No abstract

Chiral molecules play indispensable roles in advanced materials and technologies. Nevertheless, no conventional, yet reliable logical strategies are available for designing chiral molecules of desired chiroptical properties. Here, we propose a general protocol for rationally aligning multiple chiral units to boost the chiroptical responses, using hexahelicene as a prototype. In this proof-of-concept study, we align two hexahelicenes in various orientations and examine by theoretical calculations to predict the best chiroptical performance for X-shaped and S-shaped double hexahelicenes. We synthesize and optically resolve both double hexahelicenes and show that they exhibit more than a twofold increase in intensity of circular dichroism and circularly polarized luminescence, experimentally validating the protocol. The enhanced chiroptical responses are theoretically assignable to the electric and magnetic transition dipole moments of component hexahelicenes aligned in the correct symmetry. A guiding principle for designing advanced molecular and supramolecular chiral materials is further discussed.

An ideal optically active helical chromophoric polymer comprising a flexible rodlike silicon main chain and enantiopure alkyl side chains, poly{(S)-3,7-dimethyloctyl-3-methylbutylsilylene}, underwent a thermodriven helix-helix transition at -20 °C in isooctane. The transition characteristics, including transition temperature, transition width, the population of right-and left-handed helical motifs, global shape, and screwpitch, were to be characterized quantitatively by spectroscopically analyzing circular dichroism (CD) and UV absorption characteristics. This is based on the unique property of the rodlike polymer in which the CD band completely matches the corresponding UV band profile at all temperatures. Moreover, fine controlling the contents and chirality of an additional chiral silylene unit incorporated in the copolymers allows free manipulation of the transition temperature in the range from -64 to +79 °C. Molecular mechanics calculation showed remarkable differences in the potential energy curve of the main chain torsion angle between flexible and rigid rodlike polysilylenes. These results and knowledge gained should assist in designing and controlling new types of helix-helix transition polymers directed to diverse screw-sense related properties and applications.

Single-walled carbon nanotubes (SWNTs) have remarkable and unique electronic, mechanical, and thermal properties, which are closely related to their chiralities; thus, the chirality-selective recognition/extraction of the SWNTs is one of the central issues in nanotube science. However, any rational materials design enabling one to efficiently extract/solubilize pure SWNT with a desired chirality has yet not been demonstrated. Herein we report that certain chiral polyfluorene copolymers can well-recognize SWNTs with a certain chirality preferentially, leading to solubilization of specific chiral SWNTs. The chiral copolymers were prepared by the Ni(0)-catalyzed Yamamoto coupling reaction of 2,7-dibromo-9,9-di-n-decylfluorene and 2,7-dibromo-9,9-bis[(S)-(+)-2-methylbutyl]fluorene comonomers. The selectivity of the SWNT chirality was mainly determined by the relative fraction of the achiral and chiral side groups. By a molecular mechanics simulation, the cooperative interaction between the fluorene moiety, alkyl side chain, and graphene wall were responsible for the recognition/dissolution ability of SWNT chirality. This is a first example describing the rational design and synthesis of novel fluorene-based copolymers toward the recognition/extraction of targeted (n, m) chirality of the SWNTs.