Fullerene Dianions Research Articles

Direct Investigation of Excited C60 Dianion and Its Intramolecular Electron Transfer Behaviors.

For the first time, the dynamics of excited fullerene dianions and associated intramolecular electron transfer (ET) were directly investigated by using femtosecond pump-probe laser flash photolysis on selectively reduced C60, pyrrolidino[60]fullerene (C60H), and dyads including C60-naphthalenediimide (NDI) and C60-pyromellitimide (PI). Upon near-infrared laser excitation, the excited dianion of C60 or C60H displayed two states with lifetimes of less than one and several tens of ps, attributed to prompt internal conversion from the theoretically predicted Sn state. Furthermore, the ET processes from the excited C602- in dyad molecules, including C602--NDI•- and C602--PI•-, were confirmed with varied ET rate constants due to the difference in the driving force for ET. The current findings provide a clear description of the hitherto uncharted excited-state and photoinduced ET characteristics of fullerene dianions, paving the way for photochemical studies of excited multi-ions (excited multi-polarons) and their application in organic semiconducting materials.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions.

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C842-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C702- and a negative one for C602-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

Delayed electron emission from fullerene dianions: Evidence for a thermal tunneling detachment

Isolated multiply charged anions exhibit a repulsive Coulomb barrier (RCB) towards electron loss. In this paper, the role of tunneling through the RCB in the delayed electron emission is addressed. Electron kinetic energy distributions of photoexcited fullerene dianions ${\mathrm{C}}_{84}{}^{2\ensuremath{-}}$ and ${\mathrm{C}}_{90}{}^{2\ensuremath{-}}$ have been measured with a velocity-map imaging spectrometer for delays of about 150 ns after the nanosecond laser excitation. On such spectra, delayed electron emission is unambiguously separated from direct detachment. These measurements display significant delayed emission below the RCB. A detailed-balance model including electron tunneling through the RCB has been developed. The very good agreement between the measurements and the model provides quantitative evidence for a thermal tunneling electron emission.

On the Radical Anion Spectra of Fullerenes C60 and C70

The radical anion spectra of C60 and C70 can be easily generated and studied in primary aliphatic amines. In these conditions the C60 _• and C70 _• species are accompanied by the dianions C60 2− and C70 2−. In the near infrared region the electronic transitions at 1075, 1058 and 991 nm were assigned to C60 _• while those at 1020–1030 nm and at 933 nm were assigned to the fullerene dianion C60 2−. The electronic transition of C70 _• occurs at 1370 nm and that of C60 2− at 1170 nm, but in primary aliphatic amines C70 also display a broad absorption band at 814–820 nm with other features certainly due to a specific interaction of the amine with C70. The behavior of C60 was studied with primary aromatic amines, but no interaction was found. Furthermore, solutions of C60 and C70 were studied with the secondary amine dioctylamine and with the tertiary amines triethylamine and N,N,N',N'-tetramethylethylenediamine (TMEDA). FT-IR spectra of selected fullerene-amine adducts were reported.

Negative-Ion Electron Capture Dissociation: Radical-Driven Fragmentation of Charge-Increased Gaseous Peptide Anions

The generation of gaseous polyanions with a Coulomb barrier has attracted attention as exemplified by previous studies of fullerene dianions. However, this phenomenon has not been reported for biological anions. By contrast, electron attachment to multiply charged peptide and protein cations has seen a surge of interest due to the high utility for tandem mass spectrometry (MS/MS). Electron capture dissociation (ECD) and electron transfer dissociation (ETD) involve radical-driven fragmentation of charge-reduced peptide/protein cations to yield N-C(α) backbone bond cleavage, resulting in predictable c'/z(•)-type product ions without loss of labile post-translational modifications (PTMs). However, acidic peptides, e.g., with biologically important PTMs such as phosphorylation and sulfonation, are difficult to multiply charge in positive ion mode and show improved ionization in negative-ion mode. We found that peptide anions ([M - nH](n-), n ≥ 1) can capture electrons within a rather narrow energy range (~3.5-6.5 eV), resulting in charge-increased radical intermediates that undergo dissociation analogous to that in ECD/ETD. Gas-phase zwitterionic structures appear to play an important role in this novel MS/MS technique, negative-ion electron capture dissociation (niECD).

Electron emission from laser-heated fullerene dianions: Probing the repulsive Coulomb barrier

Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.

Formation of higher-order fullerene dianions in collisions with Na atoms

We have measured electron capture cross sections in collisions between higher order fullerene anions Cn- (n=76, 78, 82, 84, 86, 90 and 96) and Na atoms. The ions were produced in an electrospray ion source (ESI) and accelerated to an energy of 50 keV. The measured cross section for dianion formation is three times larger for C96 than that for C60. The latter cross section was earlier found to be 36 A2. The dramatic increase of the cross section with fullerene size is explained by means of the curve crossing model for electron transfer.

Photodetachment of fullerene monoanions and dianions in a Penning trap: Probes of delayed electron emission and associated activation barriers

Singly and doubly charged fullerene anions were introduced into gas phase by means of electrospray starting from a solution containing a very efficient electron donor compound. Following isolation and trapping, mass selected anions were irradiated with a ns-pulse from a Nd:YAG laser at the third harmonic ( hν = 3.5 eV) in order to study (delayed) photoemission. We present results on the laser-fluence dependence of photoemission for singly charged C 76 − and C 84 − as well as for doubly charged C 76 2−. Results are discussed in terms of statistical electron emission as opposed to direct detachment. They are further analyzed to give information on the number of photons necessary to induce delayed electron emission on the experimental time scale. The averaged absolute absorption cross-sections obtained were about 0.6–0.8 Å 2 for all species probed. The agreement between experimental data and statistical modelling shows that electronic excitations which rapidly relax to vibrationally hot ions provide the dominant contributions to the 355 nm absorption cross-sections. As a corollary, we show that direct single-electron detachment is a minor process for the singly charged fullerene anions having a cross-section <0.02 Å 2. For C 76 2−, electron emission follows an absorption law in which single-photon excitation is the leading contribution. From the data we deduce a Coulomb barrier height for C 76 2− of 1.20 eV.

Publisher’s Note: “Electron transfer collisions between isolated fullerene dianions and SF6” [J. Chem. Phys. 123, 074318 (2005)

Publisher’s Note: “Electron transfer collisions between isolated fullerene dianions and SF6” [J. Chem. Phys. <b>123</b>, 074318 (2005)

Electron transfer collisions between isolated fullerene dianions and SF6

Electron transfer collisions of trapped doubly charged fullerene anions C76(2-), C(78)2-, and C84(2-) with SF6 are studied in a Fourier transform ion cyclotron resonance mass spectrometer at center-of-mass collisional energies ranging from thermal energy to 77 eV. Collision energy dependencies manifest threshold energies for (nominally exoergic) single electron transfer onto SF6 of 1.46+/-0.3 eV, 1.56+/-0.3 eV, and 1.63+/-0.3 eV for C(76)2-, C78(2-), and C(84)2-, respectively. Kinetics studies reveal charge-transfer cross sections of up to 430+/-200 A2 for C84(2-) at a collision energy of 77 eV. The mechanism and the energetics are discussed in terms of classical electrostatic model calculations. Additionally, we rationalize the collision energy dependencies of the charge-transfer cross sections using the two-state Landau-Zener formalism to describe the associated resonant electron tunneling probability.

Formation of fullerene dianions in a Penning trap

Fullerene dianions in the range C702- to C902- have been created by subjecting trapped fullerene monoanions to low energy electrons in a Penning trap. The dianion production was found to be a function of the trapping-potential depth and the time of interaction between the simultaneously stored monoanions and electrons. Under similar conditions the dianion yield depends on the size of the fullerenes with more than 10% of the trapped C90- ions forming dianions while the corresponding relative yield for C702- was less than 0.1%. The large difference can be explained by the repulsive Coulomb barrier and the second electron affinity of the fullerenes.

Electrochemical Synthesis and Transformations of Substituted Methano[60]fullerenes

61‐(Dimethoxyphosphoryl)‐61‐(methoxycarbonyl)methano[60]fullerene was produced by using the joint electrochemical reduction of fullerene C60 and (dimethoxyphosphoryl)methoxycarbonyldibromomethane. A few steps of reduction were detected for phosphorylated methano[60]fullerenes. When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro‐Bingel reaction). This process involves the step‐by‐step cleavage of two C–C bonds of exo‐carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion.

Novel experimental arrangement developed for direct fullerene analysis by electrospray time‐of‐flight mass spectrometry

A novel electrospray setup was found effective for direct analysis of fullerene solutions by electrospray (ES) mass spectrometry. The electrospray capillary needle used for the analysis is equipped with a thin metal (copper, platinum or stainless steel) wire installed inside the capillary. The wire tip protrudes slightly from the capillary end. In this configuration the high electrical field formed by the wire tip stimulates a specific electrospray mode with a fine spray originating from the tip. The correlation of the acquired mass spectra with the magnified view of the spray at the capillary tip was investigated. The effective formation of fullerene ions in both negative and positive ion modes was observed in mass spectra only in the specific case of the electrospray originating from the wire tip. The fullerene di-anions observed in the negative ES mass spectra provide evidence for the electrochemical nature of this process occurring at the ES capillary tip. Observation of fullerene ions in mass spectra obtained using the suggested electrospray arrangement is assumed to be a consequence of the fine spray originating from the sharp metal wire tip. In this case the liquid/metal interface is near the Taylor cone apex.

Electron attachment to negative fullerene ions: a Fourier transform mass spectrometric study

Fullerene dianions were prepared by low-energy electron attachment to negatively charged fullerene ions C 76 − and C 86 − stored in a commercial Penning trap spectrometer under UHV conditions. Up to 85% of trapped monoanions can be converted to dianions upon long-term irradiation with electrons having kinetic energies averaging 6 eV corresponding to an attachment cross-section on the order of 100 Å 2 for C 76 −. The lifetime of the resulting C 76 2− was determined to be >10 4 s—limited by collisions with residual gas molecules. A model for statistical electron emission based on Klots’ theory is used to provide an upper limit for the dianion temperature.

On the generation and stability of isolated doubly negatively charged fullerenes

Intense fullerene anion beams were generated by electrospray of dichlorobenzene solutions of fullerenes and tetrakis(dimethylamino)ethylene (TDAE). Ions were probed with Fourier transform mass spectrometry (FTMS) about one second after the electrospray step. In addition to monoanions, large quantities of fullerene dianions were detected. The efficiency of dianion formation and detection was found to increase dramatically with cage size. While C 60 2− could not be observed, dianions of trace fullerenes up to C 124 2− were readily accessed. Among all fullerene dianions probed only C 70 2− was found to be metastable under collision-free conditions on the measurement timescale with a half-life of about 80 s towards electron autodetachment. Observation and stability of fullerene dianions are discussed in terms of rough estimates of their second electron affinities.

Lifetimes of metastable spherical carbon cluster dianions

Semiclassical calculations are made of the lifetimes of metastable spherical carbon cluster dianions using a model potential based on electrostatics. The metastability is attributed to shape resonances resulting from the combination of the long range Coulomb repulsion and the shorter range electron-molecule interaction. Results for fullerene dianions show a strong dependence of the lifetimes on the molecular size. The transition from stable to metastable dianions is estimated to occur when the radius of the spherical molecule is about 5.5 Å. This simple model potential might provide a starting point for experimental and theoretical searches for the existence of stable dianions

Retro-Bingel reaction in the electrochemical reduction of bis(dialkoxyphosphoryl)methanofullerenes

Four steps of reduction were detected for bis(diethoxyphosphoryl)- and bis(diisopropoxyphosphoryl)methano[60]fullerenes (1, 2) and bis(diethoxyphosphoryl)methano[70]fullerene (3) by cyclic voltammetry in the o-dichlorobenzene—DMF (3 : 1, v/v)/Bu4NBF4 (0.1 mol L–1) system on a glass-carbon electrode. At the first step the reversible transfer of one electron affords stable radical anions 1 and 2 (g = 1.9999, ΔH = 1.9 G). When two electrons per molecule are transferred, the methano fragment is rapidly eliminated (retro-Bingel reaction). This process involves the step-by-step cleavage of two C—C bonds of exo-carbon with the fullerene shell in combination with the stepwise transfer of other two electrons and a proton to form finally the carbanion of the methano fragment and fullerene dianion. For all studied compounds, the elimination rate is much higher than that for bis(alkoxycarbonyl)- and dialkoxyphosphoryl(alkoxycarbonyl)methano[60]fullerenes, which makes it possible to propose bisphosphorylmethane groups as protective in synthesis of new fullerene derivatives.

Molecular bilayer-based superstructures of a fullerene-carrying ammonium amphiphile: structure and electrochemistry.

The synthesis of a water-soluble C60-carrying single-chain ammonium amphiphile, 10- (N-methyl-2-fulleropyrrolidyl)decyltrimethylammonium bromide (1) as well as the characterization of aqueous solutions and cast films of 1 are described. X-ray diffraction study suggests that cast films of 1 form a multilayer structure based on biomembrane-like molecular bilayers. Electron microscopy has revealed that 1 produces both fibrous and disk-like aggregates with 10-12 nm of thickness through self-organization of 1 in aqueous solution. Differential scanning calorimetry, dynamic light scattering, FTIR, and UV-visible absorption studies were also carried out to characterize aqueous solutions and cast films of 1. Electrochemistry for an aqueous solution and for cast films of just 1 and 1 incorporated in lipid films on electrodes was conducted. It was found that films of just 1 and of 1/lipid cast on electrodes showed electron transfer reactions leading to the generation of the fullerene dianion or trianion. In contrast, electrochemistry of aqueous solution of 1 at a bare electrode gives a cathodic current near -0.5 to -0.6 V against SCE; however, an anodic current for the solution did not appear.

Synthesis and Isolation of σ-Bonded Fullerene Metal Derivatives from Reactions of Fullerene Dianion C602- with Organometal Halides

Two novel fullerene derivatives (μ-C60)Mo2(CO)6(η5-C5H4CO2Et)2 (1) and (μ-C60)W2(CO)6(η5-C5H4CO2Me)2 (2) were isolated from reactions of a tetrahydrofuran (THF) solution of dianion C602-, derived f...

Preparation of networks with C 60 knots using anionic polymers.

Two methods to prepare controlled three dimensional structures including Cso molecules as knots are described. For the first network synthesis, THF soluble C 60 2−(K +) 2 is reacted with a “living” polystyrene bearing a carbanion at each end. The use of the fullerene dianion is necessary to avoid complications originating from electron transfer from the carbanions to C 60. The second method, developed to increase the functionality of the knots, consists of reacting C 60 with high functionality polystyrene stars where each branch bears at its end a carbanion. The properties of these networks based on cm, compare well with classical PS gels.