphyrin-C60 oligomer

Photoinduced electron transfer (ET) between C₆₀ and porphyrin (P) in a new polymer containing porphyrin, poly(p-phenylenevinylene), and pendant fullerene units has been investigated by nanosecond transient absorption and phosphorescence spectroscopy. Compared to the physically doping material systems, binding porphyrin/C₆₀ through chemical bonds in a polymer detains the formation of the triplet states of porphyrins and C₆₀. The formation of intermediate charge transfer state (CSS) of P⁺-C₆₀ ^? was observed, which led to the delayed formation of triplet states of porphyrins and C₆₀. The reduced opto-electronic properties, such as optical limiting performance, were also observed, which resulted from the delayed formation of triplet states. The results presented in this article are significant in understanding the complicated spectral characteristics of the triplet state and charge transfer of the porphyrin and C₆₀ complexes, and are therefore related to the controllable performance of the new materials in applications.     

Mixed-ligand iridium(IV) complexes with amino acids,adenine and hypoxanthine

The complexation in iridium(IV)-purine base (adenine, hypoxanthine)-amino acid (α-alanine, aspartic acid, lysine) systems was studied by pH titration. The stability constants of 1: 1: 1 complexes were determined. The stability of 1: 1: 1 mixed-ligand complexes with hypoxanthine and adenine increases in the series Ala < Lys < Asp. Reactions between aqueous solutions gave the following coordination compounds: [Ir(C₅H₄N₄O)(C₃H₆NO₂)Cl]Cl₂, [Ir(C₅H₄N₄O)(C₄H₅NO₄)]Cl₂, [Ir(C₅H₄N₄O)(C₆H₁₃N₂O₂)]Cl₃, [Ir(C₅H₅N₅)(C₃H₆NO₂)]Cl₃, [Ir(C₅H₅N₅)(C₄H₅NO₄)]C_l2, and [Ir(C₅H₅N₅)(C₆H₁₃N₂O₂)]Cl₃. The individual character of the complexes was established by chemical and thermogravimetric analyses and powder X-ray diffraction. The complexes were characterized by NMR, IR, and X-ray photoelectron spectroscopy. Alanine and lysine in mixed-ligand iridium(IV) complexes are bidentate (α-NH₂ and COO⁻ groups), aspartic acid is tridentate, and purine bases function as polydentate ligands through heterocycle N atoms and functional groups (NH₂ in adenine and C=O in hypoxanthine).     

Synthesis and properties of fullerene (C70) complexes of 2,6-bis(porphyrin)-substituted pyrazine derivatives bound to a Pd(II) ion

2,6-Bis(porphyrin)-substituted 3,5-dimethylpyrazine and its zinc complex bound C₇₀ to yield 1:1 inclusion complexes, which were characterised by ESI-MS, UV–vis, fluorescence and NMR spectroscopies. Association constants of the C₇₀ complexes were determined by fluorescence and NMR spectral analyses. A decrease in absorbance of the Soret band of the pyrazine derivative by the effect of C₇₀ was observed, suggesting the existence of a charge transfer interaction between C₇₀ and porphyrin. Experimentally reliable values for the association constants were obtained by the NMR method and were about six times larger than those of the corresponding C₆₀ complexes. Palladium complexation of the porphyrin–pyrazine ligand was found to enhance the association with fullerene. The association constant of 2,6-bis(porphyrin-Zn)-substituted 3,5-dimethylpyrazine-Pd(II) complex with C₇₀ was determined to be 8400 ± 900 M^? 1. From the comparison of the association constants, it was found that inclusion room for C₇₀ in the Pd(II) complex was maintained, juxtaposed between porphyrins attached to the opposite sides of the pyrazine ligands.

     

Radiation-cured polyisoprene/C60 fullerene nanocomposite. Part 1: Synthesis in hexane and in toluene

Fullerene C₆₀ can be grafted onto polyisoprene chains by γ irradiation in n-hexane or in toluene. The resulting polyisoprene/C₆₀ nanocomposites can be easily recovered from the solutions and have been characterized by electronic absorption and Fourier transform-infrared (FT-IR) spectroscopy. The electronic absorption spectroscopy clearly shows that C₆₀ undergoes addition reaction from the polyisoprene macroradicals formed by γ radiolysis. FT-IR spectroscopy supports the grafting reaction of polyisoprene on C₆₀ and additionally it shows that C₆₀ acts as a sensitizer for the partial cis–trans isomerization of the polyisoprene monomeric units. Relatively high levels of C₆₀ lead the polyisoprene-based nanocomposite to a crosslinked gel.The thermal behaviour of the polyisoprene/C₆₀ nanocomposites has been studied by the thermogravimetric analysis (TGA-DTG). It is shown that the radical adducts polyisoprene/C₆₀ are thermally reversible and regenerate free C₆₀ during the pyrolysis under N₂.     

Photophysical and electrochemical properties of heteroleptic tris-cyclometallated Ir(III) complexes

Some new heteroleptic tris-cyclometallated iridium(III) complexes have been synthesized and fully characterized. Among these iridium(III) complexes, bis(1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (3) and bis(3-methyl-1-phenylpyrazolato-N,C^2′)iridium(III)[5-(2′-pyridyl)tetrazolate] (4) show excellent quantum yields at room temperature, the electron density being perturbed by introducing the pyridyltetrazole ligand, making k_r > k_nr. This destroys the concept of phenylpyrazole based iridium complexes.     

A Supramolecular [10]CPP Junction Enables Efficient Electron Transfer in Modular Porphyrin–[10]CPP⊃Fullerene Complexes

Efficient photoinduced electron transfer was observed across a [10]cycloparaphenylene ([10]CPP) moiety that serves as a rigid non‐covalent bridge between a zinc porphyrin and a range of fullerenes. The preparation of iodo‐[10]CPP is the key to the synthesis of a porphyrin–[10]CPP conjugate, which binds C₆₀, C₇₀, (C₆₀)₂, and other fullerenes (K_A>10⁵ m ^?1). Fluorescence and pump–probe spectroscopy revealed intramolecular energy transfer between CPP and porphyrin and also efficient charge separation between porphyrin and fullerenes, affording up to 0.5 μs lifetime charge‐separated states. The advantage of this approach towards electron donor–acceptor dyads is evident in the case of dumbbell‐shaped (C₆₀)₂, which gave intricate charge‐transfer behavior in 1:1 and 2:1 complexes. These results suggest that [10]CPP and its cross‐coupled derivatives could act as supramolecular mediators of charge transport in organic electronic devices.     

Separation of nanocarbons by molecular recognition

This review article surveys the author’s work during the last ten years. The research works have been carried out in the interdisciplinary fields of supramolecular, synthetic organic and materials chemistry. This review consists of the following 5 topics; (1) novel synthetic methodology for constructing benzyl ether-linked oxacyclophanes and oxacalixarenes by reductive coupling reactions, (2) preferential precipitation of C₇₀ over C₆₀ with p-halohomooxacalix[3]arenes prepared by the above reductive coupling reactions, (3) highly practical purification of fullerenes by filtration through activated carbon thin layer, (4) host-guest chemistry of C₆₀ and C₇₀ with porphyrin monomers and dimers in solution, and (5) optical resolution of carbon nanotubes through preferential complexation with chiral diporphyrin nanotweezers. New terminology is also proposed in the definition of the structures and stereochemistry of carbon nanotubes.     

Molecular Complexes of Fullerenes C60 and C70 with Saturated Amines

New molecular complexes of fullerenes C₆₀ and C₇₀ with leuco crystal violet (LCV, 1-3); leucomalachite green (LMG, 4-6); crystal violet lactone (CVL, 7); N,N,N′,N′-tetrabenzyl-p-phenylenediamine (TBPDA, 8, and 9); N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPDA, 10, and 11); triphenylamine (TPA, 12, and 13); and substituted phenotellurazines (EPTA and TMPTA, 14, and 15) have been synthesized. Crystal structures have been solved for C₆₀ complexes with LMG (5, 6) TBPDA (8), TMPDA (10), and TPA (12). The C₆₀ molecules form closely packed double layers in 5 and 6, hexagonal layers in 10 and quasi-three-dimensional layers in 8 and 12. The substitution of disordered solvent molecules in the complexes with LMG (4, 5) by naphthalene ones results in the ordering of the C₆₀ molecules. According to IR-, UV-visible-NIR and ESR-spectroscopy the complexes have a neutral ground state. The spectra of 1-8, and 10 show intense charge transfer bands in the visible and NIR-range. On photoexcitation by white light (light-induced ESR (LESR) spectroscopy), 1 and 10 were shown to have an excited ionic state. The LESR signals were generated at light energies <2.25 eV indicating that the excited states in the complexes are realized mainly by direct charge transfer from donor to the C₆₀ molecule.     

Synthesis, structure, and reactions of iridium(I) complexes with 5,10,15,20-tetraphenyl-21H,23H-porphine and 5,10,15,20-tetraphenyl-21H,23H-porphine dianion

One-step joint synthesis of two iridium porphyrin complexes, a donor-acceptor SAT (sitting a top) complex μ-(5,10,15,20-tetraphenylporphine)-bis-chloroiridium(I) and the covalent complex (5,10,15,20-tetraphenylporphinato)chloroiridium(III) by the reaction of free porphyrin and chloroiridic acid (H₃O)₂IrCl₆ in boiling phenol was studied. The structure of complexes was confirmed by spectroscopy (UV/Vis, IR, 1H NMR) and TLC. The iridium(III) SAT complex with the hydride ligand in the first coordination sphere, (acetato)hydrido(5,10,15,20-tetraphenylporphine)iridium(III), was obtained by oxidative addition reaction, which is quite rare for porphyrin complexes. The thermodynamic stability of the complexes to oxidants (aerated acids) was studied by spectrophotometric titration.     

Supramolecular triads bearing porphyrin and fullerene via ‘two-point’ binding involving coordination and hydrogen bonding

Supramolecular triads composed of fullerene (C₆₀) as primary electron acceptor, zinc porphyrin (ZnP) as primary electron donor, and either a ferrocene (Fc), or N,N-dimethylaminophenyl (DMA), or N,N-diphenylaminophenyl (DPA) entity as a second electron donor were constructed via a ‘two-point’ binding motif involving axial coordination and hydrogen bonding. The B3LYP/3-21G(*) optimized structures revealed disposition of the three entities of the triads in a triangular fashion. The redox behavior of the different components was studied using cyclic voltammetry in o-dichlorobenzene containing 0.1 M (n-C₄H₉)₄NClO₄. The oxidation potentials of the second electron donor followed the trend: Fc<DMA<DPA, and the free-energy calculations suggested the possibility of the occurrence of sequential hole transfer in these triads. Efficient electron transfer from the excited singlet state of zinc porphyrin to the fullerene entity was observed in all of the studied triads in o-dichlorobenzene. Longer charge-separated states were observed for zinc porphyrin with a carboxylic acid compared with that having an amide group. The ratios of the experimentally determined forward to reverse electron transfer rates, k_CS/k_CR were evaluated to be 10³ for triads formed by zinc porphyrin with a carboxylic acid, suggesting charge stabilization in these triads.     

Supramolecular complexes obtained from porphyrin-crown ether conjugates and a fullerene derivative bearing an ammonium unit

A methanofullerene derivative with an ammonium subunit (1) has been prepared and its ability to form a supramolecular complex with a porphyrin-crown ether conjugate evidenced by NMR, UV-vis, electrospray mass spectrometry (ES-MS) and luminescence experiments. Interestingly, in addition to the ammonium-crown ether recognition, intramolecular stacking of the fullerene moiety and the porphyrin subunit has been evidenced. Due to this additional recognition element, the association constant for the supramolecular complex is increased by two orders of magnitude when compared to the K_a values found for the complexation of 1 with benzo-18-crown-6. Finally, non-covalent systems resulting from the association of cation 1 with porphyrin derivatives bearing two crown ether subunits have been investigated. Intramolecular C₆₀-porphyrin interactions have also been evidenced within these supramolecular complexes. As a result, the 2:1 complexes are very stable as shown by the ES-MS studies.     

A Tunable Cyclic Container: Guest‐Induced Conformational Switching,Efficient Guest Exchange,and Selective Isolation of C70 from a Fullerene Mixture

An adaptable cyclic porphyrin dimer with highly flexible linkers has been used as an artificial molecular container that can efficiently encapsulate various aromatic guests (TCNQ/C₆₀/C₇₀) through strong π–π interactions by adjusting its cavity size and conformation. The planar aromatic guest (TCNQ) can be easily and selectively exchanged with larger aromatic guests (C₆₀/C₇₀). During the guest‐exchange process, the two porphyrin rings switch their relative orientation according to the size and shape of the guests. This behavior of the cyclic container has been thoroughly investigated by using UV/Vis spectroscopy, NMR spectroscopy, and X‐ray crystal structure determination of the host–guest assemblies. The electrochemical and photophysical studies demonstrated the occurrence of photoinduced electron transfer from bisporphyrin to TCNQ/C₆₀/C₇₀ in the respective host–guest assemblies. The cyclic host can form complexes with C₆₀ and C₇₀ with association constants of (2.8±0.2)×10⁵ and (1.9±0.3)×10⁸ m ⁻¹, respectively; the latter value represents the highest binding affinity for C₇₀ reported so far for zinc(II) bisporphyrinic receptors. This high selectivity for the binding of C₇₀ versus C₆₀ allows the easy extraction and efficient isolation of C₇₀ from a C₆₀/C₇₀ fullerene mixture. Experimental evidence was substantiated by DFT calculations.     

Remarkably Efficient Photocurrent Generation Based on a [60]Fullerene–Triosmium Cluster/Zn–Porphyrin/Boron–Dipyrrin Triad SAM

A new artificial photosynthetic triad array, a [60]fullerene–triosmium cluster/zinc–porphyrin/boron–dipyrrin complex ( 1 , Os₃C₆₀/ZnP/Bodipy), has been prepared by decarbonylation of Os₃(CO)₈(CN(CH₂)₃Si(OEt)₃)(μ₃‐η²:η²:η²‐C₆₀) ( 6 ) with Me₃NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy ( 5 ) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one‐electron redox couples, that is, the sum of those of 5 and 6 . DFT has been applied to study the molecular and electronic structures of 1 . On the basis of fluorescence‐lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C₆₀; the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 ( 1 /ITO; ITO=indium–tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo‐octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X‐ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1 /ITO have been investigated by a standard three‐electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron‐transfer arrays, in that C₆₀ is linked by π bonding with little perturbation of the C₆₀ electron delocalization.     

Controlled Synthesis of meso‐Veratrylporphyrins under Acid‐Catalyzed Scrambling Conditions

5,15‐Bis(veratryl)porphyrin (1), 5,10,15‐tris(veratryl)porphyrin (2), and 5,10,15,20‐tetrakis(veratryl)porphyrin (3), C₃₆H₃₀N₄O₄, C₄₄H₃₈N₄O_6, and C₅₂H₄₆N₄O₈ were synthesized by an acid‐catalyzed, one‐pot reaction of meso‐dipyrromethane, 3,4‐di‐OCH₃‐benzaldehyde (veratraldehyde), and trifluoroacetic acid (TFA). These three different products were obtained as a result of scrambling reactions. The molecular structures have been characterized by elemental analysis, ESI mass spectra, 1D and 2D NMR techniques (¹H and ¹³C), and UV‐visible spectroscopy.     

3D vs. turbostratic: controlling metal–organic framework dimensionality via N-heterocyclic carbene chemistry

Using azolium-based ligands for the construction of metal–organic frameworks (MOFs) is a viable strategy to immobilize catalytically active N-heterocyclic carbenes (NHC) or NHC-derived species inside MOF pores. Thus, in the present work, a novel copper MOF referred to as Cu-Sp5-BF₄, is constructed using an imidazolinium ligand, H₂Sp5-BF₄, 1,3-bis(4-carboxyphenyl)-4,5-dihydro-1H-imidazole-3-ium tetrafluoroborate. The resulting framework, which offers large pore apertures, enables the post-synthetic modification of the C² carbon on the ligand backbone with methoxide units. A combination of X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ssNMR) and electron microscopy (EM), are used to show that the post-synthetic methoxide modification alters the dimensionality of the material, forming a turbostratic phase, an event that further improves the accessibility of the NHC sites promoting a second modification step that is carried out via grafting iridium to the NHC. A combination of X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) methods are used to shed light on the iridium speciation, and the catalytic activity of the Ir–NHC containing MOF is demonstrated using a model reaction, stilbene hydrogenation.

A new MOF with a saturated N-heterocyclic carbene ligand undergoes a series of structural transformations to produce a turbostratic material, which serves as a better support for an iridium hydrogenation catalyst, when compared to the parent material.     

Mechanism of complexation of the phenothiazine dye methylene blue with fullerene C60

The complexation of fullerene C₆₀ with the aromatic dye methylene blue (MB) in aqueous solution was studied. Spectrophotometric titration revealed a reasonably strong interaction between C₆₀ and MB molecules with an equilibrium constant K = 2110 L/mol and the binding of up to five dye molecules with the surface of C₆₀. The energy analysis of the MB-C₆₀ system showed that the intermolecular and hydrophobic interactions were dominant in the energy profile of the complexation, and while the electrostatic factor played an insignificant role.     

π-Electronic Charge-Transfer Interactions in Inclusion Complexes of Fullerenens with Cyclic Dimers of Metalloporphyrins

Spectroscopic and voltammetric analyses on inclusion complexes of a fullerene such as C₆₀ with a cyclic dimer of a free-base porphyrin (1-M; M = 2H) and its metal complexes (M = Co, Ag, and Zn) showed that a charge-transfer interaction is partly responsible for the complexation.     

Fluorescent Bis-Calix[4]arene-Carbazole Conjugates: Synthesis and Inclusion Complexation Studies with Fullerenes C60 and C70

Supramolecular chemistry has become a central theme in chemical and biological sciences over the last decades. Supramolecular structures are being increasingly used in biomedical applications, particularly in devices requiring specific stimuli-responsiveness. Fullerenes, and supramolecular assemblies thereof, have gained great visibility in biomedical sciences and engineering. Sensitive and selective methods are required for the study of their inclusion in complexes in various application fields. With this in mind, two new fluorescent bis-calix[4]arene-carbazole conjugates (4 and 5) have been designed. Herein, their synthesis and ability to behave as specific hosts for fullerenes C₆₀ and C₇₀ is described. The optical properties of the novel compounds and their complexes with C₆₀ and C₇₀ were thoroughly studied by UV-Vis and steady-state and time-resolved fluorescence spectroscopies. The association constants (K_a) for the complexation of C₆₀ and C₇₀ by 4 and 5 were determined by fluorescence techniques. A higher stability was found for the C₇₀@4 supramolecule (K_a = 5.6 × 10⁴ M⁻¹; ΔG = −6.48 kcal/mol). Evidence for the formation of true inclusion complexes between the host 4 and C₆₀/C₇₀ was obtained from NMR spectroscopy performed at low temperatures. The experimental findings were fully corroborated by density functional theory (DFT) models performed on the host–guest assemblies (C₆₀@4 and C₇₀@4).     

Electrochemical oxidation and reduction of rhodium and iridium complexes with fullerenes C60 and C70

The electrochemical behavior of rhodium and iridium complexes with fullerences C₆₀ and C₇₀ was studied by cyclic voltammetry in a THF—toluene mixture. The complexes were found to be capable of oxidation and reduction. It was demonstrated that thein situ generation of metallofullerene complexes in the electrochemical cell by the interaction of C₆₀ and C₇₀ with hydridocarbonylphosphine complexes of rhodium and iridium, HM(CO)(PPh₃)₃, is possible. The influence of structural factors and the action of CO₂ on changes in the redox properties of fullerene complexes was considered.