With the principal motivation of making nanoscale coils for generating magnetic field, Takuzo Aida and co-workers at the Japan Science and Technology Agency ERATO-SORST, Tokyo have synthesised molecular coils with helical sense or ‘handedness’. Producing nanocoils necessitates that the structures twist in either one helical sense or the other, and that the coils are good conductor of electric currents that produce the magnetic fields. Here, Aida’s group have achieved these important characteristics in a molecular system1.

According to Takuya Yamamoto, one of the scientists of this group, the most exciting finding is that, “even a nano-object with such complex morphology and properties— coiled pathways, one-handedness and electroconductivity—can be obtained by precisely controlled self-assembly using two assembling modules.”

Fig. 1: Scanning electron microscopy images of nanocoils formed from hexabenzocoronene co-assemblies.

The assembled modules used to build these nanocoil structures were composed of two differently substituted hexabenzocoronene (HBC) molecules. The appropriate combination of co-assemblies of amphiphilic HBC molecules resulted in the selective formation of helical coils of one-handedness (Fig.1).

Self assembly of amphiphilic HBC molecules resulted in the creation of graphite-like nanotube arrangements and the incorporation of a norbornene group on some of the side chains led to the formation of helical nanocoils. A mixture of coils with both helical senses was formed, however, and coils of only one helical sense were formed by the introduction of a chiral centre on the norbornene side chains. The chirality determines the handedness of nanocoils—that is, one enantiomer twisting it in one direction and the other enantiomer in the opposite sense.

The nanocoils were the kinetic products of reactions, whereas the simpler tubular assemblies were formed due to thermodynamic processes. To ensure isolated nanocoil structures, the arrangement must be covalently locked. This is where the double bond functionality of the norbornenes—with responsiveness to ring opening polymerisation—comes into play. The norbornenes react and the new covalent bonds ‘cement’ the nanocoils together. Here, the polymerised nanocoils were about 30nm in diameter, with a helical pitch and tape width of 60 and 20 nm, respectively.

The films consist of polymerised assemblies of the amphiphilic molecules that conduct electricity when doped with iodine.

These nanostructured materials could be used in electronic and optoelectronic applications, of which molecular solenoids are one possibility.