Helical Polysilanes Research Articles

Smectic–Smectic Phase Segregation Occurring in Binary Mixtures of Long and Short Rigid-Rod Helical Polysilanes

Small-angle X-ray scattering and atomic force microscopy were used to observe smectic–smectic phase segregation in binary mixtures of rigid-rod-like helical poly[n-decyl-(S)-2-methylbutylsilane] (PDMS) polymers with molecular-weight ratios of 4.82 and narrow molecular-weight distributions. Phase segregation is attributed to entropic effects as the homopolymers in the binary mixture differ only in molecular weight. Entropy-driven segregation in smectic phases was theoretically predicted in mixtures of rodlike particles with different lengths under high pressure. Binary mixtures of long and short PDMS with broader molecular-weight distributions, which do not form smectic phases, showed no such segregation, verifying that the driving force for segregation is the entropy gained through smectic-phase formation. The binary mixture of a long PDMS with a narrow molecular-weight distribution and a short PDMS with a broad molecular-weight distribution showed segregation of each component, indicating that the entrop...

Aggregation-induced chiroptical generation and photoinduced switching of achiral azobenzene-alt-fluorene copolymer endowed with left- and right-handed helical polysilanes.

The left and right helicities of azobenzene (Azo)-containing main-chain polymer (PF8Azo) were successfully controlled with an enantiomeric pair of rigid rod-like helical polysilanes carrying (S)- and (R)-2-methylbutyl groups (PSi-S and PSi-R, respectively) as their hetero-aggregates in a mixture of chloroform and methanol solvents and in the solid state. Optimizing the good and poor cosolvents and their volume fractions showed that the molar ratio of PF8Azo to PSi-S/-R and the molecular weight of PF8Azo were crucial to boost the CD amplitudes of PF8Azo/PSi-S and PF8Azo/PSi-R hetero-aggregates. The photoresponsive trans–cis transformation caused noticeable changes in the sign and magnitude of the chiroptical behavior due to the hetero-aggregates. Moreover, the optically active PF8Azo homo-aggregates were produced by complete photoscissoring reactions at 313 nm, which could be assigned to the Siσ–Siσ* transitions of PSi-S and PSi-R.

Time-evolved, far-red, circularly polarised luminescent polymer aggregates endowed with sacrificial helical Si–Si bond polymers

Non-charged helical polysilanes (PSi) were spontaneously endowed with CPL on the order of gem = ±0.02 at 700 nm when non-helical red- and far-red π-conjugated polymeric (PF8DBT) aggregates were employed after ageing at 20–25 °C for 24 h, followed by PSi-selective photoscissoring at 313 nm for 60 s.

Aggregation-induced scaffolding: photoscissable helical polysilane generates circularly polarized luminescent polyfluorene

An enantiopair of rigid rod-like helical polysilanes as a photoscissible scaffold allowed the production of CPL- and CD-active dioctylpolyfluorene aggregates associated with complete removal by a polysilane-selective photoscissoring reaction at 313 nm.

Novel design, facile synthesis and low infrared emissivity properties of single-handed helical polysilanes

Helical polysilane copolymers, consisting of functional groups, were prepared and exhibited lower infrared emissivity values at wavelengths from 8 to 14 μm. The properties of helical polysilanes were systematically investigated.

Asymmetrically Tilted Alignment of Rigid-Rod Helical Polysilanes on a Rubbed Polyimide Surface

The homogeneous alignments of helical rod-like polysilanes on a rubbed polyimide alignment layer were investigated by polarized optical microscopy (POM) and atomic force microscopy (AFM) analyses. The POM and AFM observations determined that polysilanes with a series of aliphatic side chains helically arranged around the main chains were tilted to the right and left by 33° from the rubbing direction when the handedness of the side-chain helical array is left and right, respectively. It is interesting to note that the side-chain arrays run perpendicular to the rubbing direction on the polyimide surface, which is different from intuitive "knob and hole" packing of the extended polyimide chain and the helical grooves between the side-chain arrays surrounding the polysilane backbone. More surprisingly, both right- and left-tilting smectic domains were simultaneously observed with an equal probability for an achiral polysilane, which apparently has the interconverting right- and left-handed helical segments separated by helical reversals. This might be the first observation of the chiral segregation of dynamic helical polymers.

Circularly Polarized Light Enhancement by Helical Polysilane Aggregates Suspension in Organic Optofluids

Circularly polarized (CP) light may play key roles in the migration and delocalization of photoexcited energy in optically active macroscopic aggregates of chiral chlorophylls surrounded by an aqueous fluid in the chloroplasts under incoherent unpolarized sunlight. Learning from the chiral fluid biosystem, we designed artificial polymer aggregates of three highly luminescent helical polysilanes, 1-S, 2-S, and 2-R (Chart 1). Under specific conditions (molecular weights and good-and-poor solvent ratio), 1-S aggregates with ∼5 μm in organic fluid generated an efficient circularly polarized luminescence (CPL) with gCPL = −0.7 at 330 nm while retaining a high quantum efficiency (ΦPL) ∼53% at room temperature under incoherent unpolarized photoexcitation at 290 nm. This huge gCPL value was the consequence of the intense bisignate circularly dichroism (CD) signals (gCD = −0.35 at 325 nm and +0.31 at 313 nm) due to coupled oscillators with electric-dipole-allowed-transition origin. Also, 2-S and 2-R aggregates gav...

Mirror Symmetry Breaking in Helical Polysilanes: Preference between Left and Right of Chemical and Physical Origin

From elemental particles to human beings, matter is dissymmetric with respect to mirror symmetry. In 1860, Pasteur conjectured that biomolecular handedness— homochirality—may originate from certain inherent dissymmetric forces existing in the universe. Kipping, a pioneer of organosilicon chemistry, was interested in the handedness of sodium chlorate during his early research life. Since Kipping first synthesized several Si-Si bonded oligomers bearing phenyl groups, Si-Si bonded high polymers carrying various organic groups—polysilanes—can be prepared by sodium-mediated condensation of the corresponding organodichlorosilanes. Among these polysilanes, optically active helical polysilanes with enantiomeric pairs of organic side groups may be used for testing the mirror symmetry-breaking hypothesis by weak neutral current (WNC) origin in the realm of chemistry and material science. Several theoretical studies have predicted that WNC-existing chiral molecules with stereogenic centers and/or stereogenic bonds allow for distinguishing between image and mirror image molecules. Based on several amplification mechanisms, theorists claimed that minute differences, though still very subtle, may be detectable by precise spectroscopic and physicochemical measurements if proper chiral molecular pairs were employed. The present paper reports comprehensively an inequality between six pairs of helical polysilane high polymers, presumably, detectable by (chir)optical and achiral 29Si-/13C- NMR spectra, and viscometric measurements.

Alternating Thick and Thin Layers Observed in the Smectic Phase of Binary Mixtures of Rigid-Rod Helical Polysilanes with Different Molecular Lengths

Alternating Thick and Thin Layers Observed in the Smectic Phase of Binary Mixtures of Rigid-Rod Helical Polysilanes with Different Molecular Lengths

Modeling and Understanding Smectic-Phase Formation in Binary Mixtures of Rodlike Polysilanes: Comparison of Onsager Theory and Experiment

We study a binary mixture of short and long parallel hard rods in the smectic A phase to understand the role of length ratio (weight ratio) in the formation of three different types of ordering which were observed in binary mixtures of helical polysilanes. Using Onsager’s second virial theory of parallel hard rods, and modeling the short and long polymers as hard cylinders of different lengths (L1 ≠ L2) and same diameters (D), we show that the normal smectic A phase (S1) forms for length ratios (l = L2/L1) between 0.58 and 1, the microsegregated smectic structure (S2) takes place for 0.32 < l < 0.56 and 0 < l < 0.39 in short-rod-rich and long-rod-rich phases, respectively, and the two-in-one ordering (S3) is stable only in mixtures rich in long rods for 0.33 < l < 0.58. These results are in very good agreement with the available experimental data of binary mixtures of polysilanes [Okoshi et al. Macromolecules 2009, 42, 3443]. In addition, the theory predicts the existence of a partially microsegregated smectic structure (S4), in long-rod-rich mixtures, for 0.3 < l < 0.39. Our theoretical results show that two or even three smectic order parameters should be measured, for all types of smectic phases, in order to obtain the correct positional distribution functions from the X-ray diffraction pattern.

Stability of the columnar and smectic phases of length-bidisperse parallel hard cylinders

The effect of length-bidispersity on the stability of nematic, smectic and columnar phases of rod-like particles is studied in the perfect alignment limit using Onsager's second virial theory. The rod-like particles are modelled as hard cylinders of equal diameters (D) but different lengths (). Three different smectic structures are observed: (i) the conventional smectic (S1) phase, where both components accommodate in the same layer; (ii) the microsegregated smectic (S2) phase, which can be considered as an alternation of fluid layers rich in short and long rods, respectively; and (iii) two layers of short rods accommodate inside one layer of long rods, which gives the third smectic (S3) structure. Due to the inefficient packing of the short and long rods into a layered structure along the symmetry axes of the rods, the smectic phase is destabilised with respect to nematic and columnar phases upon mixing the short and long components. With decreasing length ratio () the smectic phase is destabilised with respect to the nematic phase at compositions rich in short rods and two forms of smectic phases, namely S1 and S2, take place in alternation. The alternation of the structure is the consequence of the minimisation of the number of overlapping layers of the short rods with one long rod. In mixtures rich in long rods, the short and long rods are in the same layer up to l = 0.39, while the short rods can accommodate into the interstitial region of long rods for l < 0.39 and the system forms a S2 phase. The S3 phase is observed in the range 0.57 < l < 0.39 and is due to the efficient packing of two layers of short rods inside one smectic layer of long rods. Our theoretical predictions for the three smectic structures are in close agreement with the smectic phase behaviour of a binary mixture of short and long helical polysilanes (Okoshi et al., Macromolecules 42, 3443 (2009)). It seems reasonable that the helical polysilanes can be considered as an ideal system for testing the hard-body theories. Finally, it is interesting that the stabilisation process of the columnar phase with respect to the smectic ordering with decreasing length ratio turns over at l = 0.3.

Entropically-Driven Formation of Smectic A1, A2, and A3 phases in Binary Mixtures of Rigid-Rod Helical Polysilanes with Different Molecular Weights

The smectic liquid crystal arrangements of binary mixtures of rod-like polymers, i.e., poly[n-decyl-(S)-2-methylbutylsilane]s, with narrow molecular weight distributions, were investigated by synchrotron radiation small-angle X-ray scattering (SR-SAXS) and atomic force microscopy (AFM) analyses. Depending on the molecular weight ratio, r (higher molecular weight/lower molecular weight), and the mixing ratio, three types of smectic phases are formed with different accommodations by two polymers. In the binary mixture of the shorter and longer polymers with the lower r values of 1.0−1.7, the two polymers are randomly mixed within a layer, forming the conventional smectic A1 phase with a layer thickness corresponding to an average length of the two polymers. In contrast, when two kinds of polymers with the higher r values of 1.7−2.8 are mixed, different types of smectic A structures are formed. Upon the addition of the shorter polymer to the abundant longer polymer, a smectic A2 phase in which a pair of smec...

Switching in molecular shapes: main chain length driven rod–circle transition of isolated helical polysilanes

Unique conformations such as rod, semicircle, and circle structures of isolated semi-flexible helical polysilanes were observed by atomic force microscopy (AFM); the chain topology was significantly related to the chain length (molecular weight) on the surfaces.

Spectroscopic evidence for diastereomeric helical segments of polysilane bearing enantiopure (S)‐ and (R)‐3,7‐dimethyloctyl groups

AbstractWe synthesized two optically active helical polysilanes, poly[(S)‐3,7‐dimethyloctyl‐3‐phenylpropylsilane] (PS1) and poly[(R)‐3,7‐dimethyloctyl‐3‐phenylpropylsilane] (PS2), bearing a flexible and rodlike silicon main chain and enantiopure alkyl side chains with (S)‐ and (R)‐chiral centers, respectively, at the γ‐positions. PS1 and PS2 underwent a thermodriven helix–helix transition at 10 °C in isooctane. Circular dichroism (CD) and UV studies demonstrated the transition characteristics, such as the transition temperature, population of right‐ and left‐handed helical motifs, global shape, and screw pitch. At −80 °C, the dissymmetry ratio suggested that a preferential right‐handed or left‐handed screw sense was present in the polymer chains of PS1 and PS2, respectively. However, above the transition temperature, the appearance of a bisignate cotton band in the CD spectra suggested that both right‐handed screw‐sense, tight helical segments and left‐handed screw‐sense, loose helical segments coexisted in the same chains of PS1 and PS2. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4518–4527, 2004

Cooperative C-F...Si interaction in optically active helical polysilanes.

Cooperative amplification of the C-F...Si weak interaction between side chains and the main chain was found to afford rigid rodlike helical polysilanes with a preferential screw sense.

Highly Oriented Langmuir−Blodgett Films of Helical Polysilanes and Their Optical Properties

Thin films of a helical polysilane with chiral substituents were fabricated using a Langmuir−Blodgett (LB) method. The polarized absorption and photoluminescence spectra show the high anisotropy for the LB films with the polymer backbone lying in-plane along the dipping direction. The anisotropy of the lowest exciton absorption band can be enhanced by annealing the LB films. With use of the oriented LB film, we have revealed the one-dimensional exciton structures characteristic of polysilanes with helical conformations by measurements of electroabsorption spectra, compared with those of polysilanes of other conformations such as trans-planar and alternating trans-gauche backbone structures. The large binding energies of the lowest exciton are generic in the one-dimensional regular polysilanes, irrespective of the conformations.

Photoluminescence from pendant dye molecules mediated by exciton transport on helical polysilane chains

We have synthesized helical polysilanes on which the pendant groups are partly substituted by dye molecules. The introduction of a few percent of dye molecules into the substituents enhances the luminescent efficiency of the visible region, by behaving as luminescence centers. We have demonstrated that the excitation energy is transferred efficiently by free excitons on the Si backbones to reach the luminescence centers, where energy transfer occurs from the exciton to the dye molecule, and as a result sensitized luminescence occurs. The exciton transfer is found to occur over 50–100 monomer units on the polysilane backbone. This approach will open the way to fabricating a wavelength-tunable luminous device by utilizing the exciton transport on Si chains.

Near-UV, circular dichroism, and fluorescence spectra of a rigid rodlike helical polysilane bearing trietheral moiety in ethanol/water.

An optically active, rigid rodlike helical polysilane with 6, 9, 12-trioxatetradecyl and (S)-2-methylbutyl substituents (1) was newly obtained as a very high molecular weight polymer of several million. Due to the presence of trietheral substituent, 1 was readily soluble in a polar solvent such as ethanol and a mixture of ethanol and water, but was insoluble in pure water. Polysilane 1 in pure ethanol at room temperature exhibited an intense and narrow ultraviolet (UV) and circular dichroism (CD) absorptions at 323 nm, associated with an almost mirror imaged fluorescence (FL) at 328 nm, that are characteristics of rigid rodlike, single-screw-sense helical polysilanes reported previously. When solution temperature was changed from 60 degrees C to -104 degrees C, a global shape of 1 expanded associated with an increase of segment length, whereas a screw pitch tended to be wound tightly. On the other hand, as a solvent polarity became poor, a global shape of 1 shrunk associated with an decrease of segment length and formed a chiral motif with an M-helicity between two helical segments with a kink. At a ratio of 50% of ethanol/water of 50:50 (v/v), 1 became insoluble and formed aggregates.

Ideal Exciton Spectra in Single- and Double-Screw-Sense Helical Polysilanes

Ideal Exciton Spectra in Single- and Double-Screw-Sense Helical Polysilanes