The fullerenes: powerful carbon-based electron acceptors

Summary

The gaseous, solution and solid state experimental evidence for electron addition to the fullerenes is reviewed and it is shown that this class of molecules function as powerful electron acceptors. The topological character of C₆₀ as described by Hiickel molecular orbital theory suggests that the molecule will undergo facile reduction, but comparisons with planar conjugated hydrocarbons show that this feature alone cannot account for the very low half-wave reduction potential of C₆₀. Because of the curvature of the surface, fullerene hybridization falls between graphite (sp²) and diamond (sp³) and these new carbon allotropes are therefore of intermediate, and perhaps variable hybridization. According to POAVI theory the carbon atoms in C₆₀ are of sp^2.28 hybridization. It is concluded that rehybridization plays an important role in determining the electronic structure of the fullerenes and it is the combination of topology and rehybridization that together account for the extraordinary ability of C₆₀ to accept electrons.

Introduction

The ability of the fullerenes to function as electron acceptors has been recognized since the first investigation of their chemistry (Haufler et al. 1990). Even before the isolation of bulk quantities of C₆₀ (Kratschmer et al. 1990), a large electron affinity was demonstrated for this molecule in gas phase experiments (Curl & Smalley 1988).

This trend has continued with the development of the physics, chemistry and materials science of the fullerenes. In this paper some of the experiments that have thrown light on the ability of the fullerenes to accept electrons are summarized and qualitative explanations for their extraordinary electron affinity are discussed.