Michiya Fujiki

Optically Active Polysilylenes: State‐of‐the‐Art Chiroptical Polymers

Controlled synthesis, chiroptical characterization, and manipulation of artificial helical polymers are challenging issues in modern polymer stereochemistry. Although many artificial polymers adopting a preferential screw-sense helical structure have been investigated, optically active polysilylenes bearing chiral side chains may be among the most suitable to elucidate the inherent nature of the helical structure, since these polymers offer powerful spectroscopic probes as a result of their ideal chromophoric and fluorophoric main chain properties around 300–330 nm. The present paper will review comprehensively the helix-property-functionality relationship between side chain structure, global and local main chain conformation, (chir)optical properties, electronic properties, several helical cooperative phenomena, the effects of temperature and solvent polarity, and molecular imaging. This knowledge and understanding of the nature of the polysilylene helix might constitute a bridge between artificial polymers and biopolymers and will assist in designing and controlling new types of helical polymers directed to diverse screw-sense-related properties and applications in the future.

Rational Concept To Recognize/Extract Single-Walled Carbon Nanotubes with a Specific Chirality

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.

Circularly Polarized Luminescence of Rhodamine B in a Supramolecular Chiral Medium Formed by a Vortex Flow

Sucked into the vortex: Hydrogels with embedded Rhodamine B dye showed stir-induced circularly polarized luminescence (CPL; see picture), the sense of which can be controlled by switching the stir direction from clockwise (CW) to counterclockwise (CCW) with slow cooling from the sol to gel states. The chiral alignment of the dye was erased by heating the sample above the gel-sol transition temperature.

Limonene magic: noncovalent molecular chirality transfer leading to ambidextrous circularly polarised luminescent π-conjugated polymers

Solvent chirality transfer using (S)- and (R)-limonenes, which are candidates for renewable volatile bioresources (bp 176 °C/760 Torr or bp 94 °C/68 Torr), allowed for the successful production of optically active poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-bithiophene] (F8T2) particles with circular dichroism (CD) and circularly polarised luminescence (CPL) properties. The particles were rapidly produced by CD-silent F8T2 with the aid of solvent chirality transfer at 25 °C. The present paper demonstrates the following: (i) through weak intermolecular forces, such as π/π, van der Waals and CH/π interactions, CD-/CPL-active F8T2 aggregates successfully emerge in a chiral tersolvent system of chloroform (a good solvent), alkanol (a poor solvent) and limonene (a chiral solvent); (ii) the alkanol and the enantiopurity of the limonene greatly affect the magnitude and sign of CD-/CPL-signals; (iii) aggregate size considerably affects the magnitude of the induced CD amplitude; (iv) clockwise and counter-clockwise stirring during preparation do not affect the magnitude of these signals; (v) stirring speed weakly affects the induced CD amplitude and (vi) the order of addition of limonene and methanol to the chloroform solution of F8T2 greatly affects the magnitude of the induced CD amplitude. To prove the renewability of limonenes, we re-used (S)-limonene purified by distilling very impure (S)-limonene-containing chloroform, methanol and F8T2, which was used and stored in a number of limonene chirality transfer experiments. As expected, the CD-/UV-vis spectra of F8T2 particles utilizing the renewed (S)-limonene gave similar CD-/UV-vis spectra to those using the fresh (S)-limonene. Moreover, limonene chirality transfer was possible to obtain two CD-active polymers from CD-silent, poly(9,9-di-n-octylfluorenyl-2,7-diyl) (F8) and poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-thiophene] (F8T1). The protocol may provide an environmentally friendly, safe and mild process to rapidly produce ambidextrous light-emitting polymers with a minimal loss of starting polymers at ambient temperature, from CD-silent polymers without any specific chiral substituents or chiral catalysts.

Ambidextrous circular dichroism and circularly polarised luminescence from poly(9,9-di-n-decylfluorene) by terpene chirality transfer

Solvent chirality transfer using enantiomeric pairs of limonene and α-pinene allowed for the successful production of optically active poly(9,9-di-n-decylfluorene) (PF10) aggregates with circular dichroism (CD) and circularly polarised luminescence (CPL) properties. The aggregates were rapidly produced by CD-/CPL-silent PF10 with the aid of solvent chirality transfer at 25 °C. We demonstrate that: (i) through weak forces, such as π/π, van der Waals, and CH/π interactions, CD-/CPL-active PF10 aggregates successfully emerge in a chiral tersolvent system of chloroform (as a good solvent), alkanol (as a poor solvent), and terpenes (as a chiral solvent), (ii) the molecular weight of PF10, alkyl chain length of PFs, terpene structure and enantiopurity, solvent type, and solution temperature greatly affect the magnitude and sign of the CD-/CPL-signals, (iii) the vortex and aggregate size only mildly affect the magnitude of these signals, and (iv) a new CPL band at 434 nm from PF10 aggregates, arising from the CD/UV-vis band at 428 nm, may result from a chiral β-phase. The terpene chirality transfer demonstrated herein may provide a new environmentally friendly, safer, and milder process to rapidly produce ambidextrous chiroptical polymeric materials at ambient temperature from CD-/CPL-silent polymers without any specific chiral catalysts or substituents.

Chiroptical generation and inversion during the mirror-symmetry-breaking aggregation of dialkylpolysilanes due to limonene chirality

We observed the emergence and inversion of chiroptical handedness in three chiroptically silent dialkylpolysilanes during aggregation in limonene-methanol-THF tersolvents.

Switching handedness in optically active polysilanes

Abstract Chain-like optically active polysilanes comprise a helical main chain of SiSi single bonds and chiral and/or achiral side groups. They exhibit unique absorption, circular dichroism, and fluorescence spectra around 300–400 nm due to the σ-conjugation. Since the first brief report of optically active polysilane synthesis in 1992, this research 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 account focuses on screw-sense switchable, optically active poly(dialkylsilane)s among the above optically active polysilanes, including (i) (chir)optical properties, (ii) quantitative population analysis of right- and left-handed helices based on Kuhns dissymmetry ratio, (iii) capability of screw-sense inversion, and (iv) chiroptical switch and memory, as consequences of side group internal interactions, double-well potential energy, and external stimuli. Such knowledge and understanding might stimulate a further polysilane research, and also may advance these polymers to the realization of Si-based nanomaterials and Si-based nanodevices in the future.

Room-temperature near-ultraviolet electroluminescence from a linear silicon chain

Single-layer light emitting diodes (LEDs) were fabricated using poly[bis(p-butylphenyl)silane] as the emissive layer. An efficient and stable electroluminescence with a maximum at 407 nm was observed at room temperature under a forward electric field greater than 6×105 V/cm. The coincidence of electroluminescence with photoluminescence suggests the origin of the electroluminescence in an excited silicon chain segment. The high external quantum efficiency (0.1% photons/electron), narrow emission (full width at half maximum=15 nm), improved operating stability, and good solubility in organic solvents provide the possibility of using polysilanes for ultraviolet LEDs.

Structural defects in poly (methylphenylsilylene)

The Si-based structural defects in poly(methylphenylsilylene) backbones were detected as two new 29 Si NMR signals and as two new Si-Si vibrational IR bands. The defects, consisting of an organosilyne unit and about three methylphenylsilylene monomer units near the branch, were presumed to be in a fairly lengthened Si-Si bonding state. The empirical linear relationship between the relative intensity of broad photoluminescence and the defect density predicts the disappearance of the broad luminescence for a defect density of less than 1%.

A chiral π-stacked vinyl polymer emitting white circularly polarized light

An optically active, π-stacked poly[2,7-bis(4-tert-butylphenyl)dibenzofulvene] having a preferred-handed helical conformation was synthesized by anionic polymerization. A thin film sample of the polymer exhibited broad-band white circularly polarized light (CPL) emission on photo excitation.