EPR Studies Associated with the Electrochemical Reduction of C₆₀ and Supramolecular Complexes of C₆₀ in Toluene−Acetonitrile Solvent Mixtures

Abstract

Electron paramagnetic resonance (EPR) studies at 295 K of the radical anion of buckminsterfullerene (C60 •-) generated electrochemically by bulk-controlled potential electrolysis of neutral C60 in mixed toluene−acetonitrile solvents (5−20% v/v acetonitrile with 0.1−0.2 M n-Hex4NPF6 or n-Bu4NPF6 as the supporting electrolyte) have shown that at least three different forms of C60 •- exist in solution. The radical anions have different g values and can thus be detected by EPR spectroscopy in the form of single line signals with different line widths. The familiar species with the broad line width signal (ΔH pp = 6 mT) was present in a high percentage (ca. >95%) along with two species with narrow line width (ΔH pp = 0.16 and 0.07 mT) signals that were present between 1 and 5% of the total concentration of C60 •-. The concentrations of the species with narrow line width EPR signals increased with increasing time so that over a 20-h period their EPR signal intensity approximately doubled. The purity of the toluene was found to be very important in determining the number of species detected with sharp EPR signals. Although two species were always detected with sharp EPR signals, when lower purity toluene (<99%) was used, the number of species with narrow line width signals increased to >2. The increase in the sharp EPR signals with increasing time is not inconsistent with a slow reaction between C60 •- and low levels of impurities in the solvent to form lower symmetry paramagnetic species, the narrow line width signals being very similar to those reported for substituted fullerene radicals. EPR and cyclic voltammetry experiments were conducted on C60 in the presence of several complexing agents, cyclotriveratrylene (CTV), the symmetrical tris-allyl substituted analogue of CTV [CTV(allyl)3], and p-benzyl-calix[5]arene. The intensity of the narrow and broad line width EPR signals decreased when C60 •- was electrochemically generated in the presence of the complexing agents, indicating that the host−guest π−π interactions were sufficiently strong to alter the EPR signals of C60 •-. Cyclic voltammetry experiments performed on C60 in the presence of the complexing agents showed that the first four reduction processes of C60 split into two new processes upon complexation, with the time allowed for the host−guest reaction being critical in determining the voltammetric behavior.

Discrete Fulleride Anions and Fullerenium Cations

“…Substantial negative shifts in redox potentials can be observed when C 60 is complexed by calixarenes, presumably because fulleridestabilizing solvent dipoles are excluded from the immediate solvation sphere. 79 Reductive electrochemistry is frequently reported as part of the routine characterization of derivatized fullerenes. In an early systematic study, Suzuki and co-workers 80 showed that the usual effect of derivatization is to make the fullerene somewhat more difficult to reduce (0.1-0.3 V), although some electropositive addenda such as silyl groups, quaternary nitrogen centers, 81 or organocynanides 82 can move the redox in a positive direction.…”

“…There is no convincing evidence that environmental influences or supramolecular interactions can bring about such a dramatic change in signal characteristics. Studies with calixarenes show that complexation of C 60 -leads to a loss of the broad signal 362 and some of the sharp signal, 79 but this is most easily explained by the effects of spin coupling in noncovalent aggregates. The removal of the (t 1u ) 1 degeneracy of C 60 -, sufficient to change the broad signal into a sharp signal, can be accomplished by the formation of a persistent covalent bond to one or more carbon atoms.…”

“…A recent study reports that increased levels of sharp signals appear when low-purity toluene solvent was used, and this was taken as evidence for the formation of σ-bonded adducts by reaction with impurities. 79 However, there are alternate explanations. For example, the impurities could change solubilities leading to precipitates with different EPR properties.…”

Selective Triplet‐State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene)

“…This situation provides the opportunity to examine equilibration between the two triplets, especially since the isolated triplets are relatively long-lived in deoxygenated solution. Furthermore, C 60 has high electron affinity [30] and should enter into electron-transfer reactions with the excited-singlet state of Bodipy. By linking together these units through a short spacer and building in some degree of orthogonality at the linkage, we might anticipate [12] that charge recombination within the initially-formed radical ion pair would favour triplet formation.…”

Studies on the solution-phase ESR spectra of the C60 monoanion under varying experimental conditions

“…Explanations as to the identity of the sharp signals, generally present in <2% of the total number of spins, have included (i) strong ion pairing between C 60 ž and the supporting electrolyte cation used during electrochemical reduction experiments, 1 (ii) the presence of the C 60 2 paramagnetic species, 2b (iii) impurities in the C 60 , 3b,5e (iv) a percentage of a thermally excited state of C 60 ž , 5a (v) a reaction between C 60 ž and * Correspondence to: R. D. Webster, Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia; e-mail: webster@rsc.anu.edu.au O 2 to form C 60 O 2 ž , 7 (vi) presence of a dimer radical 8d and (vii) a reaction between C 60 ž and low levels of impurities in the solvent to form substituted fullerene radicals. 10 The interpretation of the broad and sharp signals became even more complicated when the conclusion that the narrow linewidth signals were present in only a small percentage of the total number of spins was challenged by several groups who reported conditions where only narrow linewidth signals were obtained. 8, 9 Stasko and coworkers 8 detected two sharp linewidth signals and were the most contentious in their assignment of one of the sharp signals as the real C 60 ž radical, with the second narrow ESR signal being assigned as a radical dimer, C 60 2 2 .…”