Enhanced Degenerate Four-Wave Mixing in an Endohedral Metallofullerene Through Metal-to-Cage Charge-Transfer

Abstract

Since the initial discovery and development of a technique for macroscopic preparation of the archetypal and most abundant fullerene C60, a wealth of fullerene-based structures have been produced including higher fullerenes, charge-transfer complexes, fullerene derivatives, superconducting exohedral-doped fullerenes, and carbon nanotubes. One of the most intriguing fullerene classes is the endohedral fullerene in which the spheroidal molecular structure is employed to encapsulate a small number of atoms (one to four) internal to the cage.1,2 Until recently, the difficult separation process of endohedral fullerenes had limited their availability to submilligram levels. Consequently, initial studies of these materials had primarily been restricted to electron paramagnetic resonance (EPR) and linear spectroscopy. EPR measurements demonstrated that, for the case of La@C82, the La atom transfers three electrons to the fullerene cage and resides in the +3 oxidation state.2 Metal-to-cage charge transfer appears to be a common feature of transition metal-containing endohedral metallofullerenes. Meanwhile, nonlinear optical studies of empty-cage C60 and C70 have shown that these materials possess both large third order susceptibilities χ(3)(−ω4;ω1,ω2,ω3)3 (~10-11 esu) and strong optical limiting behavior.4,5 We report here the first nonlinear optical measurements of an endohedral metallofullerene and find a dramatic enhancement in the third order nonlinear optical response. Degenerate four-wave mixing (DFWM) experiments on solutions of the endohedral metallofullerene Er2@C82 show that the metal-to-cage charge transfer provides a mechanism for increasing χ(3)(−ω4;ω1,ω2,ω3) by orders of magnitude relative to empty-cage fullerenes.