Synthesis of inter-[60]fullerene conjugates with inherent chirality

Abstract

Coalescence of [60]fullerenes potentially produces hypothetical nanocarbon assemblies with non-naturally occurring topologies. Since the discovery of [60]fullerene in 1985, coalesced [60]fullerene oligomers have only been observed as transient species by transmission electron microscopy during an oligomerization process under a high electron acceleration voltage. Herein, we showcase the rational synthesis of covalent assemblies consisting of inherently chiral open-[60]fullerenes. The crystallographic analyses unveiled double-caged structures of non-conjugated and conjugated inter-[60]fullerene hybrids, in which the two [60]fullerene cages are bounds to each other through a covalent linkage. The former one further assembles via a heterochiral recognition so that four carbon cages are arranged in a tetrahedral manner both in solution and solid state. Reflecting radially-conjugated double π-surface nature, the inter-[60]fullerene conjugate exhibits strong electronic communication in its reduced states, intense absorption behavior, and chiroptical activity with a dissymmetry factor of 0.21 (at 674 nm) which breaks the record for known chiral organic molecules.

Fig. 1. [60]Fullerene assemblies.

a Classical [60]fullerene dimers (gray for C and blue for N). b Potential precursor molecules (Ar = 6-t-butylpyridin-2-yl). c Strategies for the synthesis of chiral open-[60]fullerene dimers with a radial π-conjugation (gray for C, blue for N, red for O, yellow for S, and white for H).

Fig. 2. Synthesis and structures of open-[60]fullerene dimers.

a Reaction conditions and structures. b Chemical structures of 1, 4, 5, and 6. c Crystal structure of 4 (beige for C, blue for N, red for O, yellow for S, and white for H). The inset represents a packing structure. d Crystal structure of 5 (The two independent molecules with different chiral configuration are shown with beige and sky blue for C, blue for N, red for O, yellow for S, and white for H). The solvent molecules are omitted for clarity.

Fig. 3. Electronic properties of open-[60]fullerene dimers.

a Absorption spectra of 1, 4, and 5 in toluene with selected molecular structures (gray for C, blue for N, red for O, yellow for S, and white for H) and photographs of the solutions. b Cyclic and differential pulse voltammograms (CV and DPV) of 4 (Plus signs denote half-wave potentials. 0.5 mM in ODCB, 0.1 M n-Bu₄N•BF₄, 100 mV s^–1). The y-axis is relative current. c LOL-π isosurface of 4’ (B3LYP-D3/6-31 G(d,p); color codes, pink/blue for radially distributed π-orbitals and gray for π-orbitals which are not engaged in the caged π-surface). d Optical transitions and molecular orbitals of 4’ (TD CAM-B3LYP-D3/6-31 G(d,p)//B3LYP-D3/6-31 G(d,p), transition energy was scaled by an empirical factor of 72%.). e Electrostatic potential (upper) and spin density maps (lower) of (4’)^•– (UB3LYP-D3/6-31 G(d,p)). Source data are provided as a Source Data file.

Fig. 4. Chiroptical properties of open-[60]fullerene dimer (4).

a Structures of two enantiomers (gray for C, blue for N, red for O, yellow for S, and white for H). b Chiral high-performance liquid chromatography charts (toluene, 1 mL min^–1, 50 °C, 326 nm). c Circular dichroism spectra (10 μM in toluene). d Dissymmetry factor g_abs spectra. Source data are provided as a Source Data file.