Photoelectron Spectroscopy

Photoelectron Spectroscopy

(Parte 1 de 4)

in the Gas Phase and the Generation and Characterization of C1-C60F47- and D2-C60F44 in Solution

Xue-Bin Wang,†,‡ Chaoxian Chi,§ Mingfei Zhou,§ Igor V. Kuvychko,| Konrad Seppelt,⊥ Alexey A. Popov,*,# Steven H. Strauss,*,| Olga V. Boltalina,*,| and Lai-Sheng Wang*,∇

Departmentof Physics,WashingtonState UniVersity,Richland,Washington99354; Chemicaland MaterialsSciences DiVision, Pacific NorthwestNationalLaboratory,MS K8-8, P.O. Box 9, Richland,Washington99352; Departmentof Chemistry,ShanghaiKey Laboratoryof MolecularCatalystsand InnoVatiVe Materials,AdVanced MaterialsLaboratory,Fudan UniVersity,Shanghai200433 People’sRepublicof China; Departmentof Chemistry, ColoradoState UniVersity,Fort Collins,Colorado80523; Free UniVersity Berlin,D-14195Berlin,Germany; Leibniz-Institutefor Solid State and MaterialsResearch,Dresden01069, Germany;ChemistryDepartment, Moscow State UniVersity,Moscow 119992,Russia;and Departmentof Chemistry,Brown UniVersity, ProVidence,Rhode Island 02912

ReceiVed: October 1, 2009; ReVised Manuscript ReceiVed: December 9, 2009

2- is reported. The first electron affinities for the corresponding neutral molecules, C60Fn, were directly measured and were found to increase as n increased, reaching the extremely high value of 5.6 ( 0.10 eV for C60F47. Density functional calculations suggest that the experimentallyobservedspeciesC60F17-,C 60F35-, and C60F47- were each formed by reductivedefluorination of the parent fluorofullerene, C3V-C60F18,C 60F36 (a mixture of isomers), and D3-C60F48, respectively, without rearrangement of the remaining fluorine atoms. The DFT-predicted stability of C60F47- was verified by its generation by chemical reduction from D3-C60F48 in chloroform solution at 25 °C and its characterization by mass spectrometry and 19F NMR spectroscopy. Further reductive defluorination of C60F47- in solution resulted in the selective generation of a new fluorofullerene, D2-C60F44, which was also characterized by mass spectrometry and 19F NMR spectroscopy.

Introduction

Ever since the discoveryof fullerenes,extensiveexperimental and theoretical studies have been devoted to the preparation and characterization of fluorofullerenes (FFs), which were originallyproposedas potentiallubricantsand strongoxidizers.1-3 Possible applications of these fullerene derivatives have been extended to components of (i) cathode materials for lithium batteries,4,5 (i) materials with tunable optical gaps,6 and (ii) light-harvesting materials.7 FFs can be readily prepared as mixtures of isomers with a range of compositions C60Fn, where n is one or more even integers from 38 to 46, by using sufficiently strong fluorinating agents such as F2, XeF2, or highvalent transition metal fluorides.8,9 However, the selective synthesis of specific compounds (i.e., one or only a few isomers of a single composition) with well-defined structures remains much more difficult to achieve. Only three FFs, C60F18,C 60F36, and C60F48, can be prepared in synthetically useful quantities (i.e., g0.5 g) and purity. Two of them are formed as single

One of the most interesting properties of FFs is their ability to accept electrons, which makes them ideal model systems to study charge-transfer and/or electron-capture phenomena. A number of F dianions,and even long-livedtrianions,have been produced via charge transfer in collisions of singly charged F anionswith neutralatoms or molecules.13 In general,the electron affinities (EA) of C60Fn derivatives are expected to be higher than that of C60 and to increase as n increases because of the inductive electron-withdrawing effect of the F atoms (despite the fact that the fullerene π system becomes smaller as n increases).14 So far, this has only been experimentally verified for a handful of C60Fn species, all of which have an even numbers of F atoms: EA(C60F2) ) 2.74(7) eV (by gas-phase ion-moleculeequilibria),15 EA(C60F44) ) EA(C60F46) ) 4.06(25) eV (by Fourier transform mass spectrometry (FTMS); the sample was probably a mixture of isomers of both composi- tions),16 and EA(C60F48) ) 4.06(30) eV (also by FTMS).17 In addition, electrochemical studies have shown that the well- characterized FFs studied to date (n ) 2, 4, 18, 36, 48) are easier to reduce in solution than the parent fullerene, although precisely determined reversible reduction potentials have been difficult to measure because of the rapid and irreversible loss of F atoms from electrochemically generated F- radical anions.18-21 Nevertheless, approximate E1/2 values or cathodic peak potentials were found to increase as n increased. However, note that some isomers of C60(CF3)6 and C60(CF3)12 have more negative reduction potentials than C60 (although most C60(CF3)n

* To whom correspondenceshouldbe addressed.E-mail:olga.boltalina@ colostate.edu (O.V.B.); steven.strauss@colostate.edu (S.H.S.); a.popov@ ifw-dresden.de (A.A.P.); lai-sheng_wang@brown.edu (L.-S.W.).

† Washington State University. ‡ Pacific Northwest National Laboratory. § Fudan University. | Colorado State University. ⊥ Free University Berlin. # Leibniz-Institute for Solid State and Materials Research, and Moscow

State University. ∇ Brown University.

J. Phys. Chem. A X, x, 0 A

10.1021/jp9097364 X American Chemical Society derivatives have more positive E1/2 values than C60),2 so it is conceivable that some C60Fn isomers yet to be studied may be more difficult to reduce than C60. In the gas phase, both experimental15 and theoretical studies23 suggest that C60Fn species with odd values of n should have higher EAs than the corresponding closed shell species with even values of n + 1. This has also been predicted theoretically, but not verified experimentally, for perfluorinated polycyclic aromatic hydrocarbons: the DFT-predicted EAs for C14F10 (perfluoroanthracene) and 1-C14F9 are 1.8424 and 3.14 eV,25 respectively. EAs of C60Fn species have proven to be difficult to measure experimentally. Various experimental techniques, such as surface thermal ionization, electron ionization, and laser desorption, have been employed to produce singly charged

C60Fn- anions in the gas phase.2,17,15 Recently, electrospray ionization (ESI) has been used to generate both fullerene- and resolved photoelectron spectroscopy (PES) spectra of C60-. 30,31

Kappes and co-workers have carried out PES studies of higher fullerene dianions.32,3 We have developed a low-temperature PES techniquewhich couples an ESI source with a temperaturecontrolled ion trap.34 Using low-temperature PES, we have carried out a series of studies including both singly and doubly charged fullerene and fullerene oxide anions.31,35-38 Detailed electronic structure information and vibrationally resolved PES spectra were obtained at low temperatures, yielding precise adiabatic detachment energies (ADEs) of the anions (which correspond to the first or second EAs of the corresponding neutral species). Ioffe et al. reported the first ESI-PES study of

2- and estimated the second

EAs for the correspondingneutralspeciesto be 2.4(1) and 3.2(1) eV, respectively.29 However, no PES spectra of singly charged F- anions have been reported, probably because the corresponding neutral molecules have such high electron binding energies.

Here we presenta joint PES and theoreticalstudy of a number

2-, which were produced by ESI of solutions of C3V-C60F18, a mixture of isomers of C60F36,o r D3-C60F48. The combined gas-phase and theoretical study of the ionic and neutral species also led to (i) the successful generation and characterization of the remarkably stable C60F47- anion in chloroform solution at 25 °C and (i) the observation of its slow transformation to the new F D2-C60F44.

Experimental and Theoretical Methods

Materials. The compound C3V-C60F18 was prepared from C60

(Term USA; 9.5%) and K2PtF6 as previously described.10,39 APCI mass spectrometry, 19F NMR spectroscopy, and HPLC analysis showed that the product contained 90+% C3V-C60F18 and small amounts of other FFs. The compound C60F36 was prepared from C60 and MnF3 (Apollo Scientific) as previously described.40 The APCI mass spectrum of the product showed that the only composition present was C60F36 (9+%). The 19F NMR spectrumshowedthe presenceof the threeknownisomers, compound D3-C60F48 was prepared by heating C60 with F2 gas at 330 °C for 24 h in a quartz flow reactor as previously described.12,41 The one-electron reducing agent 1,1′,3,3′-tetrakis(2-methyl-2-nonyl)ferrocene (DEC) was prepared from ferrocene (Sigma-Aldrich) and 2-chloro-2-methylnonane as previously described.42

Low-Temperature Photoelectron Spectroscopy. The PES experiments were performed with a low-temperature apparatus that couples an ESI source and a temperature-controlled ion trap to a magnetic-bottle time-of-flight photoelectron spectrometer.34 Fresh spray solutions were prepared by adding an organic electron donor compound (either tetramethylphenylenediamine or tetrakis(dimethyamino)ethylene) to 10-3 M stock solutions of each F sample dissolved in a mixture of o-dichlorobenzene and acetonitrileas previouslydescribed.20,2 The anionsproduced by the ESI source were guided by RF-only devices into the 3-D ion trap. The trap was attached to the cold head of a cryostat, which consists of a closed-cycle helium refrigerator and a feedback heating system that allows the temperature to be controlled from 10 to 350 K. In this work the trap was operated at either 12 or 20 K. The ions from the ESI source were trapped and collisionally cooled for 0.1 s by ca. 0.1 mTorr

80/20 He/H2 background gas before being pulsed out into the extraction zone of the time-of-flight mass spectrometer.

During each PES experiment, anions of interest were mass selected and decelerated before being intercepted by a detachment laser beam in the interaction zone of a magnetic-bottle photoelectron analyzer. Spectra of the C60F17- monoanion and

2- dianions were obtained at both 266 and 193 nm (4.661 and 6.424 eV, respectively). Spectra of the collected with high efficiency by the magnetic bottle and analyzedin a 5.2 m long electronflight tube. Photoelectrontimeof-flightspectrawere collectedand convertedinto kineticenergy spectra, which were calibrated using the known spectra of I- and ClO2-. Electron binding energy spectra were obtained by subtracting the kinetic energy spectra from the detachment photon energies. The energy resolution (∆E/E) was estimated to be 2% (i.e., 20 meV for 1 eV electrons).

NMR Spectroscopyand ESI Mass Spectrometry.Fluorine- 19 NMR spectra of chloroform-d solutions were recorded at 25 °C using a Varian INOVA-unity 400 spectrometer operating at 376.5 MHz. In some measurements, a small amount of hexafluorobenzene was added as an internal standard (δ -164.9). ESI mass spectra were recorded using a ThermQuest Finnagan LCQ-DUO spectrometer. Chloroform-d was used as the solvent.

Theoretical Calculations. DFT calculations were performed using the GGA PBE density functional43 and the original TZ2P- quality basis set with {6,3,2}/(11s,6p,2d) contraction schemes implemented in the PRIRODA package.4 The code used expansion of the electron density in an auxiliary basis set to accelerate evaluation of Coulomb and exchange-correlation terms.45 Single-pointenergy calculationsat the B3LYP/6-311G* level and optimization of the structures at AM1 level were performed using PC GAMESS/Firefly package.46 Analysis of the electron localization function (ELF) was performed using TopMoD,47 and ELFs and spin density distributions were visualized with Molekel 4.3.48

Results and Discussion

Photoelectron Spectra. Our home-built ESI source did not produce sufficiently strong signals for F anions by direct spray of solutions of the F precursors unless an organic electron donor compound was added. With a donor compound added, the most abundant ESI-generated anions were singly charged species formed by removal of an odd number of F atoms from the parent monoanion and doubly charged dianions formed by removal of two F atoms from the parent dianion.These included

B J. Phys. Chem. A, Vol. x, No. x, X Wang et al.

C60F48). The observed compositionsare consistentwith previous reports18,28,29,49 and are related to the greater stability of closed- shell F- anions with an odd number of F atoms and closedshell FF2- dianions with an even number of F atoms relative to homologous open-shell anions.23 We had previously shown, but

only qualitatively, that EA(C60F) > EA(C60F2).15 However, before this work there were no quantitative EA measurements for FFs with odd numbers of F atoms.

Figure 1 shows the 12 K PES spectra of C60F17- at 266 and 193 nm. Relatively weak and broad features between 2.3 and

4.2 eV were observed at both wavelengths. We found that the relative intensities of these features depended on the square of the photon flux, suggesting that they arise from two-photon processes, either photodetachment from anionic fragments of the parent anion (after fragmentationby the first photon) or from excited states of the parent anion (following excitation by the first photon). A strong and sharp transition (X) was observed at 4.285 eV in the 266 nm spectrum as well as in the 193 nm spectrum, which defines the ADE of C60F17- (or the EA of neutral C60F17). In the 193 nm spectrum, a second band (A) was observed with a sharp transition at 5.71 eV, which represents an excitation energy of 1.42 eV above the ground state. Vibrational structures are resolved for the X band at both wavelengths.

Only 193 nm spectra could be recorded for C60Fn- with n g 3 due to their high electron binding energies. Figure 2 shows

of C60F33-. A shoulder is partially resolved in the threshold

region for C60F35-, yielding an ADE of 4.98 eV. A very weak and broad feature is discernible between 3.5 and 4.5 eV in the

spectrum of C60F33- (see Figure 2a), most likely due to multiphoton processes as described above for C60F17-. The 193 nm PES spectra of C60F43-,C 60F45-, and C60F47- are shown in Figure 3. Only a broad band was observed in each case. Since

no vibrational structures were resolved for these species, their ADEs were estimated by drawing a straight line along the rising edge of the peak and then adding the instrumental resolution to the intersection with the binding energy axis.

The experimental ADEs of the F- monoanions with n ) 17, 3, 35, 43, 45, and 47, which correspond to the EAs of the corresponding neutral species, are listed in Table 1. Figure 4 is a plot of these EA values vs n and also includes EA values for is now no doubt that C60Fn species with odd n values are much stronger electron acceptors than homologous species with one more or one fewer F atom. For example, the differences factor of 2 larger. There is a reasonably good linear correlation

3.40(13) eV, respectively). If one extrapolates back to n ) 1, the value of EA(C60F) predicted by the linear fit is 3.45(13) eV, which is 0.7(13) eV higher than the experimental value at (a) 266 nm (4.661 eV), and (b) 193 nm (6.424 eV). Note that the low-binding-energy features below 4 eV are assigned to two photon processes (i.e., either detachment of fragments from ground-state

C60F17- or photoexcitation of C60F17- and subsequent photodetachment from the excited-state anion).

Figure 2. Low-temperature (12 K) photoelectron spectra at 193 nm

Figure 3. Low-temperature (12 K) photoelectron spectra at 193 nm

Photoelectron Spectroscopy of C60Fn- and C60Fm 2- J. Phys. Chem. A, Vol. x, No. x, X C

of EA(C60) and 0.67(15) eV higher than the experimental value of EA(C60F2).

experimental gas-phase electron affinities reported for organic molecular species. For comparison, the experimental EA for the trifluoroacetyl radical (CF3COO) is 4.20(27) eV.50 The term “superhalogen” has been used51,52 to describe molecular species with EAs higher than 3.613 eV, the electron affinity of a Cl atom.53 Superhalogens with the highest experimental EAs

include PtF6 (7.0(35) eV)54 and LaCl4 (7.03(10) eV).5 The data in Figure 4 suggest that all FFs with n g 3, possibly including C60F34 and C60F36, can be classified as superhalogens. Other fluorocarbons with DFT-predicted LUMO energies com- parable to that of C3-C60F36 (-5.363 eV) include particular isomers of C60(CF3)8 and C60(CF3)10,2 and these superhalogen candidates should have their electron affinities measured in the near future.

We were also able to measure the PES spectra of the dianions

(Parte 1 de 4)

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