A three-dimensionally π-extended material, fullerene C₆₀, is one of attractive motifs of π-conjugation and can be transformed into higher-order structures such as dimers, trimers, and oligomers, all of which are linked by a single bond and thus the π-conjugation among cages are disrupted. To create a fully π-conjugated dimer, we focused on chemical modification of a cage-opened C₆₀ derivative possessing 1,2-dicarbonyl and α,β-unsaturated carbonyl moieties by introducing a phosphonium substituent as shown in Figure 1.
First, the reaction of phosphine with 1 was examined at room temperature. Soon after addition of trimethylphosphine into the chloroform solution of 1, the color changed from reddish orange to dark brown. The ¹H NMR and mass spectroscopic analyses of this solution clearly showed the quantitative conversion of 1 into phosphonium oxabetaine 2. By heating this solution, β-oxo-phosphorus ylide 3 was found to be formed. These phosphonium-substituted derivatives are considered as active intermediates which could be served for a variety of structural transformation. Under the heating conditions of 1 with phosphine, we achieved the one-pot synthesis of the first cage-opened C₆₀ dimer (4) linked via a double bond, in which the structure was unambiguously determined by single crystal X-ray analysis. The explicit electronic communication between the two C₆₀ cages were observed in the cyclic voltammogram showing stepwise six-electron reduction. The supramolecular properties arising from the chiral structure of the dimer would be presented in detail.

<b>Figure 1</b>. Synthesis of phosphonium-substituted derivatives (<b>2</b> and <b>3</b>) and cage-opened C<sub>60</sub> dimer (<b>4</b>).

Figure 1. Synthesis of phosphonium-substituted derivatives (2 and 3) and cage-opened C₆₀ dimer (4).

Shu Okamoto

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