Photoinduced electron transfer reactivity of aza[60]fullerene: three discrete functionalization pathways with a single substrate

Azafullerenium carbocation, C59N+, shows photoinduced electron transfer (PET) reactivity toward benzyltrimethylsilane. The reaction between (C59N)2 and benzyltrimethylsilane gives three different aza[60]fullerene monoadducts depending on the reaction conditions used.     

Photosynthetic Antenna‐Reaction Center Mimicry with a Covalently Linked Monostyryl Boron‐Dipyrromethene–Aza‐Boron‐Dipyrromethene–C60 Triad

An efficient functional mimic of the photosynthetic antenna‐reaction center has been designed and synthesized. The model contains a near‐infrared‐absorbing aza‐boron‐dipyrromethene (ADP) that is connected to a monostyryl boron‐dipyrromethene (BDP) by a click reaction and to a fullerene (C₆₀) using the Prato reaction. The intramolecular photoinduced energy and electron‐transfer processes of this triad as well as the corresponding dyads BDP‐ADP and ADP‐C₆₀ have been studied with steady‐state and time‐resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge‐separated states. Such calculations show that electron transfer from the singlet excited ADP (¹ADP*) to C₆₀ yielding ADP^.+‐C₆₀^.? is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from ¹BDP* to ADP in the dyad BDP‐ADP and electron transfer from ¹ADP* to C₆₀ in the dyad ADP‐C₆₀. Sequential energy and electron transfer have also been clearly observed in the triad BDP‐ADP‐C₆₀. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10¹¹ s^?1). The dynamics of electron transfer through ¹ADP* has also been studied by monitoring the formation of C₆₀ radical anion at 1000 nm. A fast charge‐separation process from ¹ADP* to C₆₀ has been detected, which gives the relatively long‐lived BDP‐ADP^.+C₆₀^.? with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge‐separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP‐ADP and ADP‐C₆₀, and the triad BDP‐ADP‐C₆₀ are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.     

A Long‐Lived Azafullerenyl Radical Stabilized by Supramolecular Shielding with a [10]Cycloparaphenylene

A major handicap towards the exploitation of radicals is their inherent instability. In the paramagnetic azafullerenyl radical C₅₉N^., the unpaired electron is strongly localized next to the nitrogen atom, which induces dimerization to diamagnetic bis(azafullerene), (C₅₉N)₂. Conventional stabilization by introducing steric hindrance around the radical is inapplicable here because of the concave fullerene geometry. Instead, we developed an innovative radical shielding approach based on supramolecular complexation, exploiting the protection offered by a [10]cycloparaphenylene ([10]CPP) nanobelt encircling the C₅₉N^. radical. Photoinduced radical generation is increased by a factor of 300. The EPR signal showing characteristic ¹⁴N hyperfine splitting of C₅₉N^.? [10]CPP was traced even after several weeks, which corresponds to a lifetime increase of >10⁸. The proposed approach can be generalized by tuning the diameter of the employed nanobelts, opening new avenues for the design and exploitation of radical fullerenes.     

Fullerene encapsulation with calix[5]arenes

This paper presents the synthesis of the fullerene hosts based on the calix[5]arenes and their binding properties. Calix[5]arenes 1a, 2, 3a bind C₆₀ or C₇₀ in organic solvents. The solvent effect of the fullerene complexation was clearly observed; the association constant decreases in a solvent with high solubility for C₆₀. Covalently linked double-calix[5]arenes 4-6 were also investigated on their binding properties for fullerenes in organic solvents. Their binding abilities for both C₆₀ and C₇₀ are extremely high in toluene solution. Higher binding selectivity toward C₇₀ is observed by all the double-calix[5]arenes. The selectivity of 5a toward C₇₀/C₆₀ is highest in toluene with a value of 10. The structures of the supramolecular complexes of the calix[5]arene hosts and C₆₀ or C₇₀ were investigated by using ¹H and ¹³C NMR studies. The molecular mechanics calculation and X-ray structure reveal that the interior of the calix[5]arene is complementary to the exterior of C₆₀ molecule. In contrast, the host-guest complexes of C₇₀ with the simple calix[5]arenes take many conformational options due to its less symmetric shape. The molecular mechanics calculation and our chemical shift simulation nicely worked to estimate the reliable structures; the calix[5]arene cavity takes up C₇₀ molecule, and the C₇₀ molecule tilts significantly from the C5 axis of the calix[5]arene. In the case of the host-guest complex of C₇₀ with the double-calix[5]arene, the molecular dynamics simulation of the host-guest complex represented the realistic movement of the bound C₇₀ inside the cavity. The combination of the molecular dynamics simulation and the chemical shift simulation of the host-guest complex suggested that the C₇₀ molecule rapidly moves inside the cavity.     

Copolymerization of Fullerene (C60) and Methyl Methacrylate (MMA) using Triphenylbismuthonium Ylide as a Novel Initiator and Characterization of the Copolymers (C60-MMA)

Copolymerization of fullerene (C₆₀) with methyl methacrylate (MMA) was carried out using triphenylbismuthonium ylide (abbreviated as Ylide) as a novel initiator in dioxan at 60°C for 4 h in a dilatometer under a nitrogen atmosphere. The reaction follows ideal kinetics: R_p∝ [Ylide]^0.5[C₆₀]^?1.0[MMA]^1.0. The rate of polymerization increases with an increase in concentration of initiator and MMA. However, it decreases with increasing concentration of fullerene due to the radical scavenging effect of fullerene. The overall activation energy of copolymerization was estimated to be 57 KJ mol^?1. The fullerene-MMA copolymers (C₆₀-MMA) were characterized by FTIR, UV–Vis, NMR and GPC analyses.     

A Ruthenium Complex–Porphyrin–Fullerene‐Linked Molecular Pentad as an Integrative Photosynthetic Model

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long‐lived hole–electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited‐state of a zinc porphyrin (¹ZnP*) was quenched by fullerene (C₆₀) to afford a radical ion pair, ^1,3(ZnP^.+‐C₆₀^.−). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (Ru^II) to ZnP^.+ to give the long‐lived charge‐separated state, Ru^III‐ZnP‐C₆₀^.−, with a lifetime of 14 μs. The ZnP worked as a visible‐light‐harvesting antenna, while the C₆₀ acted as an excellent electron acceptor. As a consequence, visible‐light‐driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.     

Photophysical Properties and Photoinduced Electron Transfer between[60]Fullerene—containing Cyclic Sulphoxide [C60—C60H8SO]and Tetrathiafulvalene（TTF） by Laser Flash Photolysis

Photoinduced electron transfer(PET) processes between C60-C6H8SO and Tetrathiafulvalene(TTF) have been studied by nanosecond laser photolysis.Quantrm yiekds(φet) and rate constants of electron transfer(ket) from TTF to excited triplet state of[60] fullerene-containing cyclic sulphoxide in benzonitrile(BN) have been evaluated by observing the transient absorption bands in the NIR region.With the decay of excited triplet state of [60]fullerene-containing cyclic suplhoxide,the rise of radical anion of [60]fullerene-containing cyclic sulphoxinde is observed.     

Laser flash photolysis of fullerols of C60 and of C70 in aqueous solutions

Fullerols of C₆₀ and of C₇₀ [C₆₀(OH)_n, C₇₀(OH)_m], water-soluble fullerene derivatives, unlike some other fullerene derivatives (such as C₆₀ (C₄H₆O), C₆₀ (C₃H₇N) and C₆₀ [C(COOEt)₂]_x), do not result in excited triplet state but in ionization via monophotonic process in aqueous solutions with 248 nm laser. The quantum yields of formation of hydrated electron (Φ_e ⁻) are determined to be 0.08 and 0.11 for fullerols of C₆₀ and of C₇₀ respectively at room temperature (ca. 15°C) with KI solution used as reference. By laser flash photolysis and oxidation of sulfate radical anion SO₄ ⁻, the fullerol radical cation or neutral radical of C₆₀ is confirmed to be existent and the transient absorption spectra of fullerol radical cation of C₇₀ are observed for the first time. Project supported by the National Natural Science Foundation of China     

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.     

Photoinduced electron transfer between C60 and N,N,N′,N′-tetramethylbenzidine (NTMB). Fourier transform electron paramagnetic resonance study

Fourier transform EPR spectroscopy was employed in studying the electron transfer (ET) reaction and the quenching mechanisms of the photoexcited triplet state of C₆₀ as electron acceptor and N,N,N′,N′-tetramethylbenzidine (NTMB) as electron donor in benzonitrile solution. The ET reaction product, the cation radical NTMB^*+, interacts with ^3*C₆₀, leading to photoinduced electron polarization of NTMB^*+ via triplet-doublet mixing mechanism combined with triplet mechanism. The quenching of ^3*C₆₀ and the polarization behavior of NTMB^*+ are discussed.     

Reactions of [60]fullerene with 2-azidopyrimidines

The reaction of [60]fullerene with 2-azidopyrimidines affords fullerenoimidazopyrimidines, whose electron affinity is higher than that of nonmodified C₆₀.     

[6,6]‐Open and [6,6]‐Closed Isomers of C70(CF2): Synthesis,Electrochemical and Quantum Chemical Investigation

Novel difluoromethylenated [70]fullerene derivatives, C₇₀(CF₂)_n (n=1–3), were obtained by the reaction of C₇₀ with sodium difluorochloroacetate. Two major products, isomeric C₇₀(CF₂) mono‐adducts with [6,6]‐open and [6,6]‐closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X‐ray analysis and high‐level spectroscopic techniques. The [6,6]‐open isomer of C₇₀(CF₂) constitutes the first homofullerene example of a non‐hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C₇₀(CF₂) isomers showed that it is substantially higher for the [6,6]‐open isomer (the 70‐electron π‐conjugated system is retained) than the [6,6]‐closed form, the latter being similar to the electron affinity of pristine C₇₀. In situ ESR spectroelectrochemical investigation of the C₇₀(CF₂) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter‐conversion between the [6,6]‐closed and [6,6]‐open forms of a cage‐modified fullerene driven by an electrochemical one‐electron transfer. Thus, [6,6]‐closed C₇₀(CF₂) constitutes an interesting example of a redox‐switchable fullerene derivative.     

[60]Fullerene diol issued from pentaerythritol derivatives

A new [60]fullerene diol is synthesized in good yield, in two steps starting from reaction of C₆₀²⁻ anion with the benzylideneacetal derived from 2,2-bis(iodomethyl)-1,3-propanediol. The corresponding [60]fullerene bis-mesylate is also formed in a similar way starting from bis-iodo bis-mesylate compound in the same series. The scope of this fullerene diol in synthesis is exemplified by its easy esterification with 4-formyl benzoyl chloride.     

Excitation‐Wavelength‐Dependent,Ultrafast Photoinduced Electron Transfer in Bisferrocene/BF2‐Chelated‐Azadipyrromethene/Fullerene Tetrads

Donor–acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron‐transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor–acceptor dyads, revealing excitation‐wavelength‐dependent photochemical properties demands multimodular, photosynthetic‐reaction‐center model compounds. Here, we successfully demonstrate donor– acceptor excitation‐wavelength‐dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF₂‐chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured “closely” and “distantly” positioned donor–acceptor systems. The near‐IR‐emitting BF₂‐chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation‐wavelength‐dependent, photoinduced, electron‐transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc⁺–ADP–C₆₀^.? charge‐separated state upon C₆₀ excitation, and Fc⁺–ADP^.?–C₆₀ formation upon ADP excitation is demonstrated.     

Pentamethylated derivatives of [60]fullerene: X-ray structure of the C60Me5H and ESR spectroscopy evidence for the stable radical C60Me5

Single-crystal X-ray study of 6,9,12,15,18-pentamethyl-1,6,9,12,15,18-hexahydro(C₆₀-I_h)[5,6]fullerene (C₆₀Me₅H) has been reported. In crystal packing, the stacking self-organization of molecules is realized. It is concluded that the formation of such polar columns is a general rule for crystals of C₆₀R₅H independent of the nature of the R group. An ESR spectrum of the stable fullerenyl radical of the cyclopentadienyl-type, C₆₀Me₅, was observed in a sample of the pentamethylated[60]fullerene. Rotation of methyl groups around C-C bonds is restricted on the ESR scale time and therefore protons within each methyl group are non-equivalent.     

Anionic copolymerization of [60]fullerene with styrene initiated by sodium naphthalene

The novel C₆₀–styrene copolymers with different C₆₀ contents were prepared in sodium naphthalene-initiated anionic polymerization reactions. Like the pure polystyrene, these copolymers exhibited the high solvency in many common organic solvents, even for the copolymer with high C₆₀ content. In the polymerization process of C₆₀ with styrene an important side reaction, i.e., reaction of C₆₀ with sodium naphthalene, would occur simultaneously, whereas crosslinking reaction may be negligible. ¹³C-NMR results provided an evidence that C₆₀ was incorporated covalently into the polystyrene backbone. In contrast to pure polystyrene, the TGA spectrum of copolymer containing ∼ 13% of C₆₀ shows two plateaus. The polystyrene chain segment in copolymer decomposed first at 300–400°C. Then the fullerene units reptured from the corresponding polystyrene fragments attached directly to the C₆₀ cores at 500–638°C. XRD evidence indicates that the degree of order of polymers increases with the fullerene content increased in terms of crystallography. Incorporation of C₆₀ into polystyrene results in the formation of new crystal gratings or crystallization phases. In addition, it was also found that [60]fullerene and its polyanion salts [C₆₀ⁿ⁻(M⁺)_n, M = Li, Na] cannot be used to initiate the anionic polymerization of some monomers such as acrylonitrile and styrene, etc.© 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2653–2663, 1998     

Synthesis and photophysics of porphyrin-fullerene donor-acceptor dyads with conformationally flexible linkers

The synthesis and photophysics of a series of porphyrin-fullerene (P-C₆₀) dyads in which the two chromophores are linked by conformationally flexible polyether chains is reported. Molecular modeling indicates the two moieties adopt a stacked conformation in which the two chromophores are in close proximity. Photoexcitation of the free base dyads in polar solvents such as tetrahydrofuran and benzonitrile, causes electron transfer (ET) to generate charge-separated radical pair (CSRP) states, which were directly detected using transient absorption (TA) techniques. In nonpolar solvents such as toluene, where CSRP states were not directly detected, fullerene triplet state states were formed, according to TA studies as well as singlet oxygen sensitization measurements. The low value of the quantum efficiency for sensitized formation of singlet molecular oxygen [O₂(¹Δ_g)] in toluene and chloroform indicates that singlet energy transduction to give H₂P-¹C₆₀*, followed by intersystem crossing to H₂P-³C₆₀* and energy transfer to ³O₂, is not the operative mechanism. Rather, a mechanism is proposed involving ET to give CSRP states followed by exergonic charge recombination to eventually generate fullerene triplets. Such a mechanism has been demonstrated experimentally for structurally related P-C₆₀ dyads. For the corresponding ZnP-C₆₀ dyads with flexible linkers, only photoinduced ET to generate long-lived CSRP states is observed. Photoinduced charge separation in these dyad systems is extremely rapid, consistent with a through space rather than through-bond mechanism. Charge recombination is up to three orders of magnitude slower, indicating this process occurs in the inverted region of the Marcus curve that relates ET rates to the thermodynamic driving force. These observations once again demonstrate the advantages of incorporating fullerenes as electron acceptor components in photosynthetic model systems.     

Classical Valence Similarity in Fullerene Derivatives

Summary.  The generalized Pauling bond order was enumerated in the C₆₀ fullerene cage molecule (truncated icosahedral symmetry). This index measures chemical similarity in fullerene derivatives such as dihydrofullerene (C₆₀H₂), anionized monohydrofullerene (C₆₀H⁻), N-substituted monohydrofullerene (C₅₉NH), the fullerene dimer ((C₆₀)₂), and the dianionic fullerene dimer ((C₆₀)₂ ²⁻). It is also useful in judging the chemical stability of isomers. Received October 9, 2001. Accepted November 9, 2001     

Electrochemical Studies of Calixarene-[60]Fullerene Inclusion Processes

[60]Fullerene (C₆₀)-calix[n]areneinteractions were studied in toluene:MeCN = 10:1 (v/v)using an electrochemical method. In the presence ofhomooxacalix[3]arenes and calix[5]arenes, bothE_pc and _pa shifted to the negativepotential, whereas they were scarcely affected by theaddition of calix[4]arenes. The₂values electrochemically determined forthe association with C₆₀ ^- are generallysmaller than the ₁ valuesspectroscopically determined for the association withC₆₀. The tendency indicates that thecharge-transfer-type interaction is one of thedriving forces for C₆₀ inclusion.thanks|Author for correspondence     

DFT study on [4+2] and [2+2] cycloadditions to [60] fullerene

The functionalisation of C₆₀ fullerene with 2,3-dimethylene-1,4-dioxane (I) and 2,5-dioxabicyclo [4.2.0]octa-1(8),6-diene (II) was investigated by the use of density functional theory calculations in terms of its energetic, structural, field emission, and electronic properties. The functionalisation of C₆₀ with I was previously reported experimentally. The I and II molecules are preferentially attached to a C—C bond shared and located between two hexagons of C₆₀ via [4+2] and [2+2] cycloadditions bearing reaction energies of ?15.9 kcal mol^?1 and ?72.4 kcal mol^?1, respectively. The HOMO-LUMO energy gap and work function of C₆₀ are significantly reduced following completion of the reactions. The field electron emission current of the C₆₀ surface will increase after functionalisation of either the I or II molecule.