Azafullerene C59N in Donor–Acceptor Dyads: Synthetic Approaches and Properties

Energy conversion schemes have attracted considerable attention in recent years. A large amount of research effort has focused on fullerenes, particularly C₆₀ and its derivatives, as suitable electron acceptors, owing to their outstanding properties. In this context, C₅₉N‐based donor–acceptor systems have lately attracted attention, owing to their exceptional energy‐and electron‐transfer properties. As a result, chemical derivatization of C₅₉N plays an important role in the realization of the aforementioned systems. The current Minireview aims to familiarize researchers with the main aspects of azafullerene synthesis, chemistry, and photophysical properties, while it mainly focuses on the synthetic methodologies employed for as well as on energy conversion schemes of azafullerene‐based donor–acceptor systems.     

Polyethylenimine‐assisted Synthesis of Au Nanoparticles for Efficient Syngas Production

The ability to capture, store, and use CO₂ is important for remediating greenhouse‐gas emissions and combatting global warming. Herein, Au nanoparticles (Au‐NPs) are synthesized for effective electrochemical CO₂ reduction and syngas production, using polyethylenimine (PEI) as a ligand molecule. The PEI‐assisted synthesis provides uniformly sized 3‐nm Au NPs, whereas larger irregularly shaped NPs are formed in the absence of PEI in the synthesis solution. The Au‐NPs synthesized with PEI (PEI?Au/C, average PEI Mw=2000) exhibit improved CO₂ reduction activities compared to Au‐NPs formed in the absence of PEI (bare Au NPs/C). PEI?Au/C displays a 34 % higher activity toward CO₂ reduction than bare Au NPs/C; for example, PEI?Au/C exhibits a CO partial current density (j_CO) of 28.6 mA cm^?2 at ?1.13 V_RHE, while the value for bare Au NPs/C is 21.7 mA cm^?2; the enhanced j_CO is mainly due to the larger surface area of PEI?Au/C. Furthermore, the PEI?Au/C electrode exhibits stable performance over 64 h, with an hourly current degradation rate of 0.25 %. The developed PEI?Au/C is employed in a CO₂‐reduction device coupled with an IrO₂ water‐oxidation catalyst and a proton‐conducting perfluorinated membrane to form a PEI?Au/C|Nafion|IrO₂ membrane‐electrode assembly. The device using PEI?Au/C as the CO₂‐reduction catalyst exhibits a j_CO of 4.47 mA/cm² at 2.0 V_cell. Importantly, the resulted PEI?Au/C is appropriate for efficient syngas production with a CO ratio of around 30–50 %.     

Chloro Derivatives of Azafullerenes C59NCl5 and Non‐Classical C97NCl21

High‐temperature chlorination of HPLC fullerene fractions containing mainly C₉₂/C₉₄ and C₁₀₂/C₁₀₄ resulted in the isolation and X‐ray structural characterization of chloro derivatives of azafullerenes, C₅₉NCl₅ and C₉₇NCl₂₁. It was assumed that formation of azafullerenes in the arc‐discharge synthesis of fullerenes occurred due to air leakage into the reactor. The molecule of C₅₉NCl₅ contains an isolated aromatic pyrrole ring on the fullerene cage and possesses C _5v symmetrical shape typical of other known C₅₉NR₅ derivatives. The molecule of non‐classical (NC ) C₉₇N(NC )Cl₂₁ contains an NC₆ heptagon on the azafullerene cage that assumes its formation by a C₂ loss from C₉₉N in the course of chlorination. The chlorination pattern is characterized by the presence of stabilizing isolated aromatic systems and isolated double C=C bonds on the C₉₇N(NC ) azafullerene cage.     

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.     

Azafullerene C59N–Phthalocyanine Dyad: Synthesis,Characterisation and Photoinduced Electron Transfer

The synthesis of a new azafullerene C₅₉N–phthalocyanine (Pc) dyad is described. The key step for the synthesis of the C₅₉N–Pc dyad was the formation of the C₅₉N‐based carboxylic acid, which was smoothly condensed with hydroxy‐modified Pc. The structure of the C₅₉N–Pc dyad was verified by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, UV/Vis spectroscopy and MS measurements. The photophysical and electrochemical properties of the C₅₉N–Pc dyad were investigated in both polar and non‐polar solvents by steady state and time‐resolved photoluminescence and absorption spectroscopy, as well as by cyclic voltammetry. Different relaxation pathways for the photoexcited C₅₉N–Pc dyad, as a result of changing the solvent polarity, were found, thus giving rise to energy‐transfer phenomena in non‐polar toluene and charge‐transfer processes in polar benzonitrile. Finally, the detailed quenching mechanisms were evaluated and compared with that of a C₆₀–Pc dyad, which revealed that the different excited‐state energies and reduction potentials of the two fullerene spheres (i.e. C₅₉N vs. C₆₀) strongly diverged in the deactivation pathways of the excited states of the corresponding phthalocyanine dyads.     

pH‐Switchable Redox Reactions and Bioelectrocatalytic Processes Using Au Nanoparticles‐Modified Electrodes

Boronate ester complexes generated between methylene blue (MB⁺)‐functionalized Au nanoparticles (NPs) and electrode surfaces are implemented to stimulate the bioelectrocatalyzed reduction of H₂O₂ in the presence of horseradish peroxidase (HRP). Two kinds of Au NPs are prepared: Class I includes MB⁺/phenylboronic acid as a modifying layer, whereas Class II includes MB⁺/dithiothreitol as a mixed capping layer. The Class I or II NPs form boronate ester complexes with a dithiothreitol‐ or phenylboronic acid‐functionalized Au electrodes, respectively. By the cyclic loading of the NPs on the electrodes (pH 8.1), and the removal of the NPs (pH 1.5), switchable bioelectrocatalyzed reduction of H₂O₂ is demonstrated.     

Axially Substituted Silicon Phthalocyanine as Electron Donor in a Dyad and Triad with Azafullerene as Electron Acceptor for Photoinduced Charge Separation

The synthesis of a donor–acceptor silicon phthalocyanine (SiPc)‐azafullerene (C₅₉N) dyad 1 and of the first acceptor–donor–acceptor C₅₉N‐SiPc‐C₅₉N dumbbell triad 2 was accomplished. The two C₅₉N‐based materials were comprehensively characterized with the aid of NMR spectroscopy, MALDI‐MS as well as DFT calculations and their redox and photophysical properties were evaluated with CV and steady‐state and time‐resolved absorption and photoluminescence spectroscopy measurements. Notably, femtosecond transient absorption spectroscopy assays revealed that both dyad 1 and triad 2 undergo, after selective photoexcitation of the SiPc moiety, photoinduced electron transfer from the singlet excited state of the SiPc moiety to the azafullerene counterpart to produce the charge‐separated state, with lifetimes of 660 ps, in the case of dyad 1 , and 810 ps, in the case of triad 2 . The current results are expected to have significant implications en route to the design of advanced C₅₉N‐based donor–acceptor systems targeting energy conversion applications.     

Visible‐Light‐Induced Electron Transport from Small to Large Nanoparticles in Bimodal Gold Nanoparticle‐Loaded Titanium(IV) Oxide

A key to realizing the sustainable society is to develop highly active photocatalysts for selective organic synthesis effectively using sunlight as the energy source. Recently, metal‐oxide‐supported gold nanoparticles (NPs) have emerged as a new type of visible‐light photocatalysts driven by the excitation of localized surface plasmon resonance of Au NPs. Here we show that visible‐light irradiation (λ>430 nm) of TiO₂‐supported Au NPs with a bimodal size distribution (BM‐Au/TiO₂) gives rise to the long‐range (>40 nm) electron transport from about 14 small (ca. 2 nm) Au NPs to one large (ca. 9 nm) Au NP through the conduction band of TiO₂. As a result of the enhancement of charge separation, BM‐Au/TiO₂ exhibits a high level of visible‐light activity for the one‐step synthesis of azobenzenes from nitrobenzenes at 25 °C with a yield greater than 95 % and a selectivity greater than 99 %, whereas unimodal Au/TiO₂ (UM‐Au/TiO₂) is photocatalytically inactive.     

Tether-Directed Regioselective Synthesis of an Equatorialface Bisadduct of Azafullerene Using Cyclo-[2]-octylmalonate

Bisazafullerene (C₅₉N)₂ has been functionalized under aerobic conditions with cyclo-[2]-octylmalonate through a Mannich-type reaction, furnishing the corresponding monoadduct. A regioselective tether-directed Bingel cyclopropanation reaction was then carried out on the azafullerene core to yield a single bisadduct. Spectroscopic analysis of the formed bisadduct showed it to have a C₁ symmetrical structure, making it inherently chiral. Single-crystal X-ray analysis revealed the addition pattern of the azafullerene bisadduct to be equatorial_face. Examination of the optical properties showed the evolution of new absorption bands, which can be used as signatures for the characterization of equatorial_face bisadducts of C₅₉N. Finally, redox assays showed that the LUMO level of the equatorial_face bisadduct was increased compared to that of the parent (C₅₉N)₂.     

Amine‐Functionalized Polyglycidyl Methacrylate Microsphere as a Unified Template for the Synthesis of Gold Nanoparticles and Single‐Crystal Gold Plates

Polyglycidyl methacrylate (PGMA) microspheres, crosslinked and surface‐functionalized by amine, can be used as a solid‐state template for the synthesis of gold (Au) crystals in the forms of either nanoparticles (NPs) or plates. It is discovered that the polymer microsphere acts as an internal template to cultivate Au NPs inside the microsphere or an external template to generate the single‐crystal plates depending on the critical concentration (C_cr) of gold ions. The ion–dipole interaction and the structure‐dependent solubility of gold induce two distinct gold nanostructures in the presence of the functionalized polymer microspheres. The catalytic activity and long‐term storage of the developed gold nanostructures that can be easily scaled‐up for mass production through the developed novel methodology is demonstrated.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]?n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was ?67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.     

Adsorption of polyfunctional 5‐fluorouracil and 2,4‐dithio‐5‐fluorouracil on Au(111) surface: Structure,energy, and electronic transmission

Adsorption of 5‐fluorouracil (5‐FU) and 2,4‐dithio‐5‐fluorouracil (2,4‐DT‐5‐FU) on Au(111) surface at low coverage is studied by using periodic‐slab‐density functional theory calculation. Isolated 5‐FU molecule adsorbs preferentially at bridge site in a vertical configuration via N? H group by forming the N? H···Au nonconventional H‐bond. The formation of the anchor Au? O bond is not observed. Substitution of oxygen atoms of 5‐FU with sulfur strongly influences the nature of adsorption and leads to the Au? S anchor bond and the N? H···Au nonconventional H‐bond of single 2,4‐DT‐5‐FU molecule on Au(111) surface. The adsorption site and orientation of 2,4‐DT‐5‐FU molecule on the surface are similar to those of 5‐FU. The metal–molecule coupling effects at asymmetric Au/S(N? H)S/mol/C? H/Au and Au/N? H/mol/O/Au transport junctions and symmetric Au/S(N? H)S/mol/mol/S(N? H)S/Au and Au/O/mol/mol/O/Au transport junctions are also investigated. The electronic structure is analyzed in detail, and the obtained results are used for illustrating the electron transmission in metal–molecule–metal systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Magnetic Nanohybrids Loaded with Bimetal Core–Shell–Shell Nanorods for Bacteria Capture,Separation, and Near‐Infrared Photothermal Treatment

A novel antimicrobial nanohybrid based on near‐infrared (NIR) photothermal conversion is designed for bacteria capture, separation, and sterilization (killing). Positively charged magnetic reduced graphene oxide with modification by polyethylenimine (rGO–Fe₃O₄–PEI) is prepared and then loaded with core–shell–shell Au–Ag–Au nanorods to construct the nanohybrid rGO–Fe₃O₄–Au–Ag–Au. NIR laser irradiation melts the outer Au shell and exposes the inner Ag shell, which facilitates controlled release of the silver shell. The nanohybrids combine physical photothermal sterilization as a result of the outer Au shell with the antibacterial effect of the inner Ag shell. In addition, the nanohybrid exhibits high heat conductivity because of the rGO and rapid magnetic‐separation capability that is attributable to Fe₃O₄. The nanohybrid provides a significant improvement of bactericidal efficiency with respect to bare Au–Ag–Au nanorods and facilitates the isolation of bacteria from sample matrixes. A concentration of 25 μg mL^?1 of nanohybrid causes 100 % capture and separation of Escherichia coli O157:H7 (1×10⁸ cfu mL^?1) from an aqueous medium in 10 min. In addition, it causes a 22 °C temperature rise for the surrounding solution under NIR irradiation (785 nm, 50 mW cm^?2) for 10 min. With magnetic separation, 30 μg mL^?1 of nanohybrid results in a 100 % killing rate for E. coli O157:H7 cells. The facile bacteria separation and photothermal sterilization is potentially feasible for environmental and/or clinical treatment.     

One‐step Synthesis of AuAg Alloy Nanodots and its Electrochemical Studies towards Nitrobenzene Reduction and Sensing

Efficient, stable, and low‐cost electrocatalysts for the degradation and sensing of environment pollutants are essential components of clean environment monitoring. Here we report, one‐step synthesis and characterization of 1–3 nm diameter sized bi‐metallic AuAg nanodots (NDs) embedded in amine functionalized silicate sol‐gel matrix (SSG) and its electrochemical studies toward nitrobenzene. The SSG was used as a reducing agent as well as stabilizer for the prepared mono‐ and bi‐metallic nanoparticles (NPs). From the HRTEM, STEM‐EDS and XPS analyses, the bi‐metallic AuAg NDs were identified as an alloy and not the mixtures of Au and Ag NPs. Characteristic surface plasmon resonance (SPR) band between the Au and Ag NPs SPR absorption region was noticed for the prepared AuAg NDs. The AuAg alloy NDs with different concentrations of Au and Ag (Au₂₅Ag₇₅, Au₅₀Ag₅₀ and Au₇₅Ag₂₅ NDs) modified electrodes exhibited synergistic electrocatalytic effect than did the Au and Ag NPs towards nitrobenzene reduction and detection. Together with ultra‐small size and exceptional colloidal stability features within these SSG‐AuAg NDs pave convenient way for nanotechnology‐based catalysts development and sensor applications.     

A Hexagonal Covalent Porphyrin Framework as an Efficient Support for Gold Nanoparticles toward Catalytic Reduction of 4‐Nitrophenol

A hexagonal porphyrin‐based porous organic polymer, namely, CPF‐1, was constructed by 3+2 ketoenamine condensation of the C₂‐symmetric porphyrin diamine 5,15‐bis(4‐aminophenyl)‐10,20‐diphenylporphyrin and 1,3,5‐triformylphloroglucinol. This material exhibits permanent porosity and excellent thermal and chemical stability. CPF‐1 can be employed as a superior supporting substrate to immobilize Au nanoparticles (NPs) as a result of the strong interactions between Au NPs and the CPF support. An Au@CPF‐1 hybrid was synthesized by an interfacial solution infiltration method with NaBH₄ as reducing agent. Au NPs (5 nm) grew on CPF‐1 and were distributed without aggregation. Moreover, Au@CPF‐1 exhibits superior catalytic activity compared to many other reported Au‐based catalysts for the reduction of 4‐nitrophenol in the presence of NaBH₄. In addition, Au@CPF‐1 has excellent stability and recyclability, and it can be reused for three successive reaction cycles without loss of activity. The dense distribution of phenyl rings on the channel walls of the CPF support can reasonably be regarded as the active sites that adsorb the 4‐nitrophenol molecule through hydrogen‐bonding and C?H ??? π interactions, as was confirmed by the X‐ray structure of model compound DAPP‐Benz.     

Functionalization of azafullerene C59N. Radical reactions with 9-substituted fluorenes

An efficient reaction between the azafullerene dimer, (C₅₉N)₂ and 9-substituted fluorenes leads to the formation of four new azafullerene monoadducts.     

Exploring the interactions between gold nanoparticles and analytes through surface‐assisted laser desorption/ionization mass spectrometry

Surface‐assisted laser desorption/ionization mass spectrometry (SALDI‐MS) is applied to provide strong evidence for the chemical reactions of functionalized gold nanoparticles (Au NPs) with analytes – Hg²⁺ ions induced MPA?Au NPs aggregation in the presence of 2,6‐pyridinedicarboxylic acid (PDCA) and H₂O₂ induced fluorescence quenching of 11‐MUA?Au NDs. PDCA‐Hg²⁺‐MPA coordination is responsible for Au NPs aggregation, while the formation of 11‐MUA disulfide compounds that release into the bulk solution is responsible for H₂O₂‐induced fluorescence quenching. In addition to providing information about the chemical structures, SALDI‐MS is also selective and sensitive for the detection of Hg²⁺ ions and H₂O₂. The limits of detection (LODs) for Hg²⁺ ions and H₂O₂ by SALDI‐MS were 300 nM and 250 µM, respectively. The spot‐to‐spot variations in the two studies were both less than 18% (50 sample spots). Our results reveal that SALDI‐MS can be used to study analyte‐induced changes in the surface properties of nanoparticles. Copyright © 2010 John Wiley & Sons, Ltd.     

One‐Dimensional Carbon Nanotube/SnO2/Noble Metal Nanoparticle Hybrid Nanostructure: Synthesis,Characterization, and Electrochemical Sensing

Herein we report a facile and efficient method for self‐assembling noble‐metal nanoparticles (NPs) to the surface of SnO₂‐coated carbon nanotubes (CNT@SnO₂) to construct CNT@SnO₂/noble metal NP hybrids. By using SnCl₄ as the precursor of the SnO₂ shell on the surface of CNTs, the hydrolysis speed of SnCl₄ was slowed down in ethanol containing a trace amount of urea and water. The coaxial nanostructure of CNT@SnO₂ was confirmed by using X‐ray powder diffraction (XRD) and transmission electron microscopy (TEM). It was found that the coating layer of SnO₂ was homogeneous with the mean thickness of 8 nm. The CNT@SnO₂/noble‐metal NP hybrids were obtained by mixing noble‐metal NPs with as‐prepared CNT@SnO₂ coaxial nanocables by means of a self‐assembly strategy. With the amino group terminated, the CNT@SnO₂ coaxial nanocable can readily adsorb the as‐prepared noble‐metal NPs (Au, Ag, Au? Pt, and Au? Pd NPs). The presence of an amino group at the surface of SnO₂ was proved by use of X‐ray photoelectron spectroscopy (XPS). In addition, H₂O₂ sensing by amperometric methods could serve as detection models for investigating the electrocatalytic ability of as‐prepared hybrid materials. It was found that wide linear ranges and low detection limits were obtained by using the enzyme‐free CNT@SnO₂@Au? Pt modified electrode, which indicated the potential utilizations of the hybrid based on CNT@SnO₂ for electrochemical sensing.     

Electrochemical Biosensor Utilizing Supramolecular Association of Enzyme on Sol−gel Matrix Embedded Gold Nanoparticles Supported Reduced Graphene Oxide−cyclodextrin Nanocomposite

Herein we present β‐cyclodextrin (CD)‐functionalized reduced graphene oxide (RGO) nanosheets supported on silicate sol‐gel matrix‐embedded gold nanoparticles (Au NPs) modified electrode as a new affinity binding nanocomposite. The modified electrode is fabricated through layer‐by‐layer drop casting followed by immobilization of chemically modified enzyme conjugate (horse radish peroxidase (HRP)?adamantane carboxylic acid (ADA)). This affinity system is based on the supramolecular association between CDs and HRP?ADA and is mimicking the biological avidin?biotin interactions. CDs‐functionalized RGO (RGO?CD) functions as a macrocyclic host to form stable supramolecular inclusion complexes with enzyme conjugate. Besides Au NPs improve the interfacial interaction with RGO?CD nanosheets, and thus exhibit synergistic electrocatalytic effect toward H₂O₂ reduction in the presence of 1 mM hydroquinone.     

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity,Stability, and Temperature‐ Dependent Activation Properties

Nanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs₄[α‐SiW₁₂O₄₀]⋅n H₂O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was −67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Au^δ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.