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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.

ABSTRACT:In the family of optically active synthetic polymers, optically active polysilanes, which comprise a helical main chain of silicon-silicon single bonds and chiral and/or achiral side groups, exhibit unique absorption, circular dichroism, and fluorescence spectra around 300-400 nm due to σ-conjugation. Since the first brief report of optically active polysilane synthesis in 1992, the field has now widened to include various homo-and copolymers of optically active poly(dialkylsilane)s, poly(dialkoxysilane)s, poly[alkyl(aryl)silane]s, and poly(diarylsilane)s. This review comprehensively covers work on (i) the relationship between side chain structure, (ii) local structure-global shape relationship, (iii) (chir)optical properties, (iv) (semi)quantitative population analysis of right-and left-handed helices based on Kuhn's dissymmetry ratio, (v) several helical cooperativity effects, (vi) molecular imaging, (vii) inversion of screw-sense, (viii) chiroptical switch and memory, (ix) transfer and amplification of molecular chirality to aggregates, (x) cholesteric liquid crystallinity, (xi) helical supramolecular structures, and (xii) latent helicity, as consequences of side group internal interactions and other external stimuli. Such knowledge and understanding may stimulate optically active polymer research in the realm of nanomaterial science and nanotechnology at the sub-nm level as well as traditional polymer science, and may advance these polymers to new functional nanomaterials and thence to the realization of nanodevices in the future.

When a mobile backbone conformation of π-and σ-conjugating polymers is strongly coupled with an electronic structure, changes in UV-vis absorption spectra and/or chiroptical properties are observed phenomenologically as thermochromism, solvatochromism, piezochromism, electrochromism, and/or mutarotation. 1,2 Although polysilanes, being σ-type chromophores, exhibit these phenomena to a remarkable degree, their structural origins are controversial because of the limited information available on the relationship among the conformational properties of the Si backbone, excited electronic state, and actual optical characteristics. 2 We found empirically for various polysilanes in THF at 30°C that the lowest excitonic backbone peak intensity per silicon repeating unit, (Si unit/dm 3 ) -1 ‚cm -1 , increases exponentially as the viscosity index, R, increases. The R value relates primarily to the conformational properties of the chain 3 and therefore in polysilanes to the mean free path of a photoexcited electron-hole pair. This correlation would be helpful for discussing the conformational change of polysilane in any condition. It would also be useful for predicting the lower and upper limits of the peak intensity for the respective globular and perfectly extended rod polysilanes.Both optically active and inactive polysilanes, poly(R 1 R 2 Si), have excellent optical characteristics for studying such conformational structures in solution, because it is known that the characteristics of the lowest excitonic Si(σ)-Si(σ*) absorption at 3-4 eV varies sensitively with the helical torsion angle, segmentation, and regularity of the helix. 4-6 On the basis of this understanding, we recently obtained various optically active and inactive polysilanes bearing chiral and achiral alkyl and/or phenyl pendants by Wurtz condensation of the corresponding substituted dichlorosilanes. The main features of these polysilanes in THF at 30°C are that the value is broadly distributed from 5500 to 57 500 and the peak energy ranges from 3.5 to 4.2 eV (λ max ) 290-352 nm), while the R value ranges widely from 0.51 to 1.35.Twenty-one polysilane samples were examined in this study: seven optically active poly(dialkylsilane)s with four different types of chiral -, γ-, or δ-branched alkyl substituents; eleven optically inactive poly(dialkylsilane)s with six different types of achiral -, γ-, or δ-branched substituents; an optically inactive poly(dialkylsilane) copolymer prepared from a racemic chiral monomer; and two optically inactive poly(alkylphenylsilane)s. These polysilanes were classified as having shrink coil, flexible coil, stiff, and rigid rodlike conformations from their R value 3 and 29 Si-NMR line width. 6d A shrink form with restricted segmental motion, which was suggested from the low R value of 0.5-0.6 6b and broad 29 Si-NMR line width of about 50 Hz, 6d is forced by a pair of methyl and -branched alkyl pendants.Although the viscosity measurement needs a wide range of narrowly dispersed molecular weight polymer samples, today's advanced ...

Circularly polarized light (CPL) as a massless physical force causes absolute asymmetric photosynthesis, photodestruction, and photoresolution. CPL handedness has long been believed to be the determining factor in the resulting product's chirality. However, product chirality as a function of the CPL handedness, irradiation wavelength, and irradiation time has not yet been studied systematically. Herein, we investigate this topic using achiral polymethacrylate carrying achiral azobenzene as micrometer-size aggregates in an optofluidic medium with a tuned refractive index. Azobenzene chirality with a high degree of dissymmetry ratio (±1.3 × 10 at 313 nm) was generated, inverted, and switched in multiple cycles by irradiation with monochromatic incoherent CPL (313, 365, 405, and 436 nm) for 20 s using a weak incoherent light source (≈ 30 μW·cm). Moreover, the optical activity was retained for over 1 week in the dark. Photoinduced chirality was swapped by the irradiating wavelength, regardless of whether the CPL sense was the same. This scenario is similar to the so-called Cotton effect, which was first described in 1895. The tandem choice of both CPL sense and its wavelength was crucial for azobenzene chirality. Our experimental proof and theoretical simulation should provide new insight into the chirality of CPL-controlled molecules, supramolecules, and polymers.

A trans–cis photoisomerizable achiral polymer, poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-4,4′-azobenzene] (F8AZO), was designed. The chirality of (S)- and (R)-limonene used as a solvent allowed for the generation of optically active F8AZO aggregates, revealing intense circular dichroism (CD) signals in the visible region. The reversible chiroptical response was achieved upon alternating photoirradiation at 405 nm (trans-form) and 546 nm (cis-form). This ability originated from the switching between the trans-origin aggregation and cis-origin disaggregation of F8AZO in the limonene–2-propanol–chloroform tersolvent.