Self-assembled graphitic nanotubes with one-handed helical arrays of a chiral amphiphilic molecular graphene

Abstract

Self-assembly of a Gemini-shaped, chiral amphiphilic hexa-peri-hexabenzocoronene having two chiral oxyalkylene side chains, along with two lipophilic side chains, yields graphitic nanotubes with one-handed helical chirality. The nanotubes are characterized by an extremely high aspect ratio of >1,000 and have a uniform diameter of 20 nm and a wall thickness of 3 nm. The nanotubes with right- and left-handed helical senses were obtained from the (S)- and (R)-enantiomers of the amphiphile, respectively, due to an efficient translation of point chirality into supramolecular helical chirality. The (S)- and (R)-enantiomers coassemble at varying mole ratios to give nanotubes, whose circular dichroism profiles are almost unchanged over a wide range of the enantiomeric excess of the amphiphile (100-20%). The high level of chirality amplification thus observed indicates a long-range cooperativity in the self-assembling process. In sharp contrast, a hexabenzocoronene amphiphile with chiral lipophilic side chains did not form nanotubular assemblies. The present work demonstrates the majority rule in noncovalent systems and also may provide a synthetic strategy toward realization of molecular solenoids.

Fig. 1.

Molecular structures of HBC amphiphiles 1–3.

Fig. 2.

Formation of the self-assembled graphitic nanotubes. (a) Schematic illustrations of the structure of self-assembled graphitic nanotube consisting of HBC amphiphile 1. (b) Formation of chiral graphitic nanotubes with one-handed helical arrays of π-stacked HBC units through translation of point chirality into supramolecular helical chirality.

Fig. 3.

Self-assembly of chiral HBC amphiphile 2.(a) A gelatinous suspension formed at 20°C upon slow cooling of a hot MeTHF solution of chiral HBC amphiphile (S)-2 (3 mg/ml). (b) Fluorescence micrograph of fibers formed at 20°C upon slow cooling of a hot MeTHF solution of (S)-2 (1 mg/ml). A droplet of the suspension was sandwiched by glass plates and exposed to UV light.

Fig. 4.

TEM micrographs of nanotubes formed from chiral HBC amphiphile (S)-2 in MeTHF (a), ClCy (b), and MeTHF/MeOH (c). For the preparation of samples a and b, suspensions, formed by slow cooling of the hot solutions (1 mg/ml) from 50° to 20°C, were diluted by a factor of 10, applied onto a specimen grid covered with a thin carbon support film, and air-dried. For the preparation of sample c, a MeOH vapor was allowed to diffuse slowly into a dilute MeTHF solution at 20°C, and the resulting suspension was treated similarly to the above.

Fig. 5.

Time-dependent changes in electronic absorption and CD spectra of the enantiomers of chiral HBC amphiphile (S)-2 (a and b) and (R)-2 (c and d) in MeTHF (3 mg/ml) at 20°C, on rapid cooling from 50°C in a quartz cell of 0.1-mm path length. The spectra were taken with a 3-min interval. a.u., arbitrary units.

Fig. 6.

TEM micrographs of self-assembled helical coils formed from the enantiomers of chiral HBC amphiphile (S)-2 (a) and (R)-2 (b). The samples were prepared by slow diffusion of a hexane vapor at 20°C into dilute ClCy solutions of 2, applied onto a specimen grid covered with a thin carbon support film, and air-dried.

Fig. 7.

Time-dependent changes in electronic absorption (a) and CD spectra (b) of chiral HBC amphiphile (S)-3 in MeTHF (1 mg/ml) at 30°C on rapid cooling from 50°C in a quartz cell of 0.1-mm path length. The spectra were taken with a 3-min interval. a.u., arbitrary units.

Fig. 8.

Chirality amplification in the coassembly of (R)- and (S)-2. (a) CD spectra of self-assembled nanotubes formed from chiral HBC amphiphile 2 at varying mole ratios of its enantiomers in MeTHF (3 mg/ml) at 20°C, measured after 12 h upon cooling from 50°C. Enantiomeric excess = 100% (red), 80% (yellow), 60% (green), 40% (sky blue), 20% (blue), and 0% (black). (b) Change in intensity of the CD band at 423 nm.