Poly(di-n-pentylsilane). The spectral consequences of a helical conformation in the solid state

Miller, R. D.; Farmer, Barry L.; Fleming, William W.; Sooriyakumaran, Ratnam; Rabolt, John F.

doi:10.1021/ja00242a044

Cited by 88 publications

(48 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…X-ray diffraction (XRD) is among the most convenient techniques for elucidating the structures of helical polymers as clearly demonstrated for biological polymers, such as polypeptides 36 and DNA. 37 In fact, on the basis of the XRD measurements of oriented films or fibers derived from liquid crystalline (LC) helical polyisocyanates such as poly (n-butyl isocyanate) (10), 38 and polysilanes, for instance, poly (di-n-pentylsilane) (11) 39 and poly(n-decyl-(S)-2-methylbutylsilane) (12), 40 their helical structures were elucidated to be the 8/3 helix and 7/3 helix, respectively (Chart 1). However, owing to the limited number of diffractions and difficulty in obtaining crystalline samples from most synthetic helical polymers, the XRD method seems to be laborious and cannot provide important structural information, in particular, on the helical sense.…”

mentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

View full text Add to dashboard Cite

During the last decade, remarkable progress in developing synthetic helical polymers with a controlled helical sense has been achieved. However, the exact helical structures of most of the already prepared synthetic helical polymers remain unsolved. In this review, the recent progress in the synthesis of helical polymers and their structural determination, including helical pitch and handedness based on X-ray diffraction and spectroscopic measurements, together with high-resolution atomic force microscopy, is described. Polymer Journal (2010) 42, 3-16; doi:10.1038/pj.2009 Keywords: atomic force microscopy; circular dichroism; helical polymer; helical structure; X-ray diffractionThe helix is a unique structure as observed in nature from microscopic to macroscopic levels and is inherently chiral. Inspired by biological helices, such as DNA and proteins, chemists have been challenged to synthesize helical polymers with a controlled helical sense that aims not only to mimic biological helical structures but also to develop chiral materials for the separation of enantiomers and asymmetric catalysis. [1][2][3][4][5][6][7][8][9][10][11][12] The history of synthetic helical polymers with optical activity extends back to the 1960s when Pino and Lorenzi 13 investigated the structural and chiroptical properties of isotactic vinyl polymers prepared by the polymerization of a-olefins bearing optically active substituents. Although helical polyolefins are totally dynamic in nature and consist of short helical segments separated by frequently occurring helical reversals among disordered, random coil conformations, 14 this study was significant in the field of synthetic helical polymers, from which a number of helical polymers have been synthesized.On the basis of the pioneering studies by Nolte et al., 15 Okamoto et al. 16 and Green et al., 17 the existing synthetic helical polymers that exhibit an optical activity solely because of their macromolecular helicity can be classified into two categories with respect to their helix inversion barriers, that is, static and dynamic helical polymers (Figure 1). 9,12 Optically active helical polymers, such as poly(triphenylmethyl methacrylate) (1), 16 poly(t-butyl isocyanide) (2) 15 and polychloral (3), 18 have a sufficiently high helix inversion barrier and belong to the family of static helical polymers. Therefore, these helical polymers with either a right-or left-handed helix can be prepared by the helix-sense-selective polymerization of the corresponding achiral monomers using chiral catalysts or initiators under kinetic control. On the other hand, dynamic helical polymers, such as polyisocyanates (8) 17 and polysilanes (9), 19 possess a very low helix inversion barrier.As a consequence, they consist of interconvertible right-and lefthanded helical segments separated by rarely occurring helical reversals, as demonstrated by Green et al.,6,20,21 so that a preferred-handed helical conformation can be induced in the presence of a small amount of chiral residues at the pendant 6 or ter...

show abstract

mentioning

confidence: 99%

Synthesis and structure determination of helical polymers

Yashima

2010

Polym J

View full text Add to dashboard Cite

show abstract

“…In permethylated oligosilanes, the calculated preferred absolute values of backbone dihedral angles are Ϸ55°(gauche), Ϸ90°(ortho), and Ϸ165°(transoid) (9 -12). In oligosilanes with longer alkyl substituents, other angles (deviant and cisoid) can occur as well (13,14).The electronic absorption and emission spectra of these compounds are very sensitive to chain conformation, as manifested in the thermochromism (15, 16), piezochromism (17), solvatochromism (18), and related properties (19, 20) of polysilanes. They therefore represent an especially suitable vehicle for the study of the incompletely understood but clearly important phenomenon of -electron delocalization and particularly of its structural and conformational dependence.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

Magnetic circular dichroism of peralkylated tetrasilane conformers

Fogarty

Imhof

Michl

2004

Proc. Natl. Acad. Sci. U.S.A.

View full text Add to dashboard Cite

Magnetic circular dichroism (MCD) of five peralkylated tetrasilanes (1-5) conformationally constrained to angles ranging from nearly 0°to 180°and of the open chain tetrasilane Si4Me10 (6) shows a clear conformational dependence and permits the detection of previously hidden transitions. In the tetrasilane CH2Si4Me8 (1), with the smallest dihedral angle, comparison of MCD with absorption spectra reveals four low-energy electronic transitions. In the tetrasilanes 2-4, three distinct transitions are apparent. In tetrasilanes 5 and 6, MCD reveals the very weak transition that has been predicted to be buried under the first intense peak and to which the anomalous thermochromism of 6 and other short-chain oligosilanes has been attributed. Linear oligosilanes and polysilanes, Si n R 2nϩ2 (R ϭ alkyl), are fully saturated hydrocarbon analogs with very unusual optical properties and have been of potential practical interest, for instance, as nonlinear optical materials, photoresists, and charge conductors (1, 2).They are of considerable theoretical interest as well. Their most striking property is long-wavelength absorption and luminescence. For long peralkylated polysilane chains, the first absorption peak is in the near-UV range, between 330 and 380 nm (3), and the emission peak is at somewhat longer wavelengths (4). Emission from shorter oligosilane chains often peaks at the edge of the visible (5), and occasionally at wavelengths as long as 500 nm (6). The conformational behavior of these flexible chains is extraordinarily rich (7,8). Six potential energy minima are generally available for torsion about each internal Si-Si bond, many conformers tend to have comparable energies, and barriers to their interconversion are low. In permethylated oligosilanes, the calculated preferred absolute values of backbone dihedral angles are Ϸ55°(gauche), Ϸ90°(ortho), and Ϸ165°(transoid) (9 -12). In oligosilanes with longer alkyl substituents, other angles (deviant and cisoid) can occur as well (13,14).The electronic absorption and emission spectra of these compounds are very sensitive to chain conformation, as manifested in the thermochromism (15, 16), piezochromism (17), solvatochromism (18), and related properties (19, 20) of polysilanes. They therefore represent an especially suitable vehicle for the study of the incompletely understood but clearly important phenomenon of -electron delocalization and particularly of its structural and conformational dependence. This dependence is important for many properties such as charge and energy transfer and substituent effect and spin-density propagation across saturated systems. Detailed analysis and assignment of electronic transitions of oligosilanes as a function of conformation are therefore important, but they are difficult. The absorption bands are broad and closely spaced, making the detection of weakly allowed transitions difficult. Essentially nothing is known about the triplet states of these molecules; they do not contribute significantly to ordinary absorption spectra, and we ...

show abstract

“…These are attributed to the 7-residue 3-turn (7 3 ) helical conformation with a repeat length of 1.96 Å; the 4.4 Å streak is assigned to a 3rd turn-layer line, and the 1.96 Å reflection is a meridional one just on a 7th layer line. 16 The lack of the layer reflections shows the random displacement of molecules along the chain axis. Density requires that two chains run through the unit cell.…”

mentioning

confidence: 99%

Well-Defined Phase Sequence Including Cholesteric, Smectic A, and Columnar Phases Observed in a Thermotropic LC System of Simple Rigid-Rod Helical Polysilane

et al. 2002

View full text Add to dashboard Cite

Liquid crystal phases formed by rigid-rod polymers have been studied extensively in both theoretical and experimental ways. In the initial stage of theoretical approach, it has been shown that a system of long, hard molecules can exhibit an orientationally odered nematic phase if the density is efficiently high. 1 Recently, computer simulation has played an important role in understanding the liquid crystalline behavior of systems composed of hard rod molecules and has given the first indication that the smectic and columnar phases can also be formed by systems of molecules that interact through excluded-volume interaction alone. 2 Prior to this simulation, it was thought that smectic phases could only be exhibited by conventional molecules composed of aromatic mesogen groups and aliphatic tail groups which exhibit mainly attractive interactions. 3 This prompted many further theoretical studies of simple hard-rod systems. 4-6 On the experimental side, lyotropic phases have been extensively studied in biological molecular systems. The columnar and smectic liquid crystals have been reported in lyotropic liquid crystals of synthetic polypeptides, 7 DNA, 8 and the tobacco mosaic virus. 9 These liquid crystal phases observed can be easily differentiated from nematic and cholesteric phases which have so far been well studied for these kinds of rodlike polymers, but the detailed identification and nature of these new phases are still unclear because of the difficulty in managing the lyotropic system.In this study, we treat the polysilanes with chiral side chains that assume a helical rigid-rod conformation with the persistent length of around 85 nm. The rigidity results from severe restriction of main chain's internal rotation by steric hindrance between neighboring side chains. 10,11 Such polymer chain stiffness is closely correlated to the liquid crystallinity in the lyotropic system. Here, we find that these helical polysilanes can form thermotropic liquid crystals (LCs) as well as the lyotropic LCs if the long alkyl side chains are attached to the main chain as a second side group, as reported in a previous study. 12 The ability of thermotropic LC formation is due to the long flexible side chains that act as solvents in the lyotropic system. 13 A typical polysilane that forms a thermotropic liquid crystal is poly[n-decyl-(S)-2-methylbuthylsilane] (PD2MBS) with the following formula:This PD2MBS polymer, having length 1.96 × n Å (n: degree of polymerization) and diameter 15 Å, can be an ideal molecule to use to verify the theoretical predictions of the hard-rod model because the molecules interact mainly through excluded-volume interactions because no significant electrostatic intermolecular attractions are present due to its nonpolar nature. The nonpolar nature is also advantageous for the preparation of a sample with a narrow molecular weight distribution. A simple fractional precipitation method can be used due to the lack of aggregation by the molecules in solution.The results show that the thermotropic phase beha...

show abstract

Poly(di-n-pentylsilane). The spectral consequences of a helical conformation in the solid state

Cited by 88 publications

References 0 publications

Synthesis and structure determination of helical polymers

Synthesis and structure determination of helical polymers

Magnetic circular dichroism of peralkylated tetrasilane conformers

Well-Defined Phase Sequence Including Cholesteric, Smectic A, and Columnar Phases Observed in a Thermotropic LC System of Simple Rigid-Rod Helical Polysilane

Contact Info

Product

Resources

About