Helical polysilanes

Helical polysilanes whose optical activity is induced by chiral side chains are particularly suitable chiroptical polymers for elucidating the inherent nature of the polymer helix since they embody a fluorophoric and chromophoric main chain, exhibiting intense UV, CD, and FL bands due to the Sia-Sia ... 

The cut-and-paste technique has two steps (i) M (or P) screw-sense-selective photolysis at the longer kmax region in CC14 to form P (or M) screw sense telomers with Si-Cl termini, and (ii) Na-mediated recondensation of the telomers in hot toluene. Formation of an optically active helical polysilane with an almost single screw sense was evidenced by significant changes in the UV and CD spectra after the cut-and-paste technique, as shown in Figure 4.6b. 

Most recently, a simple, mild, one-pot immobilization method was developed to attach the rigid rod-like helical polysilane, poly( -decyl-/-butylsilylene), via a siloxy linkage to hydrophilic quartz or mica substrate surfaces.28 195 Triethylamine was used as a catalyst to couple the Si-H and/or Si-OR termini of the dialkylpolysilane chains (which are generated during the course of Wurtz-type synthesis and workup)51,195 with the surface -OH groups. AFM, UV, and IR data were used to analyze the reactions. 

Helical polysilanes where the side groups are partly substituted with Rhodamine B dye molecules and chiral groups (Fig. 12) have been synthesized and spread onto quartz plates by vertical dipping.77 A weak absorption peak due to the dye is observed around 2 eV in addition to the sharp exciton peak at 3.85 eV. The PL spectrum shows a new peak at 2 eV, while the original peak at 4 eV for the polysilane without the dye is greatly decreased. Strong red PL is observed. The introduction of only a few percent of dye modifies the absorption... 

FIGURE 12. Helical polysilane bearing Rhodamine B. (Reprinted from Ref. 77.)... 

Tunable Cholesteric Liquid Crystallinity of Helical Polysilane. 172... 

Poly (S)-3,7-dimethyloctyl-3-methylbutylsilane (16) maybe classified as a rod-like helical main chain polymer bearing enantiopure alkyl side groups. The polymer affords an intense, narrow UV absorption band, completely matching the corresponding CD and FL band mirror image profiles, similar to the rigid rod-like helical polysilanes described above. The occurrence of the... 

Scheme 6 Chemical structures of flexible rod-like helical polysilanes bearing 3,7-di-methyloctyl ((S)- and fR)-forms) and (S)-3-methylpentyl side groups...

Fig. 27 Schematic presentation of thermo-driven chiroptical transfer and amplification in soldier optically inactive polysilane (32) from sergeant optically active helical polysilane (10) on quartz...

This section will demonstrate the first sergeants and soldiers-type helix command surface experiment, in which thermo-driven chiroptical transfer and amplification in optically inactive polysilane film from grafted (or spin-coated) optically active helical polysilane onto quartz substrate [92]. Although helix and optical activity amplification phenomena based on the sergeants and soldiers principle was mainly investigated in polymer stereochemistry, the orientation and physical properties of a thick layer deposited onto a solid surface and controlled by a monolayer command film based on command surface principles was established in photochemical material and surface science [93,94]. Both sergeants and soldiers and command surface experiments appear to have been developed independently. 

From Fig. 28a, it is evident that both 10(g)-32(sc) and 10(g) show a bisig-nate CD band with a positive-signal at 309 nm and a negative-signal at 324 nm, which are characteristics of an exciton couplet Cotton band due to a helical polysilane a-cr transition. For 10(g)-32(sc), both positive-Cotton... 

A noncovalent chiral amplification based on the sergeants and soldiers effect between chiral and achiral polymers in the film state has been developed by Fujiki et al., which results in the formation of helix bundles (Fig. 35) [ 159]. An optically active, helical polysilane 84 with an almost single-handed screw-sense was chemically bonded or spin-coated on the surface of a quartz plate, followed by further spin-coating of an optically inactive polysilane 85 to give... 

Ebihara, K., Koshihara, S., Yoshimoto, M., Maeda, T., Ohnishi, T., Koinuma, H., and Fujiki, M. Direct observation of helical polysilane nanostructures by atomic force microscopy, jpn.. Appl. Phys., 36, L1211 (1997). 

In addition to the polymers described above, the polysilanes having aromatic side groups222 225 and the copolymers of a chiral monomer and an achiral monomer224-228 have been shown to adopt a helical conformation. A water-soluble, helical polysilane having an ammonium moiety has also been prepared.229... 

Fujiki, M. 1994. Ideal exciton spectra in single- and double-screw-sense helical polysilanes. J Am Chem Soc 116 6017. 

A helical polysilane derivative has been found to effectively emit CPL. °... 

Recently, many helical polysilanes with a preferred screw sense have been synthesized mainly by Fujiki and coworkers [74, 85-92]. These con-... 

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