Fullerene Radical Anion Research Articles

Magnetic Bistability in the Crystaline Fullerene Radical Salt

Comprehensive SummaryCompounds with magnetic bistability is highly attractive for the construction of switches, thermal sensors, information‐storage media and memory devices. Herein, we report a crystalline fullerene radical anion salt [Na(THF)5]C60 (1), obtained by the reduction of C60 with Na in solution, which exhibits magnetic bistability accompanied with one magnetostructural transition, giving low temperature (LT), high temperature (HT) phases and a metastable phase. Fullerene radical anions (C60•‐) in the structure were found arranged into a three‐dimensional close‐packed honeycomb subnetwork with hollow channels occupied by complex cations [Na(THF)5]+ as spacers. The magnetic bistability is attributed to structural transitions as the LT−IT phase transition was associated with the orientational disorder of fullerene anions and the distortion of the complex cations [Na(THF)5]+ in the 3D packing of fullerene anions.

Methanofullerene Synthesis via Photogenerated Fullerene Radical Anion Intermediates.

This work describes the synthesis of PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) derivatives and other methanofullerene derivatives via generation of fullerene radical anions under photoirradiation and controlled by photoswitching, without preparation, a strong reducing agent, or precise control of the reaction conditions.

Light-Induced EPR Study of the Effect of Coumarin Trace Additives on Spin Dynamics in the P3DDT/PC61BM Polymer Composite

The paper presents the results of an EPR study of the magnetic, relaxation, and dynamic parameters of spin charge carriers photogenerated by light in photovoltaic polymer composites, based on poly(3-dodecylthiophene) (P3DDT) and 6,6-phenyl-C61-butyric acid methyl ester (PCBM), in the presence of coumarin admixed to the system. It has been shown that the concentration of polarons and fullerene radical anions is characterized by a nonmonotonic dependence on the energy of incident photons with extrema at 1.9 and 2.7 eV. The largest increase in the concentration of mobile charge carriers was observed in composites with a coumarin content of 3 or 6 wt %. A significant slowdown of the carrier recombination process in the composites containing 3–6 wt % coumarin was observed after switching off the light. The temperature dependences of the concentrations of mobile and localized charge carriers for P3DDT/PCBM samples with a coumarin content of 3 wt % showed enhancement of the exchange interaction between charge carriers. The data obtained lead to the conclusion that coumarin affects spin interactions in the polymer composite.

Sonochemical synthesis of novel C60 fullerene 1,4-oxathiane derivative through the intermediate fullerene radical anion.

We have discovered a selective and efficient method for the synthesis of previously unknown 1,9-(1',4'-oxathiano)-1,9-dihydro-(C60-Ih)[5,6]fullerene, a compound with the direct attachment of the sulfur atom to the fullerene core. The method is based on the reaction of C60 with 1,2-hydroxythiols in the presence of inorganic bases in air under ultrasonication. The significance of ultrasound has been exemplified with the comparative conventional methods. The title compound has been identified with mass-(MALDI TOF/TOF), one- and two-dimensional NMR 1H and 13C (COSY, HSQC, HMBS), IR, UV-Vis spectroscopic techniques. Using the direct EPR method, we have detected radical anion C60- (g=2.0046 and ΔH=2 G) as a key reaction intermediate of the sonochemical reaction. Based on the experimental results and quantum chemical calculations, we have proposed a mechanism for the conversion of C60 and 2-mercaptoethanol to the C60-1,4-oxathiane adduct.

Influence of Photogeneration Frequency on the Transport of Spin Charge Carriers in the Copolymer–Methanofullerene Composite: EPR Study

The photovoltaic composite formed by narrow-gap copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] and methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been studied in a wide energy range of generating photons, 1.32–3.14 eV at T = 77 K, by the light-induced electron paramagnetic resonance. It has been shown that some polarons are captured by spin traps formed in the copolymer matrix, and the concentration and depth of such traps are determined by the photon energy. The recombination kinetics of polarons and fullerene radical anions after turning off the light can be described in the framework of a second-order bimolecular process. The formation of spin traps in the copolymer matrix and the exchange interaction between different spin packets cause the extreme sensitivity of magnetic resonance and electronic parameters of charge carriers to the number and energy of generating photons.

Photoexcitation of self-n-doped fullerene ammonium halides: The role of halide ion and a possible synergistic dual-redox cycle mechanism within their aggregate

Halide ion’s doping of fullerene core is crucial on the properties of highly conductive self-n-doped fullerene ammonium halides, which are a kind of promising electron transport material for photovoltaics. Herein, to understand their photoexcited electron transfer (ET) property, antimicrobial photodynamic inactivation (aPDI) activities of these materials were tested based on their unique electronic structure and bacterial electrokinesis. We observed that illuminated self-n-doped fullerene ammonium iodides (PCBANI and PCBDANI) could exhibit significant improvement of activity against two important plant pathogenic fungi, Sclerotinia sclerotiorum and Fusarium graminearum as electron donor, compared with that in dark. Through comprehensive studies, such as aPDI activity, hyphal cell morphology, ultrafast transient absorption spectroscopy, solid state nuclear magnetic resonance (ssNMR), level of total cellular reactive oxygen species (ROS), we verified that not ROS but I radical’s oxidative stress is responsible for the improved aPDI activity. Moreover, the photobiological activity of self-n-doped fullerene ammonium halides is dependent on the reducing capacity of halide anions. The ET rate from anion to an excited fullerene core decreased successively from I– to Br– and then Cl–, illustrating that the generation rate of I radical species was the fastest and consistent with the high activity of fullerene ammonium iodides. Remarkably, adding potassium iodide did not enhance PCBANI’s antifungal activity. These results disclose that the unique electronic structure of iodide and fullerene core in self-n-doped PCBANI aggregates are critical for its aPDI activity. Consequently, we propose a possible dual-redox cycles mechanism involving photoexcited fullerene radical anions and I radical species in self-n-doped fullerene ammonium iodide aggregate to exhibit sustainable aPDI activity. This work reveals that halide ions play an equally important role as fullerene core in photoexcited ET property of self-n-doped fullerene ammonium halides.

Fullerene Mediated Electrosynthesis of Silver Nanoparticles in Toluene-DMF

Using C60 and C70 fullerenes as mediators, efficient mediated electrosynthesis of silver nanoparticles (AgNP) in toluene-DMF/ 0.1 M Bu4NBF4 medium was carried out by reduction of Ag+ at the potentials of the fullerene radical anions generation. Synthesis was performed in the absence and in the presence of poly(N-vinylpyrrolidone) (PVP) in the cathodic compartment of a diaphragm-separated cell by addition of Ag+ as AgNO3 into solution and in a single compartment cell by in situ generation of Ag+ by dissolution of an Ag-anode during electrolysis. In all cases, Ag+ ions are quantitatively reduced upon consumption of the theoretical amount of electricity; the generated metal is quantitatively obtained in solution bulk in the form of spherical NP with sizes in the range from 11 ± 2 to 24 ± 9 nm. In electrolysis in a single compartment cell, dispersion of the Ag-anode occurs along with its dissolution and the total current efficiency of AgNP is 131 ÷ 163%. In the presence of PVP, individual AgNP are stabilized in the PVP shell. The isolated Ag/PVP nanocomposite is deposited on a solid support both in individual form and as three-component Ag/PVP/fullerene nanocomposite. In the absence of PVP, the Ag/fullerene nanocomposite is obtained.

EPR study of spin interactions in the poly(3-dodecylthiophene): Fullerene/polyaniline: p-toluenesulfonic acid composite

Magnetic, relaxation, and dynamic parameters of polarons P1·+, trapped in p-toluenesulfonic aciddoped polyaniline, polarons P2·+, and fullerene radical anions photoinduced in the poly(3-dodecylthiophene)/methanofullerene composite and in mixtures of these composites have been investigated using the EPR technique. It has been shown that spin charge carriers P2·+ during diffusion along poly(3-dodecylthiophene) chains interact with P1·+ trapped on adjacent polymer chains of polyaniline. The exchange interaction and spin relaxation in the ternary composites is determined by the activated hopping mobility of polarons P2·+ and depends on the composition of the material.

Cyclic Tetramers of Zinc Chlorophylls as a Coupled Light-Harvesting Antenna-Charge-Separation System.

A coupled light-harvesting antenna-charge-separation system, consisting of self-assembled zinc chlorophyll derivatives that incorporate an electron-accepting unit, is reported. The cyclic tetramers that incorporated an electron acceptor were constructed by the co-assembly of a pyridine-appended zinc chlorophyll derivative, ZnPy, and a zinc chlorophyll derivative further decorated with a fullerene unit, ZnPyC60 . Comprehensive steady-state and time-resolved spectroscopic studies were conducted for the individual tetramers of ZnPy and ZnPyC60 as well as their co-tetramers. Intra-assembly singlet energy transfer was confirmed by singlet-singlet annihilation in the ZnPy tetramer. Electron transfer from the singlet chlorin unit to the fullerene unit was clearly demonstrated by the transient absorption of the fullerene radical anion in the ZnPyC60 tetramer. Finally, with the co-tetramer, a coupled light-harvesting and charge-separation system with practically 100 % quantum efficiency was demonstrated.

Ultrafast Photoinduced Charge Separation Leading to High-Energy Radical Ion-Pairs in Directly Linked Corrole-C60 and Triphenylamine-Corrole-C60 Donor-Acceptor Conjugates.

Closely positioned donor-acceptor pairs facilitate electron- and energy-transfer events, relevant to light energy conversion. Here, a triad system TPACor-C60 , possessing a free-base corrole as central unit that linked the energy donor triphenylamine (TPA) at the meso position and an electron acceptor fullerene (C60) at the β-pyrrole position was newly synthesized, as were the component dyads TPA-Cor and Cor-C60. Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady-state fluorescence studies showed efficient energy transfer from (1) TPA* to the corrole and subsequent electron transfer from (1) corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron-transfer products, the corrole radical cation (Cor(⋅+) in Cor-C60 and TPA-Cor(⋅+) in TPACor-C60) and fullerene radical anion (C60(⋅-)), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, kCS , was found to be about 10(11) s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10) s(-1)), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge-separated species relaxed directly to the ground state in polar solvents while in toluene, formation of (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* being about 1.52 eV. That is, ultrafast formation of a high-energy charge-separated state in toluene has been achieved in these closely spaced corrole-fullerene donor-acceptor conjugates.

Fullerene van der Waals oligomers as electron traps.

Density functional theory calculations indicate that van der Waals fullerene dimers and larger oligomers can form interstitial electron traps in which the electrons are even more strongly bound than in isolated fullerene radical anions. The fullerenes behave like "super atoms", and the interstitial electron traps represent one-electron intermolecular σ-bonds. Spectroelectrochemical measurements on a bis-fullerene-substituted peptide provide experimental support. The proposed deep electron traps are relevant for all organic electronics applications in which non-covalently linked fullerenes in van der Waals contact with one another serve as n-type semiconductors.

A Charge‐Stabilizing, Multimodular, Ferrocene–Bis(triphenylamine)–Zinc‐porphyrin–Fullerene Polyad

A novel multimodular donor-acceptor polyad featuring zinc porphyrin, fullerene, ferrocene, and triphenylamine entities was designed, synthesized, and studied as a charge-stabilizing, photosynthetic-antenna/reaction-center mimic. The ferrocene and fullerene entities, covalently linked to the porphyrin ring, were distantly separated to accomplish the charge-separation/hole-migration events leading to the creation of a long-lived charge-separated state. The geometry and electronic structures of the newly synthesized compound was deduced by B3LYP/3-21G(*) optimization, while the energy levels for different photochemical events was established using data from the optical absorption and emission, and electrochemical studies. Excitation of the triphenylamine entities revealed singlet-singlet energy transfer to the appended zinc porphyrin. As predicted from the energy levels, photoinduced electron transfer from both the singlet and triplet excited states of the zinc porphyrin to fullerene followed by subsequent hole migration involving ferrocene was witnessed from the transient absorption studies. The charge-separated state persisted for about 8.5 μs and was governed by the distance between the final charge-transfer product, that is, a species involving a ferrocenium cation and a fullerene radical anion, with additional influence from the charge-stabilizing triphenylamine entities located on the zinc-porphyrin macrocycle.

A Molecular Shuttle Driven by Fullerene Radical‐Anion Recognition

We describe a electrochemically driven molecular shuttle, in which shuttling takes place by means of fullerene radical-anion recognition that results in a very low operation potential (E(1/2) =-0.580 V vs. decamethylferrocene). This has been achieved by introducing positive charges on the macrocycle, which strengthen the existing π-π interactions between the macrocycle and the electrogenerated fullerene radical anion by means of an electrostatic component. In addition, the synthesis of such a molecular shuttle has been accomplished by developing a new synthetic approach that exploits the controlled translocation of the macrocycle as a selective protecting group.

Light-Induced EPR Study of Charge Transfer in P3HT/PC71BM Bulk Heterojunctions

Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with energy of 1.98–2.73 eV in bulk heterojunctions formed by regioregular poly(3-hexylthiophene) (P3HT) with (6,6)-phenyl-C71-butyric acid ester (PC71BM) methanofullerene have been studied by the direct light-induced EPR (LEPR) method at a wide temperature region. LEPR spectra of the P3HT/PC71BM composite were deconvoluted, and the main magnetic resonance parameters of these charge carriers have been determined. A part of photoinduced polarons is pinned by large-depth traps in the polymer matrix. It was shown that magnetic resonance, relaxation, and dynamics parameters of photoinduced charge carriers depend extremely on the energy of initiated photons. Relaxation and dynamics parameters of both the charge carriers were determined separately by the steady-state saturation method. Longitudinal diffusion of polarons was analyzed in terms of spin interaction with the lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The interchain spin hopping is determined by the number and depth of the traps photoinitiated in the polymer matrix. Pseudorotation of methanofullerene molecules in a polymer matrix was shown to follow the activation Pike model. The replacement in the composite of PC61BM acceptors by PC71BM ones accelerates electron relaxation, hinders the formation of spin traps, and favors more ordered (crystalline) structure of bulk heterojunction that facilitates charge transfer in the P3HT/PC71BM composite.

Effect of the Cooling Rate on Dimerization of C60•– in Fullerene Salt (DMI+)2·(C60•–)·{Cd(Et2NCS2)2I–}

The salt (DMI(+))(2)·(C(60)(•-))·{Cd(Et(2)NCS(2))(2)I(-)} (1) containing fullerene radical anions, the anions of cadmium diethyldithiocarbamate iodide, and N,N'-dimethylimidazolium cations was obtained. Fullerenes are monomeric in 1 at 250 K and form three-dimensional packing in which each fullerene has nearly tetrahedral surroundings from neighboring fullerenes. Fullerenes with a shorter interfullerene center-to-center distance of 10.031(2) Å form spiral chains arranged along the lattice c axis. The convolution consists of four fullerene molecules. Dimerization realized in 1 within the spiral chains below 135 K manifests a strong dependence on the cooling rate. The "frozen" monomeric phase was obtained upon instant quenching of 1. This phase is stable below 95 K for a long time but slowly converted to the dimeric phase at T > 95 K. It exhibits a weak antiferromagnetic interaction of spins below 95 K (the Weiss temperature is -4 K), which results in the splitting of the electron paramagnetic resonance (EPR) signal into two components below 10 K. A disordered phase containing both C(60)(•-) monomers and singly bonded (C(60)(-))(2) dimers with approximately 0.5/0.5 occupancies is formed at an intermediate cooling rate (for 20 min). The position of each fullerene in this phase is split into three positions slightly shifted relative to each other. The central position corresponds to nonbonded fullerenes with interfullerene center-to-center distances of 9.94-10.00 Å. Two other positions are coincided to dimeric fullerenes formed with the right and left fullerene neighbors within the spiral chain. This intermediate phase is paramagnetic with nearly zero Weiss temperature due to isolation of C(60)(•-) by diamagnetic species and exhibits a strongly asymmetric EPR signal below 20 K. A diamagnetic phase containing ordered singly bonded (C(60)(-))(2) dimers can be obtained only upon slow cooling of the crystal for 6 h.

Synthesis and Photophysical Properties of a Supramolecular Host–Guest Assembly Constructed by Fullerenes and Tryptamine Modified Hypocrellin

The metal coordination polymer of hypocrellin A bearing tryptamine motif (M-DTrpHA) can include fullerene via a two-point interaction, involving π-π stacking and electron donor-acceptor interaction. The 1:1 host-guest system M-DTrpHA/fullerene exhibits a moderate association constant K(a) (6.62 × 10(4) to 6.46 × 10(5) M(-1)). Both of the metal ions in M-DTrpHA and the substituents connected to the fullerene core play important roles in stabilizing the M-DTrpHA/fullerene complex. Transient absorption spectral and NIR absorption spectral results demonstrate that, in the M-DTrpHA/fullerene system, efficient photoinduced electron transfer from the tryptamine group in M-DTrpHA to fullerene may occur, resulting in a long-lived fullerene anion radical. The observed order of quantum yield (Φ(ET)(T)) and rate constants (K(ET)(T)) for electron transfer via (3)C(60)(*) is Y(3+)-DTrpHA > La(3+)-DTrpHA > DTrpHA, consistent with their binding ability to C(60). The nanostructure of M-DTrpHA is rearranged to form an interpenetrating network after interaction with fullerene.

Effect of substituents and protonation on the mechanism of 1,3‐dipolar retro‐cycloaddition reaction of pyrrolidino[60]‐ and [70]fullerenes

The mass spectra of new substituted pyrrolidino[60]- and [70]fullerenes have been obtained using electrospray ionization conditions in the positive and negative mode of detection with two different mass spectrometers, a quadrupole ion trap and a Fourier transform ion cyclotron resonance. Radical anions M(●-) and deprotonated molecules [M-H](-) are formed under negative electrospray ionization mass spectrometry conditions, and the collision-induced dissociations of both ionic species have been studied. Either negative odd-electron ions or negative even-electron ions undergo a retro-cycloaddition process forming the corresponding fullerene product ions C(60)(●-) and C(70)(●-). The generation of fullerene radical anions from deprotonated molecules is a new exception of the "even-electron rule." In contrast, the protonated molecules [M + H](+) obtained from the positive mode of detection do not undergo this cycloreversion reaction, and the MS(n) experiment reveals a variety of eliminations of neutral molecules involving different hydrogen shifts and multiple bond cleavages that lead eventually to substituted methanofullerene fragment ions. The observed fragmentations can be correlated with the electronic character of the substituents attached to the heterocyclic moiety. The results obtained from the thermal reactions of these compounds, carried out under different pH conditions, correlate well with those obtained in gas phase. The different behaviors between protonated and unprotonated molecules and ions can be explained assuming that the retro-cycloaddition reaction takes place only when the nitrogen atom of the pyrrolidine ring (the basic center of the molecule) is unprotonated both in gas and condensed phase. The protonation of the NH group inhibits the cycloreversion process, and therefore different fragmentations take place. The detailed mechanisms of the formation and evolution of the intermediate fragments are described.

Ionic complexes simultaneously containing fullerene anions and coordination structures of metal phthalocyanines with I− and EtS− anions

The ionic complexes simultaneously containing negatively charged coordination structures of metal phthalocyanines and fullerene anions, viz., {MnIIPc(CH3CH2S−) x ·(I−)1−x }·(C60 ·−)· ·(PMDAE+)2·C6H4Cl2 (PMDAE is N,N,N′,N′,N′-pentamethyldiaminoethane, x = 0.87, 1) and {ZnIIPc(CH3CH2S−)y·(I−)1−y }2·(C60 −)2·(PMDAE+)4·(C6H4Cl2) (y = 0.5, 2) were synthesized. The both compounds were obtained as single crystals, which made it possible to study their crystal structures. In complex 1, the fullerene radical anions form honeycomb-like layers in which each fullerene has three neighbors with center-to-center interfullerene distances of 10.13–10.29 A. Rather long distances between the C60 ·− radical anions results in the retention of monomeric C60 ·− in this complex down to the temperature of 110(2) K. In complex 2, fullerenes form dimers (C60 −)2 bonded by one C-C bond. The dimers are packed in corrugated honeycomb-like layers with interfullerene center-to-center distances of 9.90–10.11 A. Manganese(II) and zinc(II) phthalocyanines coordinate iodide and ethanethiolate anions to the central metal atom to form unusual negatively charged coordination structures MIIPc(An−) (An− is anion) packed in dimers {MIIPc(An−)}2 with a short distance between the phthalocyanine planes (3.14 A in 1 and 3.27 A in 2). The pthalocyanine dimers also form layers with the PMDAE+ cations, and these layers alternate with the fullerene layers. The packing of spherical fullerenes with planar phthalocyanine molecules is attained by the insertion of fullerenes between the phenylene groups of phthalocyanines. The π-π-interactions of the porphyrin macrocycle with five- or six-membered fullerene rings are characteristic of the earlier studied ionic porphyrin and fullerene complexes. Such interactions are not observed for ionic complexes 1 and 2.

Light-induced EPR study of charge transfer in P3HT/bis-PCBM bulk heterojunctions

Radical pairs, polarons and fullerene anion radicals photoinduced by photons with energy of 1.98 – 2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) with bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6.6]C62 (bis-PCBM) fullerene derivative have been studied by direct light-induced EPR (LEPR) method in a wide temperature range. A part of photoinduced polarons are pinned in trap sites which number and depth are governed by an ordering of the polymer/fullerene system and energy of initiating photons. It was shown that dynamics and recombination of mobile polarons and counter fullerene anion radicals are governed by their exchange- and multi-trap assisted diffusion. Relaxation and dynamics parameters of both the charge carriers were determined separately by the steady-state saturation method. These parameters are governed by structure and conformation of the carriers’ microenvironment as well as by the energy of irradiating photons. Longitudinal diffusion of polarons was shown to depend on lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The energy barrier required for polaron interchain hopping is higher than that its intrachain diffusion. Pseudorotation of fullerene derivatives in a polymer matrix was shown to follow the activation Pike model.

Structural effect of electron acceptor on charge transfer in poly(3-hexylthiophene)/methanofullerene bulk heterojunctions

Radical pairs, polarons and fullerene anion radicals photoinduced by photons with energy of 1.98–2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) with bis(1-[3-(methoxycarbonyl)propyl]-1-phenyl)-[6.6]C 62 ( bis-PCBM) methanofullerene have been studied as compared with P3HT/PCBM composite by direct light-induced EPR (LEPR) method in a wide temperature range. A part of spin polarons and methanofullerene anion radicals are pinned in trap sites which number and depth are governed by an ordering of the polymer/fullerene system and energy of initiating photons. It has been shown that dynamics and recombination of mobile polarons and counter methanofullerene anion radicals are governed by their exchange- and multi-trap assisted diffusion. Relaxation and dynamics of both the charge carriers determined by the steady-state saturation method are governed by structure and conformation of their microenvironment as well as by the photon energy. Longitudinal diffusion of polarons was shown to depend on lattice phonons of crystalline domains embedded into an amorphous polymer matrix. The energy barrier required for polaron interchain hopping is higher than its intrachain diffusion. Pseudorotation of fullerene derivatives in a polymer matrix was shown to follow the activation Pike model. The replacement of PCBM by bis-PCBM provides higher anisotropy of polaron dynamics and decreases its selectivity to the photon energy. This makes spin dynamics easier and minimizes energy dispersion at charge transfer.